Chapter 3 BASIC EMBRYOLOGY
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Chapter 3
BASIC EMBRYOLOGY
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Embryology
Definition: the study of the origin anddevelopment of an organism
Prenatal period: before birth38 weeks from conception to birth (average)
“fetal” age
Gynecologic timing has been from LMPtherefore refers to 40 weeks “gestational” age
Date of conception has been difficult to time
LMP is on average two weeks before ovulation
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Traditional (artificial) division:
“Embryonic” period: first 8 weeks
All major organs formed
“Fetal” period: remaining 30 weeks
Organs grow larger and become morecomplex
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Fertilization to Implantation
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Ovulation: egg released into the
peritoneal cavity Travels down fallopian tube in which
fertilization occurs
At conception in fallopian tube, maternaland paternal genetic material join to form anew human life (zygote)
Cell division occurs with travel down thetube and into the uterus
Conception (biology) or fertilisation, the fusion of gametes to
produce a new organism of the same species (Wikipedia)
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Week 1 post conception
Zygote divides repeatedly moving down tube
toward uterus (cleavage) The daughter cells are called blastomeres
Morula : the solid cluster of 12-16 blastomeresat about 72 hours
Day 4: late 60 cell morula enters uterus, takingup fluid becoming blastocyst
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Blastocyst stage
Two distinct types of cells Inner cell mass : forms the embryo
Trophoblast : layer of cells surrounding the cavitywhich helps form the placenta
Floats for about 3 days
Implantation on about day 6 post conception
Trophoblast erodes uterine wall Takes 1 week to complete
If inner cell mass of a single blastocyst divides:monozygotic (identical) twins
_____inner cell mass
______trophoblast
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Week 2
Inner cell mass divides intoepiblast and hypoblast
2 fluid filled sacs Amniotic sac from epiblast
Yolk sac from hypoblast
Bilaminar embryonic disc :area of contact
(gives rise to the whole body)
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Bilaminar to trilaminar disc
Three primary “germ” layers: all body tissuesdevelop from these
Ectoderm
Endoderm
Mesoderm
Week 3
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Formation of the 3 “germ” layers
Primitive streak (groove) ondorsal surface of epiblast
Grastrulation: invaginationof epiblast cells
Days 14-15: they replacehypoblast becomingendoderm
Day 16: mesoderm (a new
third layer) formedin between
Epiblast cells remaining onsurface: ectoderm
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The three “germ” tissues
“Germ” as in germinate, not germs
Early specialization of cells
Are precursors Ectoderm and endoderm are epithelial tissue
(form sheets of tissue)
Mesoderm is a mesenchyme tissueMesenchyme cells are star shaped and do not attach
to one another, therefore migrate freely
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Notochord
Days 16-18
Primitive nodeepiblast cells
invaginate andmigrate anteriorly withsome endoderm cells
Rod defining the bodyaxis is formed
Future site of thevertebral column
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Neurulation
Notochord signals overlying ectoderm
Formation begins of spinal cord and brain (neurulation)
Neural plate to neural groove to neural tube: pinched offinto body
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Closure of neural tube: begins at end of week 3; completeby end of week 4 (folic acid important for this step)
Extends cranially (eventually brain) and caudally (spinalcord)
Neural crest, lateral ectodermal cells, pulled along andform sensory nerve cells and other structures
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Mesoderm begins to differentiate Lateral to notochord, week 3
Extends cranially and caudally (from head to tail orcrown to rump)
Division into three regions Somites: 40 pairs of body segments by end week 4
Intermediate mesoderm: just lateral to somites
Lateral plate: splits to form coelom (“cavity”)
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Divisions of the mesodermal lateral
plate Somatic mesoderm: apposed to the
ectoderm
Splanchnic mesoderm: apposed to theendoderm
Coelom in between will become the
serous cavities of the ventral body cavity:Peritoneal
Pericardial
Pleural
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Folding beginsAt week 4
(main difference
between the 3week embryoand the adultbody is that theembryo is stilla flat disc)
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24 dayembryro;
protrudesintoamnioticcavity
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Day 23, beginningto fold
Lateral foldswill join ventrally
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Cylindricalhuman bodyplan, day 28
Simplified
cross sectionthroughabdomen ofan adult
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Major derivatives of embryonic germ layers
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29 day embryo(this is when the heart starts pumping, about 4
weeks or 1 month, ½ cm size)
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month 3 month 5 3 month fetus
(6 cm)
Late 5th month
About 19 cm
See Table 3.1 handout
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By 8 weeks, about 2 months, allmajor organs are in place in atleast a rudimentary form; this is
why drugs early in pregnancy areso important to avoid – manycause birth defects; baby is a littleover 1” long (below right)
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