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Lecture 29: Gene Regulation I 9/9/2009 9:08:00 AM

Lactose Operon

y Lactose: disaccharide beta-D-galactopyranosyl-(1-4)-alpha-D-glucopyranose (galactose + glucose)

y Metabolism requires b-galactosidasey Proteins induced by lactose:

o Galactoside permease: brings it into the cellso Thiogalactoside transacetylase: unknown

y F plasmid for making semi-diploid cellsy Components

o lacZ = beta-galactosidaseo lacY = lactose permeaseo lacA = lactose transacetylaseo lacI = lac repressor (constitutive repressor)

Binds to the operator in absence of inducer Inducer causes conformational change Inducer is a metabolite of lactose (formed by low levels

of b-galactosidase in un-induced cells)

Repressor is a tetramer with 2-fold axis of symmetryo Oc = operator (cis-regulatory element)

Palindromey Catabolite repression

o Diauxic growth: two periods of growth separated by lag phase Bacteria prefer glucose; glucose inhibits lac operon. When glucose becomes limiting, lactose metabolism is

activated the lag time is the regulation point when the

lac operon is turned on.

o NOT transcriptional repressiono cAMP and glucose

cAMP levels are low in presence of glucose cAMP addition to cells activates lac operon transcription cAMP effects are mediated by CAP/CRP (catabolite

activator protein, a.k.a. cAMP receptor protein)

CAP binds cAMP and then DNA binds upstream of RNApolymerase

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Increases affinity of RNA polymerase for thepromoter (promoter does not quite match

consensus sequence)

Transcriptional activatory Dual Control of lac operon:

o CAP binds when glucose is present; repressor binds whenlactose is absent. The two together dually turn off the operon.

Only when both are absent is the operon transcribed.

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Lecture 30: Gene Regulation II 9/9/2009 9:08:00 AM

Bacteriophage lambda

y Temperate phagey Cloudy plaquesy Lysogenic: integration of phage genome (as a prophage) into the

bacterial genomeo bacteria resistant to productive lambda infection: result in

cloudy plaque

o infection of E.coli: formation of a closed circular form of thephage genome, then conversion to superhelical form

o 3 stages of viral protein expression early expression controls lysogenic decision

Lysogeny governed by levels of:

y cI (lambda repressor)o only phage protein made during lysogenyo binds OR1 and OL1 (operator sequences)o Mutants make clear plaques (clear I)

y cro protein (blocks cI transcription)o Binds OR3 (operator sequence)

y DNA binding proteins with both positive and negative affects ontranscription

y Induced by lambda repressor if its levels are greater than croo Lambda binding to OR1 blocks PR (promoter)

Blocks cro transcription Cro needed for early gene expression

y Lambda autoregulates its own synthesiso Binding to OR1 stabilizes binding to OR2o And binding to OR2 stimulates cI transcriptiono Binding to OR2 stabilizes binding to OR3o OR3 covers PRM and blocks cI transcription

y Cro binding to OR3 blocks Prm:o lytic cycle occurs if cro protein levels are greater than lambda

repressor

o Cro protein has highest affinity for OR3 Inhibits cI transcription

y High levels of lambda: lysogenyy High levels of cro: lytic replication

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y DNA binding proteinso Helix-turn-helix motifo Proteins bind in major grooveo Monomers bind opposite strandso Dimerso Bind palindromes

Trp Operon

y Attenuationo Leader sequence (two trp codons)o Stem:loop structure (alternative types available)o attenuator is a GC-rich hairpin followed by a U-rich tracto Translation determines which secondary structure

predominates

o When trp is present, segment 1 (leader sequence) istranslated, and regions 3 and 4 bind to terminate

transcription forming the final attenuator product

o When trp is absent, segment 1 stalls, allowing time forsegments 2 and 3 to bind, preventing final formation of the

attenuator

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Lecture 31: Chromosome & Genome Structure 9/9/2009 9:08

Only 1.5 of DNA encodes proteins

Average gene is 27000 bases.

y Average protein size is 400+ (about 13000 base pairs not27000!)

Complexity out of surprising simplicity:y Alternative splicingy Combinatorial control (small numbers of genes combined to

generate diversity).

Centromere, two telomeres, replication origins

Centromere

y Reduced recombination frequencyy Structure

o Function conserved, but not sequence If original sequence lost, new centromere will form

(neocentromere)

o Highly repetitive DNA (alpha satellites)o Unusual protein components including a histone variant called

CENP-A

y Function:o Specify kinetochore assemblyo Maintains sister chromatid cohesiono Monitors attachment of chromosomes to spindle and reports

to a spindle assembly checkpoint

Via CENP-A (which likely acts as an anchor forkinetochore assembly)

Replication origins

y AT rich; 100-200 base pairsTelomere

y Prevent chromosomal end fusiony Recruit specialized capping proteins called shelterinsy Replicative senescence: cell cycle arrest in G1 (telomeres gradually

shorten until this occurs in cells lacking telomerase)

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Solitary genes: 20-50% of genes

Gene families: set of nonidentical duplicated genes (beta-like globins)

Tandemly repeated arrays: rRNA, tRNA and histone genes are in tandemly

repeated arrays encoding identical proteins

Idiom = the G-banding pattern

Simple sequence DNAs (satellite DNAs) repetitive DNAs

y Lie near centromeresy Short (5-10 bp) repeated over and overy Mini-satellite DNA; short regions made up of 20-50 repeat units

containing 15-100 bp each.

y Differences in minisatellite DNA form the basis of DNAfingerprinting.

Moderately Repetitive DNAs

y Include tandemly repeated genes, duplicated gene families, andtransposons

y Mobile genetic elementso LINE (L1 long-interspersed elements 6-7 KB) 21% of DNA

Endonuclease that cleaves DNA RNA Pol II transcribes LINE into RNA, then translated

into proteins, and the RNA and proteins reenter the

nucleus. The endonuclease cleaves the DNA and the

reverse transcriptase inserts DNA copied from RNA.

o SINE (Short interspersed elements 300 bp) 13% of DNA Alu repeats nonautonomous (depend on LINEs) - do not encode

endonuclease/reverse transcriptase

Artificial Chromosome (AC)

y Requires: centromere, two telomeres, sequences for replicationinitiation

y HAC: Human ACo Promise as gene-therapy vectors

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o Contain a cloned, structurally defined alpha-satellite array(centromere), telomere elements, and a replication origin

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Lecture 32: Chromatin Structure/Function9/9/2009 9:08:00 AM

DNA: 10,000 fold compaction (500 fold in non-dividing cells)

Supercoiling

y Plectonemico Twisted thread (extended right-hand coils)

y Solenoidalo Negative supercoilingo Left-handed turns

Heterochromatin

y 10% of the genomey highly condensedy at centromeres/telomeres

Histones

y H1, H2A, H2B, H3, H4y H3 variant = CENP-A at centromeres needed for kinetochorey Basic (rich in arginine and lysine)y Globular, but with unstructured tails

o Tails may aid in the formation of 30nm fibero Tails extend from the nucleosomeo Tails may help attach one nucleosome to another

Nucleosomes

y Eight histone molecules; 2 each of:o H2A, H2B, H3, and H4o Repeats about 200 bp: 147 bp wrapped; rest is linker

y DNA wraps 1.7 turns in a left-handed solenoidal coily Nucleosome formation sites (NFS)

o AT in minor grooveo Allows for compression of minor groove for wrappingo Influences

Degree to which the DNA can bend Presence of other proteins bound to DNA

y 30 nm fibero 100-fold compaction

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y H1o Binds to linker DNA and contacts core histoneso One molecules per nucleosome coreo Seals two superhelical turns of DNA on nucleosome surfaceo Changes path of DNA as it exits nucleosomeo Directs relative positioning of successive nucleosomes

y Rosetteso Nuclear scaffolds and scaffold associated regions (SARs)

organize rosettes

o Persist when DNA is digestedo SARs

Found between transcription units Thus genes are primarily found within chromatin loops

Changes in chromatin structure

y ATP-driven chromatin remodeling machineso Change structure of nucleosomes so that DNA is less tightly

bound

o Some can remove/replace the H2A-H2B dimersy Covalent modifications of histone tails

o Affects stability of 30nm fiber and higher structureso Attracts specific proteins to a portion of the chromatino Charged lysines modified with acetyl, phosphate, or methyl

groups

o Consequences of histone modifications Partition genome into euchromatin/heterochromatin Change in gene expression/recruitment of proteins

involved in gene expression

Recruitment of proteins involved in DNA repair Recruitment of proteins required for DNA replication

HS4 separates active chromatin domain of beta-globin from

heterochromatin; mutation results in anemia

a-thalassemia (ATR-X); Rubinstein-Taybi; Rett and Coffin-Lowry syndromes

y ser/thr kinase mutated in Coffin-Lowry: major target in tail ofH3

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Lecture 33: Medical Genetics 9/9/2009 9:08:00 AM

Types of Genetic Diseases

y Single-gene defectso Mendelian/Mitochondrialo 2% of population / 0.36% of children / 6-8% of hospitalized

childreny Chromosome disorders

o Chromosomes are missing, duplicated, etc.o 5-7/1000 newbornso 50% of all spontaneous 1st trimester abortions

y Multifactorial disorderso Combination of multiple genetic and environmental factors

5% pediatric population 60% general population

o diabetes/schizophrenia/etc.o frequency increases with age

Impact of genetic disease

y ~50% of childhood deaths (1976)y 25% of pediatric hospital admissions

o 22% multifactorialo 3.9% single-geneo 0.6% chromosomal

Three areas of concern that bring children in:

y Major anomaly, or multiple minor anomaliesy Growth deficit

o Intrauterine (symmetric or asymmetric)o Failure to thrive

y Mental deficit (hypotonia, seizures, other neurologic abnormalitiesin neonate)

Physical:

y Inspection: observationy Palpation: touch themy Percussion: consolidation of tissue under skiny Auscultation:

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Dysmorphology: diagnostic and clinical significance of physical variants and

patterns of structural defects

Problems in Morphogenesisy Malformation: poor formation of tissue

o Leading cause of infant mortalityo 22% of neonatal deathso 20% of post-neonatal deaths

y Deformation: unusual forces on normal tissuey Disruption: breakdown of normal tissuey Dysplasia: abnormal organization of cells into tissues

2-4% of newborns have congenital anomalies

Additional 2-5% have a major malformation detected in first year of life

0.5-1.3% of newborns have >1 malformation recognized at birth

Minor malformations and variants in newborns:

y ~14% - 1 minor anomaly (no increase in frequency of majordefects)

y ~0.8% - 2 minor anomalies (major anomalies 5x more frequent)y ~0.5% - >3 minor anomalies (one or more major anomalies in

90%, 26%, and 20% in different studies

4% with one major anomaly have additional malformations likelihood

depends on first malformation detected

y hypospadias 8-9% have additional malformationsHeight/weight/and head circumference are most important measurements in

children

y Fragile X syndrome: macrocephalyy Microcephaly

Measure eyes: from inner to outer canthus (hypotelorism/hypertelorism)

Brushfield spots/epicanthal folds (abnormal after 18 months)

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Fetal Alcohol Syndrome: characteristic facial features

y Two of the following findings:o Microcephaly FOC

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Lecture 34: Cytogenetics 9/9/2009 9:08:00 AM

Constitutional abnormalities: present in the conceptus/zygote

Acquired abnormalities: somatic cell mutations/malignancies

Multifactorial = polygenic inheritance

Chromosomal disorders: excess/absence of multiple contiguous genes gene dosage imbalance

Waardenburg syndrome: mutation in PAX3 gene has multiple effects

Pleiotropy: multiple phenotypic effects of (usually) a single gene

Microdeletion syndromes: Mb stretches of DNA are deleted between

identifiable points within certain chromosomes

The fate of pregnancies with chromosomal abnormalities:

y All conceptions:o 8% begin with a chromosomal abnormalityo 15% of pregnancies spontaneously abort

y All births:o 0.7% have a chromosomal abnormality

Read through list of indications for cytogenetic testing (dont memorize):y Spontaneous abortion, stillbirth, neonatal deathy Ambiguous genitaliay Multiple congenital anomaliesy Failure to thrive (FTT), developmental delay (DD), MRy Short stature, primary amenorrhea Turner syndromey Infertility, multiple spontaneous abortions (MSA)

o one partner may carry a balanced translocation or men mayhave Klinefelter syndrome

y Previous birth with chromosome anomalyy Possible balanced translocation in a parenty Advanced maternal age (AMA)y Neoplasia, e.g., leukemia (these are acquired chromosomal

abnormalities)

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Cytogenetics Laboratory Report

y Usually analyze peripheral blood (lymphocytes)Histology: fix tissue with aldehyde to cross-link molecules. Cuts through

mitoses.

Cytogenetics: swell cells hypotonically, fix them in MeOH and acetic acid,

allow fixative to evaporate. This crushes the cells and spreads the

chromosomes out on the slide.

Late prophase and prometaphase are the best stages from which to harvest

chromosomes with good banding detail.

Short arm: p petite

Long arm: q comes after p in the alphabet

A karyotype from a good blood lymphocyte should have at least 550 bands

per haploid karyotype.

Acrocentric chromosomes tend to fuse the long arms (Robertsonian

translocations)

The ISCN is a shorthand system for describing chromosomal banding

y Der: derivative chromosome (translocations that cant be puttogether)

y Del: deletionFluorescence in situ hybridization (FISH)

y Amplify and label a sequence of DNA with a fluorescent label.y Spread denatured chromosomes and mix with probe on slide.y Probe hybridizes.y Types

o Locus specific For metaphase Probes small deletions (at the kb level)

o Repetitive sequence

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Includes satellite DNA (centromeres and subtelomeres) Centromeres of interphase cells can be probed (way

around mitosis)

o Whole chromosome paint Metaphase Useful for showing:

Whether a rearranged area contains DNA fromonly that chromosome, or from another.

cryptic translocations (hard to seetranslocations)

Smallest band visible contains 1 Mb of DNA.

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Lecture 35: Eukaryotic Gene Regulation I9/9/2009 9:08:00 AM

Similarities to bacterial gene expression

y Require transcription factors/transactivatorsy Importance of weak/reversible protein-protein interactionsy Versatility afforded by DNA looping

Reasons for euk promoters to be positively regulated

y Large size of genome makes nonspecific DNA binding a problemy More efficient: only neededgenes are transcribed

4 phases of transcription

y assemblyy initiationy elongationy termination

Know:

y TBP (TATA binding protein)y TFIID: interacts with sequence-specific DNA binding proteins

(activators)

y TFII (A, B, E, F, H) also exist.TATA box lands the polymeraseINR: initiation region lands polymerase and allows melting of DNA so

transcription can occur.

Activation of transcription

y Recruit RNA Pol II (formation of the PIC)y Recruit GTFs

o How? Pol II is not attracted to promoter sequences:Transcription factors (activators)

y Activators recruit:o Chromatin remodeling proteinso General transcription factors (TBP, etc.)o Co-activators (mediator)o RNA Pol II

y Steps:

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o Activators bind to chromatino Chromatin remodeling

Transcriptionally active chromosomal regions are DNAsesensitive, so you can probe for differences in chromatin

with a nuclease degradation only where transcription is or is about

to be occurring

Deficient in H1; the core histones are more likelyto be acetylated.

Undermethylated at CpG residues ATP-dependent chromatin remodeling complexes

Facilitate creation of an activated intermediate (I)that has loosened histone-DNA contacts.

o Covalent histone modification Histone modifying complexes brought to DNA by TFs Remodel via acetylation of histones

Affects cores affinity for DNAy neutralizes positive charge on lysiney eliminates interaction with DNA phosphates

HATs histone acetyl transferases Facilitate loading of activators onto nucleosome

templateso Additional activator proteins bound to gene regulatory region

(mediator)

o Assembly of pre-initiation complex at the promotero Transcription initiation

Assembling the pre-initiation complex

y Binding of additional activator proteinsy Mediator recruited

o Transcriptional co-activatoro Potentiates the transcriptional responseo Allows fine-tuning to result in an appropriately calibrated

output to the RNA Pol II machinery

o Brings TBP to TATA site (other GTFs come in)o Recruits RNA Pol II, assembles the PIC

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o Rearrangement of the PIC and transcription firesHow do eukaryotic TF complexes function over distances?

y Intervening DNA is looped out.y Facilitated by high mobility group (HMG) proteins

Structure of hypothetical gene regulatory protein

y DNA binding domain (binds specific promoters)y Effector domain(s): protein interaction motifs (to recruit/bind other

proteins)

Basic leucine zipper: combinatorial control

y Heterodimerization can alter DNA binding specificity

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Lecture 36: Eukaryotic Gene Regulation II9/9/2009 9:08:00 AM

Transcriptional Repression

y Repressors: gene regulatory proteins that repress transcriptiono Some are dedicatedo Most are situational repressors clues from other proteins

y 6 Mechanisms by which repressors represso competitive DNA binding

blocks activator binding siteo masking activating surface

interact with activator to block activation siteo direct interaction with the general TFs

repressor touches TFIID; prevents transcriptiono recruitment of chromatin remodeling complexeso recruitment of histone deacetylaseso recruitment of histone methyl transferases

some methyl groups will silence transcriptiono last three require co-repressors (enzymes)

Beta-globin Gene Expression

y Human beta-globin gene is part of a cluster of globin geneso 5 human beta-like globin geneso order of genes reflects sequence of their expression during

developmento expression controlled by a locus control region (LCR) located

far upstream

many genes residing in gene clusters and expressed ina differentiation/development-specific manner are

regulated by these

composed of several DNAse I hypersensitive sites thatfunction together to regulate the expression of several

cis-linked genes

o expression also controlled by proximal promoterso epsilon-globin: in embryonic yolk saco gamma-globin: when hematopoiesis is shifted to fetal livero delta/beta-globin: around birth when bone marrow becomes

major site of hematopoiesis

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Functions of LCR (basically an enhancer)

y proteins binding to hypersensitive sites in the LCR mediate theformation of modified chromatin domains and an accessible LCR

holocomplex

y Some transcription occurs through the HS sites aids inmaintaining open configuration

y If stem cell commits to another lineage, entire LCR and globin locuspacked into heterochromatin

y Transcription complexes can be found at LCR, but not at the beta-globin promoter.

y At the appropriate stage, erythroid-specific factors bind to theglobin locus, further unfold the chromatin structure and bring the

LCR and the beta-globin promoter in close proximity

y Transcription complexes are transferred to the beta-globinpromoter

Thalassemia can be caused by LCR mutations (deletion that removes all of

it; beta-globin gene is transcriptionally inactive

Rett Syndrome

y Mostly in girlsy Mental retardationy Normal development up to 1 yeary Regression: loss of speech and motor skills; seizures, microcephaly,

ataxia, autism, stereotypic hand movements (hand wringing)

y Mutation in MeCP2o Methyl CpG-binding protein 2 geneo Xq28

Expressed in the braino Lethal in males so only femaleso Methyl CpG

CpG dinucleotide found at promoter regions for Pol IIgenes

CpG islands mostly methyl-free

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Such promoters can be silenced when CpGs aremethylated

o DNA methylation can affect gene expression Direct interference of methyl CpG with DNA binding of

gene regulatory proteins Proteins which directly bind methylated CpGs

independent of surrounding sequence

o MeCP2 functions as DNA methylation dependenttranscriptional repression.

Its repression is associated with the recruitment of theSin3-HDAC complex (histone deacetylase machinery)

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Lecture 37: Chromosomal Abnormalities I10/11/2009 8:39:00 PM

Part I

Numerical Abnormalities

y dispermy or diploid egg/spermy tetraploidy: 4 copies

o failure of zygote to divide after first S-phasey Aneuploidyy Explanations

o Crossing over: recombination occurs prior to the first meioticdivision

o Synapsis of homologues in the prophase of the first meioticdivision

o Chiasma(ta): indication of crossing overo Nondisjunction

In meiosis I, the failure of homologues to separate In meiosis II, the failure of sister chromatids to

separate

o How aneuploidies arise Meiosis I

Nondisjunction Loose pairing Failure to pair Noncongression to metaphase plate (possibly

main mechanism)

Most of female meiotic errors Meiosis II

Nondisjunction Noncongression to metaphase plate Premature chromatid separation

o Uniparental disomy: both copies of a chromosome come fromone parent

y MosaicismAccuracy of a cytogenetic test depends on:

y How many chromosomal bands are clearly seen (banding resolutionor level)

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Chromosomal rearrangements occur with minimal DNAbreakage

Part II

3 most commonautosomal trisomies

y 21 (Down syndrome)y 18 (Edwards syndrome)y 13 (Patau syndrome)

Down Syndrome

y Single palmar crease (simian crease)y Fifth finger clinodactylyy Congenital heart defectsy Intestinal stenosisy Umbilical herniay Sandle footy Dictyotene prolonged phase in female meiosis between

synapsis/recombination and the two meiotic cell divisions

y Translocation Down syndromeo 4% of Down syndrome babies inherit Robertsonian

translocationo Between - of this 4% are de novo; other fraction

inherited

o Form a trivalent during synapsis Three patterns of segregation possible

o Rate of Down Syndrome with Robertsonian 14;21 carriers Should be 1/3 of births BUT:

If mother is carrier, rate is 10-15% If father is carrier, rate is 1-2%

Trisomy 18

y Prominent occiputy Lowest, malrotated earsy Overlapping fingers

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o Can be seen on ultrasound.o Caused by muscle shortening due to spasm or fibrosis, or

impairment of antagonistic muscle with loss of muscular

balance

y Dimple on the chesto Short sternum

y Prominent calcaneus (rocker-bottom feet)y Congenital heart/renal defects/malformationsy Hypotonia then hypertonia

Trisomy 13

y Cleft lip; cleft palatey Microcephalyy Prominent calcaneus (rocker-bottom foot)y Post-axial polydactylyy Umbilical herniay Congenital heart/renal defectsy Do not make it through the first year of lifey Holoprosencephaly: Cyclops

o Failure of clevage of the prosencephalon with deficits inmidline facial development no brain hemispheres

o Causative gene: Sonic hedgehog gene (SHH)o Can be inherited as AD with variable expression

(holoprosencephaly spectrum)

Trisomy 16: most common spontaneous abortion trisomies

Trisomy 22: next most common spontaneous abortion trisomy

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Lecture 38: Eukaryotic Gene Regulation III9/9/2009 9:08:00 AM

Structure and Function ofNuclear Receptors and their Ligands

Nuclear receptors

y Nuclear receptor superfamilyo Intracellular receptors for small hydrophobic signaling

molecules (steroid hormones)

y Ligand activated transcription factorso In the absence of bound ligand:

Cant bind DNA Cant activate transcription

o Some steroid hormone receptors become repressors whenligand binds (example: some thyroid hormone receptors)

y Ligands are small, hydrophobic, and freely diffusibley Steroid hormones pass across membrane and directly regulate the

receptors.

o Generalized structure from N- to C-term Activation domain DNA binding domain Ligand binding domain

y Androgen receptoro Ligand: testosterone metabolite

5alpha-dihydroxytestosterone testosterone

male sexual differentiation male behavior sperm production at puberty prostate development prostate cell proliferation Gene targets:

y PSAo Androgen insensitivity syndrome (AIS)

Mutations in areas that make direct contact with ligandor abolish binding to co-activator proteins

Typically includes evidence of feminization of theexternal genitalia at birth, abnormal secondary sexual

development in puberty, and infertility

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CAIS: complete, typical female genitalia PAIS: partial, ambiguous genitalia ( or predominately

male or female

MAIS: mild, typical male genitalia Sequencing of the AR locus can detect mutations in

95% ofCAIS cases.

Somatic mutations have been linked with prostatecancer.

y Retinoid receptorso Unliganded retinoid receptors

Bind retinoic acid response elements Recruit histone deacetylase machinery

o Ligand bound receptors Retinoid binding leads to release to HDAC Recruits of transcriptional co-activators

o Ligands All-trans retinoic acid

All-trans-retinoic acid (RA)-based therapy in acutepromyelocytic leukemia (APL)

y APL: acute myeloid leukemia; immaturewhite blood cells predominate in the

patients bone marrowy 10% of all adult acute myeloid leukemias

(1% of childhoold leukemias)

y contains a t(15;17) rearrangementy PML-RARa

o Chimeric transcription factor derivedfrom t(15;17)

o Fusion of the retinoic acid receptor tothe PML gene

o Results in the production of a chimericgene regulatory protein

o Retains the RAR DNA binding andretinoic acid (RA)-binding domains

o Contains portions of a protein calledPML

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y Retinoids are key for myeloid differentiation,vitamin A deficient mice and humans have

defects in hematopoiesis

o Genes regulated include regulators ofthe cell cycle, growth factors and theirreceptors, clotting factors, etc.

y RAR alpha responds to high-dose ATRA.o PML-RARa forms dimers and

represses the transcription of target

genes essential for granulocyte

maturation

o Repression is achieved through highaffinity interactions with the HDAC

machinery

o Can recruit DNA methylating enzymesresulting in hypermethylation of DNA

o ATRA induces a conformationalchange, making residues available for

binding to co-activators such as CBP

y Survival rate: 75-80% of young patientswith ATRA and chemotherapy

y 9 retinoic acid

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Lecture 39: Chromosomal Abnormalities II9/9/2009 9:08:00 AM

Deletion Syndromes

Cri-du-chat syndrome (del 5p)

y Hypoplastic larynxy Microcephalyy Ocular hypertelorism with epicanthal fold and downslanting

palpebral fissures

y Live to majority ofteny Severe mental retardation, but can achieve social development of a

normal 4 or 5-year-old

Wolf-Hirschhorn syndrome (del 4p)

y Frontal bossing; prominent glabella; hypoplastic orbital ridgesy Broad, flat nose; ocular hypertelorismy Greek helmet facey large, lowset, simplified ears; micrognathiay closure defects (CL/CP; cardiac)y severe mental retardationy 1/3 die in first year

Microdeletion syndromes (contiguous gene syndromes; segmental

aneusomies)y Prader-Willi s./Angelman s.: 15q11-q13

o Prader-Willi s. MR; moderate severe hypotonia at birth Almond-shaped eyes, narrow bitemporal diameter Short stature; small hands and feet Hypogonadism, microgenitalia Hyperphagia in childhood leading to obesity

o Angelman s. (del 15q11.2-13) Microcephaly, prognathism, large mouth with widely

spaced teeth; open-mouth expression

Ataxia with puppet-like gait Severe MR; no language happy disposition with inappropriate laughter

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cause: absence or mutation of E6-AP ubiquitin ligasegene

Single gene mutation; Prader-Willi requires a largerdeletion

y DiGeorge s./velocardiofacial s.: 22q11o DiGeorge s.

Ocular hypertelorism; bulbous nasal tip; lowset,protuberant ears; small mouth; micrognathia

Hypoplasia of thymus T-cell deficit, infections Absent or hypoplastic parathyroids hypocalcemia,

neonatal seizures

CP, bifid uvula Conotruncal cardiac defects

o VCFS: Nose with broad root, hypoplastic ala nasae and

bulbous tip

CP or valopharyngeal insufficiency; speech difficulties Some patients: decreased lymphoid tissue; neonatal

hypocalcemia

Conotruncal cardiac defects: development of ascendingaorta and main pulmonary artery

y Miller-Dieker s: 17p13.3 (lissencephaly)o Brain with smooth cortical surface; absent gyrio High forehead, vertical creasing, bitemporal hollowing,

midfacial hypoplasia, short nose with upturned nares, long

philtrum with protuberant upper lip

o Postnatal growth retardation, seizures, spasticityo Profound MRo Dead before age of 2

y Williams s.: 7q11.23Explanation for microdeletion syndromes: NAHR, nonallelic homologous

recombination

y LCRs (low copy repeat regions) of near homology flanking thedeletion critical regions can cause misalignment of homologous

chromosomes as they synapse in meiosis

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y Recombination within a misaligned regions results in a deletion ofone chromosome and a duplication of the other

y Unequal genetic exchangeo Rareo Flanking LCRs predispose the unequal exchanges to occur,

producing recognizable syndromes, though at low rates

y Genomic disordersy Note: Angelman depends on loss of one gene; Williams s. is caused

by a deletion to small to visualize; DiGeorge/VCSF deletion is

compatible with productive, functional life; cri-du-chat s. shows

how size of deletion affects phenotype

Charcot-Marie-Tooth disease

y Caused by NAHR resulting in a duplication of a 1500 kb region on17p11.2

y Deletion of same region causes hereditary neuropathy with liabilityto pressure palsies

Reciprocal translocation

y Carriers of balanced reciprocal translocations have an aggregateempirical risk of 12% of producing offspring with an imbalanced

translocation (a derivative chromosome)y Segregation patterns similar to Robertsonian translocation

Subtelomeric region rearrangements

y Regions near telomeres are gene rich and subject to:o Terminal deletions and balanced translocation in carriers with

transmission in imbalanced form

o Rearrangements can be cryptic (microscopically invisible)Sex Chromosome Abnormalities

y Klinefelter syndromeo 1/1000o 47, XXYo long limbso cannot diagnose at birth or at prepuberty

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o Gonads: small, fibrosedo Infertileo Verbal low; performance skills normalo Normal spatial skills; perceptiono Feminine habitus (gynecomastia in 30%)

y Turner syndromeo 45, Xo coarctation of the aortao webbed neck: result of fetal cystic hygroma (results from

general lymphedematous state during fetal development)

o short stature, widespread nipples and shield chesto low hairlineo pigmented nevi/cubitus valguso tapering fingers with convex nailso lymphedemao average or above average intelligenceo deficiencies in spatial perception, perceptual motor

organization and fine motor skills

o nonverbal IQ lower than verbal need help in math

o elevated risk of impaired social adjustmento hormone therapy for growth, secondary sex characteristicso can diagnose at birtho Gonads: streak; amenorrheao Infertileo 45, X is only 50% of diagnoseso 46, X i(Xq) is 15%o 45, X/46, XX mosaics is 15%o Some are fertile

y 47, XXXo 1/1000o taller than sibso normal fertility, XX games are rareo 10-15 points below sibs IQ; verbal IQ is less than

performance (IQ scores cluster in 85-90 range)

y 48, XXXX

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o absence of obvious dysmorphic features in 5-month-oldo severe MRo tallo hypertelorism with epicanthal foldo radioulnar synostosis

y 47, XYYo 1/1000o taller than sibso normal fertility, YY gametes are rareo IQ is less than sibs; language difficultieso Criminality: tall, mentally deficient prisoners frequently have

this karyotype

Lack of emotional control Impulsive Not particularly aggressive Lack psychopathology Convictions correlate with IQ Supportive upbringing compensates for genetics

47, XXX; 47, XXY; 47, XYY karyotypes are underrepresented in cytogenetic

studies of spontaneous abortion material

y 45, X is overrepresentedy The other three survive about as often as normal conceptionsy General principle: Sex chromosome aneuploidies are less harmful

than autosomal ones (except Turner s.)

49, XXXXY

y hypoplastic genitaliay severe MRy skeletal abnormalitiesy Some X genes must be inactivatedy About 15% of X genes (on p arm) escape inactivationy Necessary for normal phenotypey Even with 3 of the Xs inactivated, the genes that escape

inactivation are present in 2 extra copies.

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Sex reversal

y 46, XX maleo SRY translocated into the X chromosome

y 46, XY femaleo SRY translocated out of the Y chromosomeo Short statureo Primary amenorrheao Lack of secondary sex characteristics, poor breast

development, absent axillary hair

o Normal female genitaliao Uterus present, streak gonads

y Genes located at the pseudoautosomal region (synaptic region)assort in an autosomal, rather than X-linked, pattern. The Sex-

determining region Y (SRY) is located just below the

pseudoautosomal region, so that it can sometimes be translocated

(NOT a crossing over) to the X chromosome.

y 46, XY females with androgen insensitivity syndromeo testicular feminizationo apparently normal female external genitaliao blind vaginao no uterus or uterine tubeso sparse axillary and pubic hairo cause: mutation of X-linked androgen receptor gene

y 46, XY females with duplication of DAX1 gene at Xp21 suppressingthe effect ofSRY.

Abnormal genitalia may involve

y Hermaphroditism a condition in which both ovarian and testiculartissue are involved

y Pseudohermaphroditism the individual has gonadal tissues of onesex, but external genitalia of the opposite sex, ambiguous, or

bearing marks of embryological development of the opposite sex

y Ambiguous genitaliao Hypospadias (urethra opening on underside of penis or

perineum)

o Enlarged clitoris; micropenis

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o Fused labia resembling a scrotumo Blind, pseudovaginal opening

y Congenital adrenal hyperplasio ARo Defects in enzymes of the adrenal cortex required for cortisol

biosynthesis

o Excess production of androgens leads to virilization of femaleinfants (male infants are normal)

o 21-hydroxylase deficiency has a salt-losing variety which canlead to neonatal death

Chromosome Instability Syndromes

y Single gene, autosomal recessive syndromes with deficiencies ofenzymes of DNA replication and repair

y Include: ataxia telangiectasia, Bloom s., Fanconi anemia, Robertss., Xeroderma pigmentosum, and others

y Chromosomes show breaks/gaps involving one or both chromatidsy Sister chromatid exchanges can be apparent in high numbersy Remarkable radial configurations of conjoined chromosomes look

like synapsed chromosomes

y Increased rearrangementsy Physical findings:

o Neoplasiao Growth retardationo Progeriao MRo Limb reductiono Sun sensitivity (uV)o Sensitivity to ionizing radiation (X-rays)o Chromosomal translocationso Immunodeficiencyo Gonadal failure

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Pigmentation abnormalities (white forelock) PAX3 mutation

o Proteins acquisition of toxic effects Hereditary amyloidoses

Mutation in thyretin leads to protein accumulationas fibrils causing cell disruption

Huntington diseaseo Proteins acquisition of a new function

Alpha-1-antitrypsin Point mutation at 358 causes it to inhibit

thrombin, leading to a severe bleeding disorder

o Dominant negative effect Disease causing allele disrupts function of the normal

allele

Osteogenesis Imperfecta type IIy Extreme divergence of sex ratio

o Sex-limited: male-limited precocious puberty (familialtestotoxicosis)

o Sex-influenced: pattern baldness and hemochromatosisX-linked Recessive

y Anhydrotic ectodermal dysplasiao Sparse hair; frontal bossing; absent eyebrows; supraorbital

ridging; hyperpigmentation; periorbital wrinkling; saddle

nose; everted, thick lips; hypoanodontia peg teeth; hoarse

voice; hypo-anhidrosis

y No father-to-son inheritancey All daughters are carriers

X-linked dominant

y X-linked hypophosphatemic (vitamin D resistant) ricketsy Incontinentia pigmenti (male lethal)

o Bullae and verrucous papules along Blaschkos lineso Lines ofBlaschko show migration of embryologic skin-forming

cells

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Ocular Pulmonary Skin

Striae atrophicae Recurrent hernia

Duralo AD disorder of connective tissueo Mutations in fibrillin-1 gene (FBN1)

Other disorders: Neonatal Marfan syndrome Isolated skeletal findings AD ectopia lentis MASS phenotype

y Mitral valve prolapse or myopiay Aortic enlargement borderline and

nonprogressive

y Skeletal and Skin findingso Fibrillin-1 is an extracellular glycoprotein that polymerizes to

form microfibrils

Scaffolding for deposition of elastin Structural molecules in load-bearing tissues

o Incidence: 1 in 10,000o 25-35% are de novo mutations

Delayed age of onset

y Huntington diseaseo Neurologic disorder with

Chorea, dementia, and affective disorder Expansion of a CAG repeat sequence on chromosome

4p

AD with one of the lowest mutation rates in humans De novo mutations are rare.

y ADPKD: autosomal dominant polycystic kidney diseaseo Progresses to end-stage renal disease (8-10% of all end-

stage renal disease)

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Uncontrolled high blood pressure and diabetes are mostcommon reasons for end-stage renal disease

o Prevalence: 1/300 1/1000o Hypertension in 20-30% of children and 75% of adultso Delayed age of onset

Complete penetrance by age 90 Most manifest in 3rd or 4th decade

o Detection of renal cysts by ultrasound increases with age 80% to 90% by age 20 100% by 30

Variable expression

y Environmental effects environment can diminish or enhance theexpression of a phenotype

o Diet and PKUo UV light and Xeroderma pigmentosa

y Modifier genes genes that alter the expression of a gene atanother locus

y Allelic heterogeneity different alleles (mutations) at a specificlocus can produce varying expression of a disease

y Locus heterogeneity a single disease phenotype caused bymutations at different loci in different families

y ADPKD has locus heterogeneityo ADPKD1

Gene: PKD1 Polycystin 1 16p13.3-p13.1 85% of affected individuals generally more severe with earlier onset of ESRD

mean age 54

o ADPKD2 Gene: PKD2 Polycystin 2 4q21-q32 15% of affected individuals onset ESRD 20 years later (mean age 74)

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males progress to ESRD more rapidly than femalesOsteogenesis Imperfecta

y Type I collageno Triple helix composed of

Two pro-alpha-1 chains One pro-alpha-2 chain

o Assembly begins at C-terminus so mutations near C-terminalend of the pro-alpha-1 chain are more disruptive

y Type I OIo Fracturing lessens with ageo Most do not have skeletal deformityo Can also have dentinogenesis imperfectao Conductive hearing loss with ageo Mild bone fragility; half have normal stature

y Type II OIo More severeo Perinatal lethalo Respiratory failure shortly after birth

y Type III OIo Between Types I and II in severityo A lot of skeletal deformityo Also have DGI

Neurofibromatosis type I

y ADy 1/3500 individualsy large gene on proximal 17q (NF1)

o 350 kb and 60 exonso high mutation rate

y 50% of cases are new mutationso 80% are paternal in origin

y Neurofibromino Control of cellular proliferationo Tumor suppressor

y Mutations result in loss of function 80% cause protein truncation

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y Interfamilial phenotypic variability/intrafamilial phenotypicvariability

y Caf au lait spots: hyperpigmented skin patches (big freckles)o 6 or more of 5 mm or moreo 15 in adultso Axillary freckles

y Lisch nodules: benign growths on the irisy Neurofibromas: benign peripheral nerve tumors

o Increase in number at puberty in womeny Plexiform neurofibromas: extensive growths along a large nerve

sheath

y Allelic heterogeneityo Complete deletion of the gene

Large number and early onset of neurofibromas More frequent and severe cognitive abnormalities Somatic overgrowth

o A 3 bp in-frame deletion of exon 17 Pigmentary changes ofNF1, but no cutaneous or

surface plexiform neurofibromas

y Causes of variable expressiono Modifying genes at other loci

Family studies show Lisch nodules and caf-au-laitspots more similar among 1st than 2nd degree relatives

o Stochastic or random factors Acquired second hit mutations, or loss of heterozygosity

at the NF1 locus in the following tumors

Neurofibromas, malignant peripheral nerve sheathtumors, astrocytomas, and others

Sex-limited traits

y Traits expressed in only one of the sexes due to anatomicaldifferences

o Uterine/testicular defectsSex-influenced traits

y Male-pattern baldnesso AD in males

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o Females express it in the homozygous state, but milderAffected females with an X-linked recessive disorder

y Manifesting heterozygote: skewed inactivation with greaterrepresentation of the mutant allele in the active X chromosomes

o 5% of female carriers of hemophilia A have factor VIII levelslow enough to be considered mild hemophiliacs

y Turner syndromey Translocations or deletions of X chromosomey Homozygous female

Anticipation

y Progressively earlier or more severe expression of a disease in themost recent generations

o Expansion of DNA repeatsy Myotonic dystrophy

o ADo Progressive muscle deteriorationo Cardiac arrhythmiaso Testicular atrophyo Cataractso Incidence: 1/8000 most common MD in adultso Mutation: expanded CTG repeato Unaffected: 50 copieso Phenotypes:

Mild 50-150 CTG repeats cataract; mild myotonia; DM onset 20-70 years

Classic 100-1000 CTG repeats weakness myotonia cataracts

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balding cardiac arrhythmia onset 10-30 years

Congenital >2000 CTG repeats prenatally polyhydramnios and decreased fetal

movement

generalized weakness hypotonia respiratory compromise mental retardation in 50-60% facial diplegia (weakness) onset birth-10 years

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Lecture 42: Non-traditional Inheritance 9/9/2009 9:08:00 AM

Mitochondrial DNA

y 16.5 kb on a double-stranded circular molecule that encodeso 22 tRNAso 2 rRNAso 13 polypeptides

y Differ from Mendelian geneticso Heteroplasmy: presence of more than one genetically distinct

mtDNA per cell

Children can have varying proportions of genotypeso Homoplasmy: presence of only one mtDNA in a cello Replicative segregation: changes in proportions of different

mtDNA alleles as the mitochondria reproduce

Population genetics

y Study of allele frequency distribution and change influenced by:o Natural selectiono Genetic drifto Mutationo Gene flow

Phenotypic expression in mitochondrial disease

y Reduced penetrancey Variable expressiony Pleiotropy

Clinical expression for all mtDNA mutations depend on:

y Threshold effect: mutational load and vulnerability of each tissue toimpaired oxidative metabolism

o Tissues most dependent: CNS/vision/hearing/musclePleiotropy and variable expression

y A3243G mutation in tRNAleu geneo MELAS mitochondrial encephalomyopathy with lactic

acidosis and stroke-like episodes

o Diabetes/deafnesso CPEO chronic progressive external ophthalmoplegia

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o Cardiomyopathyo 0.5-1.5% of diabetes in general population

Mitochondrial DNA deletion syndromes

y Three overlapping phenotypeso Kearns-Sayre syndrome (KSS)

Onset

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o Loss of coordinationo Spasticityo Dementiao Sensorineural hearing losso Optic atrophyo Articulation defectso Renal dysfunctiono Diabeteso Cardiomyopathy

y Onset: childhood or adulty Slowly or rapidly progressivey Mutation in mitochondrial tRNAlys geney Heteroplasmic with variable expressiony Diagnosis: muscle biopsy

o Gomori trichrome stain: ragged-red fibero Cytochrome oxidase stain lack of cytochrome oxidase in

affected muscle fiber

o Mutations usually detectable in all tissues including WBC inindividuals with typicalMERRF

o Individuals having few symptoms or asymptomatic maternalrelatives

May be undetectable in leukocytes May only be detected in skeletal muscle, or sometimes

in skin fibroblasts, urinary sediment, oral mucosa, hair

follicles

y Treatment:o Supportiveo Coenzyme Q10/idebenone

Bypass block with artificial electron acceptor Minimize free radical-induced damage

o Riboflavin in complex I and/or complex II deficiency enhances residual activity

Uniparental disomy

y Nondisjunction

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y Disomic cell line containing 2 of the same chromosome inheritedfrom one parent.

o Isodisomy same chromosome (homolog) present induplicate

o Heterodisomy both homologs from one parent are presenty Possible cause of disease when:

o AR disorder with only one parent a carriero X-linked disorder with transmission from father to son

(paternal heterodisomy) or expression in homozygous form in

a female (isodisomy father affected or mother a carrier)

o Imprinted genes presentEpigenetic

y Any factor that affects gene function or expression without changein the genotype or primary DNA sequence

o DNA methylationo Chromatin configurationo Histone modificationso Transcription factor binding

y Stably transmitted through mitosisy A normal phenomenon that explains

o X inactivationo Genomic imprintingo Regulation of gene expression in the development and

maintenance of tissue differentiation

Genomic imprinting

y Parent of origin specific silencing of a subset of geneso Results in paternal/maternal alleles with different levels of

expression

y Imprint is erased between generations and new imprint establishedaccording to the sex of the germline

y Features:o Often physically linked in clusters that contain reciprocally

imprinted genes

o May be tissue specific/developmentally regulated

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o Expressed in developing embryo, placenta, and prenatal andpostnatal brain

Fetal growth control Paternally expressed genes promote growth Maternally expressed genes suppress growth

A role in postnatal maternal care before weaning and inneonatal adaptation

y Prader-Willi Syndrome (PWS)o Some cases: Lack of the paternally expressed genes in the

15q imprint domain

y Angelman syndrome (AS)o Some cases: Lack the maternally expressed genes in the 15q

imprint domain

Dynamic mutations

y CGG and CAG commony Frequently found in genes that encode transcription factors or that

regulate development

y CAG repeat expansionso occur primarily with paternal transmission

y Fragile X syndrome, Friedreich ataxia, and myotonic dystrophyo expansion primarily during maternal transmission

Unstable repeat expansions

Class 1 disorders

y Expansion of noncoding repeats cause loss of protein function byimpairing transcription of the preRNA from the affected gene

y Friedreich ataxiay Fragile X

o Non-specific phenotype HC>50% Prominent forehead Long face Large ears Prominent jaw Normal growth

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Large testes post-pubertyo Affected males: moderate MRo Affected females: mild MRo Autistic behaviorso X-linked dominant

80% penetrant in males 30% penetrant in females

o Single most common inherited cause of mental retardation 1/1250 males 1/2500 females

o FMR1 on distal Xq has a CGG repeat in 5 UTR Normal: AGG triplets every 9-10 CGG repeats; probably

to maintain repeat integrity by preventing DNA strand

slippage during replication

o 5-40 repeats (normal)o 41-58 repeats (intermediate)o 59-200 repeats (premutation)

not associated with MR FMR1-related premature ovarian failure occurs in 20%

of females with FMR1 premutation

o >200 repeats (full mutation) aberrant hypermethylation of the deoxyctidylate

residues contained in CG repeats usually occurs once

expansion reaches this level

Premutations ONLY expand to full mutations whentransmited through FEMALES

o Normal-transmitting males Daughters will be carriers Are at risk for tremor/ataxia syndrome

Late-onset, progressive cerebellar ataxia andintention tremor

Class 2 disorders

y Expansions of noncoding repeats that confer novel properties on theRNA

y Myotonic Dystrophy 1 and 2y Fragile X-associated tremor/ataxia syndrome

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Class 3 disorders

y Due to repeat expansion of a codon that confers novel properties onthe affected protein

y Spinocerebellar ataxiasy Huntington disease

o Normal: 36 CAG repeats

Reduced-penetrance HD-causing alleles: 36-39 repeats Full penetrance HD-causing alleles: >40 repeats

o chromosome 4p: long polyglutamine tract in huntingtino soluble nonaggregated mutant huntingtin interact with

transcription regulators to alter transcription of many proteins

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Lecture 43: Intro to Development 9/9/2009 9:08:00 AM

Developmental fields specify the fate of cells over a region.

y Insects can regenerate certain tissues between molts so thisdevelopmental field is still present after the leg is formed

y Answer 3: roach sees a discontinuity; fixes it does not magicallyreturn leg to normal, but returns leg to quasi-normal

y What is the upper limit in the size of developmental fields?o About 1 mmo Means that humans must be made into their correct shape in

miniature very early in development

Growth

Differentiation

y Ways of making cells different from each othero Gradients

cells along gradient develop differently Bicoid gradient in Drosophila embryo

Anterior-posterior developmento Induction

Cells brought together adopt new fate Neural tube is formed by bringing together different

cells: induced into becoming neural crest cells. Migrate away; differentiate into PNS nerves

Patterning by sequential induction Each cell induces another in turn

o Asymmetric distribution of determinants Daughter cells take on different fates example from c. elegans P granules (or polar plasm in mammals) consist of

proteins

Segregated into germline progenitor cells Then segregated into P4 to form germline cells Cells without p granules become somatic cells

Determination versus differentiation

y Cells directed down a developmental path before they differentiate.

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y When their fate is decided/restricted, they have undergonedetermination.

y Grafting experiment chick limb developmento Take leg bud; graft it onto wing budo Graft cells from before determination will become whatever

they are inserted into

o Graft cells from after determination become what theywould have been before grafting

Terms

y Totipotent: can produce all cells in an organismy Pluripotent: can produce most cell typesy Multipotent: can produce multiple cell types

o Blood stem cellsy Stem cells: primal undifferentiated cells that can differentiate into

other cells can have multiple levels of potency

y Why do scientists refer to stem cells that appear to be able todifferentiate into all cell types pluripotent, rather than totipotent?

o It is hard to prove totipotency.Morphogenesis

y Cellular basis of morphogenesiso Morphogenetic movementso Cell matrix adhesionso Intracellular adhesionso Changes in cell shapeo Cell death death of cells between fingerso Cell proliferation

y Early developmento Cleavage divisions

Approximately 24 hours after fertilization: first division Morula: 30 cells (blastomere): 96 hours No growth: all formed inside the pellucid zone, which

cannot expand

o Blastocyst

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Compaction of cells and the accumulation ofintercellular fluid leads to formation of blastocyst cavity

The embryoblast inside consists of roughly 12 cells The enveloping trophoblast, made of a single layer,

contains around 100 cellso Hatching

Around the end of day 5, the embryo frees itself of thepellucid zone

Through a series of expansion-contraction cycles, theembryo bursts the covering

Enzymes help dissolve the pellucid zone at theabembryonic pole

o Migration through fallopian tube Embryo cleaves while migrating The blastocyst reaches the uterine cavity : end of day 5 Hatching / embedding occur on day 6 in uterine cavity

o Adhesion to endometrium The cells of syncytiotrophoblast grow in between the

cells of the uterine epithelium

o Summary Day 0: spermatazoan penetrates oocyte Day 1: two cell stage Day 2: four-cell stage Day 3: eight-cell stage Day 4: morula (16-32 cells) Day 6: free blastocyst following hatching

o Formation of the primitive streak (week 3) Ectoblast cells migrate through the primitive streak and

form the mesoblast: vertebrate equivalent of

gastrulation

Embryonic disk is oval-shaped and the ectoblast isbathed in amniotic fluid

o Genesis of the notochord On the 19th day, through the invagination of epiblast

cells coming from the primitive node

y Neurulation

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o Begins when notochord induces the formation of the CNS bysignaling the ectoderm germ layer above it to form the thick

and flat neural plate

o The neural plate folds in on itself to form the neural tube, Will differentiate into spinal cord and brain (CNS) Failure to close = spina bifida

o Neural crest cells migrate away and the ectoderm fuses tomake a continuous sheet.

o Series of signaling events along both the dorsal-ventral axisand the anterior-posterior axis specifies the fate of neural

tube cells

o Notochord signals Primary neurulation occurs in response to soluble

growth factors secreted by notochord

Ectodermal cells are induced to form neuroectoderm. Ectoderm sends and receives signals ofBMP4 / BMP7

(bone morphogenic protein)

Cells which receive BMP4 and BMP7 signalsdevelop into epidermis

Inhibitory signals chordin, noggin and follistatis areneeded to form neural plate

Created and emitted by notochord Cells which do not receive BMP4 signaling due to

effects of these develop into anterior

neuroectoderm cells of the neural plate

Cells which receive Sonic hedgehog in addition toinhibitory signals form neural plate cells

o Floor (basal) plate signaling Floor plate of incipient neural tube secretes Shh

Produces a gradient Induces motor neurons Suppresses Pax genes

Forms most of the ventral part of nervous system Includes motor portion of spinal cord/brain stem

o Roof plate signaling Expresses more BMP4 and other TGF-b signals

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To form dorsal-ventral gradient along neural tube Induces slug in the future neural crest and

maintains Pax-3 and Pax-7 expression dorsally

Forms dorsal portions of nervous system Devoted to sensory processing

What is a developmental program?

y Like a Rube Goldberg device like the game mousetrapy Development must be regulated and must be redundanty Specify when and where genes are expressed

Where do developmental programs run?

y Where does information processing occur?o Transcription regulation and gene expressiono Signal transduction pathwayso Biochemical pathways

y Where is developmental information stored?o DNAo Environmento Cytoplasm

Nuclear transplantation Proves that much of developmental information is

stored in the cytoplasm

Skin cells in culture cannot grow into a frog, butthe nuclei of skin cells will grow into a frog if

placed in an enucleated oocyte.

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Lecture 44: Developmental Genetics 9/9/2009 9:08:00 AM

Congenital anomalies

y 2-3% live born infantsy >20% of infant deaths

Causes ofBirth Defectsy 6% - recognizable chromosome defecty 7.5% - monogenicy 20% - multifactorialy 6-7% - known environmental factors

o Maternal diseaseo Infectiono Teratogens

Spina bifida = malformation

Smith-Lemli-Opitz Syndrome

y Poor growthy Moderate-severe MRy Facial dysmorphismy Genital anomalies malesy Sometimes have complete sex reversal (XY female, etc.)y AR deficiency of 7-dehydrocholesterol reductasey Treatment: crystalline cholesterol

o Not curativeo Cholesterol is involved in cleavage ofSHH to N-SHH and

potentially to the interaction ofSHH with its receptor, Patched

y Type I: survivaly Type II: can have holoprosencephaly; usually diey Overlapping developmental/metabolic pathways (SHH signal

transduction pathway)

Association: group of abnormalities of unknown etiology and pathogenesis

that is seen together more often then predicted by chance.

VATERR Association

y V = vertebral defects

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y A = anal atresia +/- fistulay T-E = fistula with esophageal atresiay R = radial dysplasia

o Thumb or radial hypoplasiao Preaxial polydactylo Syndactyly

y R = renal anomalyy Also: cardiac defects, single umbilical artery, and prenatal growth

deficiency

Sequence: etiologic agent causes a single localized anomaly that causes

multiple pleiotropic defects as secondary effects of the primary anomaly

Malformation Sequence

y Robin Sequenceo Heterogeneouso Restricted mandibular growth causes posterior displacement

of the tongue and interferes with palatal closure

o U-shaped cleft palate Proves it is a secondary, not a primary defect of palatal

closure (primary defect is a V-shaped cleft palate)

Deformation Sequence

y Oligohydramnioso Cause

Chronic leakage of amniotic fluid Defect of urinary output

o Leads to: Growth deficiency Fetal compression Pulmonary hypoplasia will die of respiratory

insufficiency (lack of terminal alveoli)

Limb positioning defects Potter facies

Disruption Sequence

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y Amniotic-band disruption sequenceo Interferes with blood flowo Often affects extremities; can affect whole body when severeo Look for constriction rings

Dysplasia Sequence

y Neurocutaneous melanosis sequenceo Lack of migration of melanoblastic precursors from the neural

crest

o Melanocytic hamartosis of the skin, pia, and arachnoido Must remove the skin because it can become malignanto Requires significant grafting

y Skeletal dysplasiay Ectodermal dysplasia

Factors that influence phenotype in the presence of a mutation

y Genetic modifiersy Stochastic or probabilistic factorsy Environmental factors

o Teratogenso Protective agents

Early Development

y Regulative development ability to compensate for regions injuredor removed

y Substantial portions of a particular region of the blastocyst can beremoved and the conceptus is still able to develop normally

y Can only remove a single cell for testing for abnormalities; highererror rate

y Monozygotic twinning can occur at three times in developmento Dichorionic 0-3 days (35%)o Monochorionic; diamniotic 4-7 days (65%)o Monoamniotic >7 days (rare)

Gastrulation

y Three primary germ layers arise from the epiblast

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y Mosaic development fate of a particular cells is specifiedindependently of its environment (embryo can no longer easily

compensate for damaged or lost cells)

y Organogenesis begins after gastrulation (wks 4-8)Epigenetic regulation

y Allows the differentiated phenotype to be maintained in daughtercells in the absence of any change in DNA content or sequence

y At the molecular level, this as a s consequence of alteration inchromatin structure, DNA modification, or both.

Cell to cell communication

y Cell surface receptor and molecule that binds to ity Juxtracrine interactions require physical contact between cellsy Paracrine interactions proteins secreted by one group of cells

diffuse a small distance to induce changes in the pattern of gene

expression in adjacent cells

o Morphogens Substances that diffuse out and initiate several types of

developmental programs in surrounding cells,

depending on their location along the morphogen

concentration gradient.y Signaling molecules

o Four major families Fibroblast growth factors (FGF)

About 24 exist Angiogenesis, mesoderm formation, axon ext.

Hedgehog proteins 3 homologs

sonic, desert, indian Wingless (Wnt) Family proteins

15 cysteine-rich glycoproteins makes dorsal cells of somites into muscle polarity of limbs; GU system

Transforming growth factor B (TFG-B) 30 members

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regulate formation of ECM/cell divisiono Sonic Hedgehog (SHH)

Morphogen secreted by the notochord and floor ofdeveloping neural tube that induces and organizes

different cells in the developing brain and spinal cord AD mutations cause holoprosencephaly

Failure of forebrain and midface to develop Cleft lip/palate Hypotelorism Absence of forebrain structures Variable expression single central incisor, partial

absence of corpus callosum

Accutane (isotretinoin)

y 13-cis-retinoic acido a retinoid given by mouth for treatment of cystic acne

y isotretinoin causes congenital anomalieso CNSo Heado Limbso C-V

y Pattern of defects suggests teratogenic effects are due to inhibitionof cell migration from the cranial neural crest

y Retinoic acidso Go into the nucleuso Regulate patterning genes (e.g., HOX) with retinoic acid

response element (RARE)

y Retinoic Acid Embryopathyo 2.5 year-old boy

triangular face ocular hypertelorism malformed external ears

o Severely affected neonate Hydrocephalus Microtia/anotia

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y Adverse effects with a dose range of 0.5-1.5 mg/kg, but somedoses as low as 0.2 mg/kg

y Critical point of exposure is 2-5 weeks post-conception (>15 days)y If the fetus is exposed in the critical period:

o 23% had major malformationso 52% had intellectual deficits at 5 years of age

Fibroblast growth factor receptor mutations

y Membrane-spanning tyrosine kinase receptor with ano extracellular ligand-binding domain consisting of:

three immunoglobulin subdomains, a transmembranedomain, and a split intracellular tyrosine kinase domain.

o Between first and second Ig domains is a stretch of 4-8 acidicamino acids, termed the acid box

y Alternative splice sites result in 12+ distinct isoforms for each gene.y Mutations in FGFR3 cause skeletal dysplasias

o Hypochondroplasia (H)o Thanatophoric dysplasia (T)o Achondroplasia (A)

FGFR3 gene negatively regulates bone growth Thus, mutations might be gain-of-function that activate

the fundamentally negative growth control ofFGFR3o Gain of function mutations

y Axis specificationo Embryo

Head to tail/cranio-caudal/anterior-posterior Dorsal-ventral Left-right

o Limbs Shoulder-fingertip/proximal-distal Thumb-5th finger/anterior-posterior Dorsum-palm/dorsal-ventral

y Pattern formationo A patterning program overlays the embryo after axis

determination

o divides embryo into segments and assigns each an identity

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o Hirsuitismo Cryptorchidismo CHDo Characteristic face

Downslanting palpebral fissures Hypoplastic maxilla Prominent nose

PAX3

y 9 human genes in PAX family of transcription factorsy PAX3 expressed in CNS during neural tube closure and later in

somites

y Neural crest cellso Migrate throughout the embryoo Differentiate into many cell types melanocytes, enteric and

peripheral ganglia, Schwann cells, and populate tissues

including the facial bones and cardiac outflow tracks

y Waardenberg Syndrome Type Io Mutation ofPAX3o Heterozygosity for PAX3 loss-of-function mutation results in a

reduction of neural crest derivatives such as melanocytes in

hair, eyes, and inner ear White forelock Pale colored eyes/heterochromatic irides Sensorineural deafness

o Dystopia canthorum outward displacement of inner canthuso Upper extremity defect occasionally

Programmed cell death (apoptosis)

y Necessary for completion of morphogenesis in many structureso Remodeling of ventricular septum/outflow tracto Elimination of certain lymphocyte lineages that react to self

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Lecture 45: Embryology I 9/9/2009 9:08:00 AM

Germ cells associated with yolk sac

y Migrate from yolk sac to developing gonadsFemale

y Primordial germ cellso In yolk sac

y Oogoniumo After finding the ovaryo Undergo mitosis

y Primary oocyte in prophaseo Prophase of first meiotic divisiono 46 double-structured chromosomes

y Secondary oocyteo After first maturation divisiono 23 double-structured chromosomeso polar body produced

y Mature oocyteo After second maturation divisiono 23 single chromosomeso second polar body produced

Malesy Spermatogoniumy Primary spermatocytey Secondary spermatocytey Spermatids

In the ovary in utero

y At 4th month: oogonia produce primary oocytey At 7th month: almost all oogonia have become oocytesy Newborn: no oogonia; oocytes in diplotene stage

After puberty

y Oocyte is surrounded by follicle/epithelial cellsy Primordial follicle primary oocyte with a single layer of support

cells

y Follicle cells become cuboidal then stratified

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y Zona pellucida: laid down by follicle cells/oocyte (acellular)y Antrum: space filled with fluid inside follicley Theca interna/externa

o Layer of cells around follicleo Interna: production of hormones

y Cumula oophorus: a hill/cloud of cells over zona pelluciday Maturation of the oocyte

o Cell undergoes first maturation divisiono Enters metaphase of second maturation divisiono Completes second maturation division if fertilized

Seminiferous tubules

y Sertoli cell: support cellsy Spermatogonia: mitosisy Conversion of spermatid to sperm: Spermeogenesis

o Nucleus condenses dramaticallyo Packet of enzymes: acrosome granule

Produced by rough ER and Golgio Tail formed by a central core of microtubules from centriole

Ovulation to Implantation

y Follicle cells = granular cellsy What leaves the follicle?

o Cumulus oophorus cellso Zona pellucidao Oocyte

y What remains in the ovary?o Follicle cells converted to corpus luteum (yellow bodies)

Hormone producing element Yellow because of cholesterol

o Thecal cellsy Fallopian tube sweeps cumulus oophorus/egg off of ovary

Fusion of egg/sperm

y Tail of sperm is most important for getting into eggy Releases packet of enzymes

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o Eats through zona pellucidao When a sperm makes contact with egg membrane, changes in

zona pellucida exclude other sperm

o Egg will then undergo second maturation divisiony After joining

o Male and female pronucleuso Cleavage divisions to reduce cytoplasm of oocyteo Morula: group of cellso Blastocyst: inner mass of cells where embryo will formo Trophoblast: outer mass of cells that eats into uterine lining

Fertilization in upper regions of fallopian tubes

y By about 5-6 days, in the uterusEndometrium

y Inner lining lost at menstruationy Ovulation occurs at proliferative phase of endometriumy Corpus luteum forms at secretory phasey When corpus luteum degenerates, menstrual phase begins

o Knocks out LHy In pregnancy:

o Corpus luteum implants, maintaining LHy LH = luteinizing hormone: maintains corpus luteumy FSH = follicle-stimulating hormone: regulates ovulation

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Lecture 46: Embryology II 9/9/2009 9:08:00 AM

Second Week ofDevelopment: Bilaminar Germ Disc

Blastocyst

y Trophoblast: layer of cells surrounding embryoblasto Eats its way into uterine lining

Erodes endometriumo No matter where it is, at this stage, it will burrow into

whatever lining is nearby

After losing the zona pellucida Can implant in mesentery of abdominal cavity (in

rectouterine pouch pouch of Douglas)

Usually implants ectopically in fallopian tube Also abnormal: internal opening of the cervix (placenta

preva) placenta forms in front of fetus

o Syncytiotrophoblast Syncytio: indistinct cell boundaries Massive cytoplasm with nuclei Burrowing layer enzymes to erode endometrium Producing a hormone HCG (Human chorionic

gonadotropin)

Similar to LH (luteinizing hormone)y Which maintains corpus luteum

Implanting structure maintains corpus luteum onits own, even though LH production is cut off

HCG is tested to diagnose pregnancy Spaces develop within here: trophoblastic lacunae (9

days)

By 11 days, the maternal vessels are interrupted Blood fills the lacunae No embryonic circulation yet, but nutrients do

cross to nourish the developing structure

o Cytotrophoblast Cyto: Has distinct cell boundaries Layer undergoing mitosis Adds cytoplasm and nuclei to syncytiotrophoblast

y Amnion is beginning to form

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Buccopharyngeal membrane Primitive streak: at caudal end of embryo

Primitive streak

y From epiblast, cells peel off and work their way between hypoblastand epiblast

o Some incorporate into hypoblasto May replace these to form the endodermo Cells stream down and forward through primitive node

y Buccopharyngeal membraneo Cells cannot squeeze between hypoblast/epiblast at this pointo Cells come around and over it, but not under ito Heart will develop in this area (from the cells that go around)o Actual membrane will form the notochord:

open to endoderm initially underlying ectoderm flanked by mesoderm (intraembryonic mesoderm

connected to extraembryonic mesoderm)

y Mesoderm: new middle layer from epiblasty Endoderm: hypoblasty Ectoderm: remnant of epiblasty Cephalic part of primitive streak gives rise to notochordy Controls fate of future cells

After notochord forms

y Ectoderm foldsy Ectoderm will form neural tube above notochordy Mesoderm

o Next to axis of embryo: paraxial mesodermo Intermediate mesoderm: next to paraxial mesodermo Lateral plate mesoderm: lateral to intermediate mesoderm

Splits into two parts One part lines body wall: parietal mesoderm One part lines endoderm/yolk sac: visceral

mesoderm

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o Mesoderm cant get between/through cloacal membrane atthe caudal end

o Inadequate migration of mesoderm in primitive streak Not enough mesodermal cells to form caudal end Mermaid syndrome

Sirenomelia Fusion of limb buds, etc.

Sacrococcygeal teratoma Teratoma: tumor with derivatives of any of the

germ layers (mesodermal derivatives, etc.)

Most extreme examples: hair/teeth can be foundin these teratomas

Thought to derive from pluripotent cells migratingimproperly from primitive streak

Uterine endometrium

y Trophoblast has now interrupted blood vesselsy Syncytiotrophoblast is no longer at the interface between

trophoblast and embryo

y Instead, cytotrophoblast shell is at the interfaceo This is to keep the syncytiotrophoblast from going too far into

the endometriumo Cytotrophoblast reproduces rapidly to take over for the

syncytiotrophoblast

y Umbilical vessels are beginning to form in connecting stalky Extraembryonic mesoderm have developing embryonic vesselsy Intervillous space: repository for mothers blood, squirted in from

maternal vessels

y Chorionic villi: location of embryonic vesselsy Cross-section of a chorionic villus

o Internal surface: cytotrophoblast babys bloodo External surface: syncytiotrophoblast interacts with

maternal blood

Conjoined twins: overexpression of goosecoid? Google.

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Lecture 47: Metabolic Genetic Disorders 9/9/2009 9:08:00 AM

Most metabolic diseases are autosomal recessive

y Nuclearo Autosomal

Dominant: fam. Hypercholesterolemia, Huntington Recessive: CF, PKU, sickle sell anemia

o Sex-linked (mostly X linked) Dominant: vitamin-D resistant rickets Recessive: Lesch-Nyhan, Hunter, Fragile X

y Organellar: Mitochondrialo Maternal inheritance: MELAS, MERRF, LHON

Substitutions

y Transitionso MCAD Def A985G

y Transversionso Sickle cell anemia: GAG GTG glu6val

Deletions: propionic acidemia 419delA

Insertions: propionic acidemia 917-923insT

Frame-shift mutations: propionic acidemia

Multiple base changes

y Deletionso CF 508CTTdel, DMD

y Insertionso Tay-Sachs TATC-frameshift mutation

Repeat expansions

y Fragile X syndrome CGGy Huntington disease CAGy Myotonic dystrophy CTG

Types of Metabolic Genetic Disorders

y Enzyme defectsy Receptor protein defectsy Transport protein defectsy Structural protein defects

Aminoacidopathies

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y Classification of lysosomal storage disorders (LSD)o Complex lipidoses (sphingolipidoses)o Mucopolysaccharidoseso Glycogenoses type II (pompe)o Neuronal ceroid lipofuscinoseso Mucolipidoses (ML)o Lysosomal protein-trafficking defect

ML type II (I-cell disease)o Lysosomal transport defect (cystinosis)o Oligosaccharidoseso Sialidoses

y Pathophysiology of LSDo Disease phenotype is a consequence of the type of substrate

and its sites of turnover

Activated macrophages: Gaucher, Niemann-Pick Vascular endothelium (Fabry) Muscle (Pompe) Connective tissue (mucopolysaccharidoses)

y Glycogen storage disorders (GSD)o Pompe disease (GSD type II),a lysosomal disorder:

Severe hypotonia, mild hepatomegaly, macroglossia,globular heart, early cardiac failure

Elevated glycogen in muscle biopsy Normal blood sugar Alpha-1,4-glucosidase deficiency in lysosomes AR

y Mucopolysaccharidoseso I: Hurler (alpha-L-iduronidase deficiency) step 2

Progressive; debilitating; life-threatening High mortality

Obstructive airway disease Respiratory infection Cardiac complications

Death by age 10 MR Dysostosis multiplex, coarse facies, corneal clouding

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Lumbar gibbus Hepatosplenomegaly enlarged liver and spleen Umbilical and inguinal hernias common

o II: Hunter (iduronate sulfatase deficiency) step 1o III: Sanfilippo syndrome (Types A-D)o IV: Morquio A & B

A: missing galactose 6-sulphatase B: missing beta-galactosidase Similar to AD Achondroplasia (FGFR3 receptor defect)

o VI: Maroteaux-Lamy step 3o VII: Sly syndrome step 5o IX: Hyaluronidase deficiencyo GAGs degradation, dermatan sulfate

oo Step 4 deficiency: Sandhoff disorder

y Sphingolipidoseso Gaucher glucocerebrosidaseo Fabry alpha-galactosidaseo Niemann-Pick A/B sphingomyelinaseo Niemann-Pick C/D cholesterol traffickingo Farber ceramidaseo Krabbe galactocerebrosidaseo MLD arylsulfatase Ao GM1 gangliosidosis beta-galactosidase

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o GM2 gangliosidosis beta-hexosaminidase A,B,AP Major Tay-Sachs gene mutation in Ashkenazi Jews

4 base insertion in hexosaminidase A geneleading to frameshift

y Lysosomal enzymes transport disorders:o I-cell (inclusion bodies) disease: mucolipidosis type II

AR Newly synthesized lysosomal enzymes are secreted Serum/body fluids contain elevated levels of active

lysosomal enzymes

Enzymes are missing mannose 6P residues on their N-linked carb chains

Defect: lysosomal enzyme N-acetylglucosamine-1-phosphotransferase

No gene isolated (linkage 4q21-23) No treatment

Receptor defects

y Familial hypercholesterolemiaLDL receptor defecto ADo Xanthomaso Classes of mutation

Class 1: receptor synthesis Class 2: receptor transport ER-Golgi Class 3: LDL binding by receptor Class 4: receptor clustering in coated pit Class 5: failure to discharge LDL in endosome (recycling

defect)

Transport defects

y Cystic fibrosisy Duchenne and Becker muscular dystrophies

o DMD: calf muscle is replaced by fibrous tissueo X-linked recessive

Mitochondrial disorders

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y 13 polypeptides, OXPHOSy 1000s mtDNA/celly 100000 mtDNA/mature oocytey Management

o Treat symptomso If diagnosed in newborns, offensive substrates are restricted

by diet restriction (low phenylalanine, low galactose)

o Supplement the deficient product Arginine for citrullinemia (urea cycle disorder)

o Detoxification of offensive substrates medium-chain fatty acids using carnitine and

glycine to acyl-carnitines,acylglycines in MCAD

deficiency

o Detoxification of accumulated substrates through alternatepathway (auto-regulation by the body)

omega oxidation of medium-chain fatty acids inMCAD deficiency to form dicarboxylic acids

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Lecture 48: Embryology III 9/9/2009 9:08:00 AM

Third to Eighth Weeks: The Embryonic Period

Third week:

y Neural plate will eventually form the neural foldy Neural fold

o Cells at crest (ectodermal in origin) Form a group of cells that migrate Give rise to dorsal root ganglia

Unipolar neurons Sympathetic ganglia of sympathetic chain

o Examples of neural crest derivatives Cranial and spinal ganglia Autonomic ganglia Adrenal medulla Melanocytes Some structures of the face/neck

Fourth week:

y Tubular-shaped structurey Neural tube open at anterior/posterior end

o Anterior/posterior neuropore: last part of neural tube to sealo Anterior neuropore closes before the posterior

y Pericardial bulge heart is developingy When amnion grows enough to capture connecting stalk, it

becomes the umbilical cord

y Pharyngeal arches: on either side of head and neck; contribute toformation of face and neck structures

y Otic placode ear developmenty Lens placode eye developmenty Buccopharyngeal membrane: bilaminar (ectoderm and endoderm)

membrane derived from prechordal plate (oral membrane)

y Derivatives of ectodermo Epidermiso Neural portions ofCNS and PNSo Special sensory elements of the ear, nose and eyeo Skin including hair, nails and glands

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y Mesodermo Flat plateo Notochord is a derivativeo Heart initially develops in mesoderm at anterior end of

embryo transitions

o Paraxial mesoderm First cells to migrate, followed by intermediate, etc. Connective tissue, cartilage, bone and muscle of body

wall and limbs

Vertebral column and musculature Hypodermis and dermis of the skin Somites form

Blocks of cells Each has a particular innervation from a spinal

nerve

Sclerotome Dermomyotome

y Dermotome: forms dermisy Myotome: forms a muscle

o Lateral plate mesoderm on either side of the developing bodycavity

Parietal and visceral layer Serous membranes lining the pleural, pericardial,

peritoneal cavities

Adrenal cortex (from parietal layer) note that adrenalmedulla forms from neural crest, then migrates here

o Intermediate mesoderm Mesonephros: part of development of kidney Situated on posterior body wall Gives rise to urinary, kidney and associated structures Gives rise to gonads, etc.

o Angiogenesis: development of vessels Form in mesoderm Can form de novo Dorsal aorta originates as a pair of vessels that coalesce

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y Endodermo Incorporated into tubular-shaped structureo First part = foregut

Supplied by celiac trunk Except for the spleen, which arises de novo

o Middle = midgut connected to yolk sac via vitelline duct Vitelline duct can show up as Meckels diverticulum Supplied by the superior mesenteric artery Distalmost innervation of the vagus

Stops at the splenic flexure, usuallyo End = hindgut

Supplied by inferior mesenteric artery Sacral parasympathetics

o Derivatives Epithelial lining of the trachea, bronchi, lungs Parenchyma of the thyroid, liver, pancreas Epithelial lining of the urinary bladder and urethra Lining of stomach, esophagus, liver

o Cloaca = common opening from urinary/intestinal tracts Eventually separates Forms lining of urinary tract

o Allantois Remnant in humans In some animals: area where waste products are

excreted

From top of urinary bladder Becomes urachus/medial umbilical fold

Fourth week on:

y 4th week: substantial yolk sac; head and taily 5th week: eyes developing, nasal pit, limb buds formy 6th week: head is taking shape; umbilical cordy 6th-7th tube: can see neural tube through transparent epidermis

o somites have disappeared as they have differentiatedo amniono yolk sac smaller now; basically nonfunctional

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y 7th week:o hands/feeto fingers, but not as much in the toes (lower half lags behind)o head is taking shapeo umbilical vein and arterieso huge liver

y 8th weeko eyelids beginning to form; fuse togethero now has human characteristicso no bone formation; but cartilage is forming

Question:

Why are the third to eighth weeks of embryogenesis so important for normal

development and the most sensitive for the induction of structural defects?

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can tell how mature the lungs are by what is in theamniotic fluid (surfactant)

o swallows amniotic fluid into digestive tube some passes through wall of digestive tract; travels

back to mothers circulationo pees into amniotic fluid

but not waste products only fluid waste products processed through maternal blood

o does not defecate in utero accumulating material in last part of digestive tube

including bile = meconium

baby only defecates in utero under distress greenish-yellow stain on skin: meconium staining

y skin is thin and transparent; little faty Amniotic fluid

o Volume fluctuates by swallowing/urinationo Turned over every 3 hourso Polyhydramnios

Too much amniotic fluid If a fetus cannot swallow, there is excess amniotic fluid Esophageal atresia

No lumen in esophagus Pyloric stenosis Other abnormalities in development of digestive tubes Anencephaly

Cannot swallow because they do not have theneural centers necessary

o Oligohydramnios Too little amniotic fluid Defect in urinary system

5.5-6 months

y lanugo hair has formedo usually disappears around birth

y definitive hair in eyebrows and eyelashesy vernix caseosa

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o coats the surface of the fetuso sebaceous gland secretions mixed with sloughed epidermal

cells

y weight gain in final trimester7 months

y should be able to surviveo lungs are developed enough (have enough surfactant)o surfactant: lines the alveoli of the lungs, allowing them to

expand more easily without it, like blowing up a balloon for

the first time, every time

chorionic plate

y has chorionic villio develop all the way around, but are restricted later to one

side (the side attached to the mothers circulation)

o barrier initially formed by: syncytium cytotrophoblast connective tissue endothelium

o barrier later formed by: syncytium endothelium changes to allow faster/easier exchange of

blood/nutrients

y embeds into endometrium which is now the decidua capsulariso decidua as in deciduous trees will be lost

y as placenta develop, villi develop elaborate branching patternsy early in development

o chorion frondosum: villus choriono c