8/3/2019 Ch03 Embryo
1/68
1
Human Anatomy, First EditionMcKinley & O'Loughlin
Chapter 3 :
Embryology
8/3/2019 Ch03 Embryo
2/68
3-2
Embryology The study of the developmental events
that occur during the prenatal period
8/3/2019 Ch03 Embryo
3/68
3-3
Embryology Begins with Fertilization:
A single fertilized cell divides by mitosisto produce all of the cells in the body.
8/3/2019 Ch03 Embryo
4/68
3-4
The Prenatal Period The first 38 weeks of human development
between fertilization and birth.
The pre-embryonic period: first 2 weeks of development
zygote becomes a spherical, multicellularstructure.
The embryonic period:
third through eighth weeks
all major organ systems appear.
8/3/2019 Ch03 Embryo
5/68
3-5
The Prenatal Period The Fetal Period:
Includes the remaining weeks of
development prior to birth
The fetus continues to grow
Its organs increase in complexity
8/3/2019 Ch03 Embryo
6/68
3-6
The Stages of Embryogenesis
Cleavage: zygote divides by mitosis forms a multicellular structure called a blastocyst.
Gastrulation: blastocyst cells form three primary germ layers basic cellular structures from which all body
tissues develop.
Organogenesis: three primary germ layers arrange themselves in
ways that give rise to all the organs within thebody.
8/3/2019 Ch03 Embryo
7/68
7
8/3/2019 Ch03 Embryo
8/683-8
Gametogenesis Following birth, an individual undergoes
maturation.
the body grows and develops
the sex organs become mature
the sex organs then begin to produce
gametes
8/3/2019 Ch03 Embryo
9/683-9
Chromosomes Human somatic cells contain 23 pairs of
chromosomes for a total of 46. 22 pairs ofautosomes one pair ofsex chromosomes.
Autosomes contain genetic information
for most human characteristics. Homologous chromosomes:
pair of similar autosomes
8/3/2019 Ch03 Embryo
10/683-10
Diploid CellsA cell is said to be diploid if it contains
23 pairs of chromosomes.
2N = 46
8/3/2019 Ch03 Embryo
11/683-11
The Sex Chromosomes The pair of sex chromosomes
determines whether an individual is
female (XX) or male (XY). One member of each pair of
chromosomes is inherited from each
parent.
8/3/2019 Ch03 Embryo
12/683-12
Gametogenesis Begins with meiosis.
Produces secondary oocytes in thefemale.
Produces sperm in the male.
8/3/2019 Ch03 Embryo
13/683-13
MeiosisA type of cell division
Starts with a diploid parent cell
Produces haploid daughter cells (spermor eggs/ova).
8/3/2019 Ch03 Embryo
14/68
3-14
Meiosis I Meiosis results in the formation of gametes
(sex cells).
In meiosis I: homologous chromosomes are separated after
synapsis
crossing over occurs.
In meiosis II:
sister chromatids are separated
sequence of phases resembles mitosis.
8/3/2019 Ch03 Embryo
15/68
3-15
Prophase I Homologous, double-stranded
chromosomes in the parent cell form
pairs (synapsis). Tetrad:
Pair of homologous chromosomes
Crossing over occurs between the maternal and paternal
chromosomes.
8/3/2019 Ch03 Embryo
16/68
3-16
Metaphase I Homologous pairs of chromosomes line
up above and along the equator of the
cell. Forms a double line of chromosomes.
Alignment is random with respect to
maternal or paternal origin.
8/3/2019 Ch03 Embryo
17/68
3-17
Anaphase I Pairs of homologous chromosomes
separate and are pulled to the opposite
ends of the cell.
8/3/2019 Ch03 Embryo
18/68
3-18
Telophase I and Cytokinesis Nuclear division finishes
The nuclear envelopes re-forms
The cytoplasm divides
Two new haploid cells are produced
8/3/2019 Ch03 Embryo
19/68
19
8/3/2019 Ch03 Embryo
20/68
3-20
Prophase II Resembles the prophase stage of
mitosis.
In each of the two new cells:
nuclear membrane breaks down
chromosomes collect together.
Crossing over does not occur in thisphase.
8/3/2019 Ch03 Embryo
21/68
3-21
Metaphase II The double-stranded chromosomes
form a single line in the middle of the
cell. Spindle fibers extend from the
centrioles at the poles to the
centromere of each double-strandedchromosome.
8/3/2019 Ch03 Embryo
22/68
3-22
Anaphase II The sister chromatids of each double-
stranded chromosome are pulled apart
at the centromere. Each chromatid (single strand) is pulled
to the opposite pole of the cell.
8/3/2019 Ch03 Embryo
23/68
23
8/3/2019 Ch03 Embryo
24/68
3-24
Telophase II and Cytokinesis The single-stranded chromosomes arrive at
opposite ends of the cell.
A cleavage furrow forms Cytoplasm in both cells divides
Produces a total of four haploid daughtercells.
These daughter cells mature: sperm in males
oocytes in females.
8/3/2019 Ch03 Embryo
25/68
25
8/3/2019 Ch03 Embryo
26/68
3-26
Oogenesis In females, the sex cell produced is
called the secondary oocyte.
This cell will have 22 autosomes andone X chromosome.
8/3/2019 Ch03 Embryo
27/68
3-27
Oogenesis Oogonia:
parent cells that produce oocytes reside in the ovaries are diploid cells.
All the oogonia start the process of meiosisand form primary oocytes prior to birth.
They are arrested in Prophase I and remain
this way until the female reaches puberty. Each month usually only one becomes a
secondary oocyte.
8/3/2019 Ch03 Embryo
28/68
3-28
8/3/2019 Ch03 Embryo
29/68
3-29
Oogenesis When the primary oocyte completes the first meiotic
division, two cells are produced.
Division of the cytoplasm is unequal.
The secondary oocyte receives the bulk of thecytoplasm and is the cell that is arrested inMetaphase II.
The second cell, which receives only a tiny bit of thecytoplasm, is called a polar body.
The polar body is a nonfunctional cell and eventuallydegenerates.
8/3/2019 Ch03 Embryo
30/68
3-30
Oogenesis Only the secondary oocyte has the potential
to be fertilized.
The secondary oocyte is ovulated The corona radiata and the zona pellucida
form protective layers around the secondaryoocyte.
8/3/2019 Ch03 Embryo
31/68
3-31
Oogenesis If the secondary oocyte is not fertilized, it
degenerates about 24 hours after ovulation, stillarrested in metaphase II.
If the secondary oocyte is fertilized, it first finishesthe process of meiosis. Two new cells are produced,and as before, the division of the cytoplasm isunequal.
The cell that receives very little cytoplasm becomesanother polar body and eventually degenerates.
The cell that receives the majority of the cytoplasmbecomes an ovum which can be fertilized.
8/3/2019 Ch03 Embryo
32/68
3-32
Oogenesis Typically, only one secondary oocyte is
expelled (ovulated) from one of the two
ovaries each month. The left and right ovaries alternate
ovulation each month.
8/3/2019 Ch03 Embryo
33/68
3-33
Spermatogenesis The parent or stem cells that produce
sperm are called spermatogonia.
Spermatogonia are diploid cells thatreside in the the testes.
Each one first divides by mitosis to
make an exact copy of itself called aprimary spermatocyte.
8/3/2019 Ch03 Embryo
34/68
3-34
8/3/2019 Ch03 Embryo
35/68
3-35
Spermatogenesis Primary spermatocytes then undergo meiosis
and produce haploid cells called spermatids.
Spermatids contain 23 chromosomes, butthey still must undergo further changes toform a sperm cell.
In spermiogenesis, spermatids lose much oftheir cytoplasm and grow a long tail called aflagellum.
8/3/2019 Ch03 Embryo
36/68
3-36
Spermatogenesis The newly formed sperm cells are
haploid cells that exhibit a distinctive
head, a midpiece, and a tail. From a single spermatocyte, four new
sperm are formed.
All sperm have 22 autosomes and eitheran X chromosome, or a Y chromosome.
8/3/2019 Ch03 Embryo
37/68
3-37
Fertilization
Two sex cells fuse to form a new cellcontaining genetic material derived from both
parents. Restores the diploid number of chromosomes.
Determines the sex of the organism.
Initiates cleavage.
Occurs in the widest part of the uterine tube(the ampulla).
8/3/2019 Ch03 Embryo
38/68
38
8/3/2019 Ch03 Embryo
39/68
3-39
Fertilization Millions of sperm cells are deposited in the
female reproductive tract during intercourse.
Only a few hundred have a chance atfertilization.
Only the first sperm to enter the secondaryoocyte is able to fertilize it.
The remaining sperm are prevented frompenetrating the oocyte.
8/3/2019 Ch03 Embryo
40/68
3-40
Cleavage Shortly after fertilization, the zygote
begins to undergo a series of divisions.
Divisions increase the number of cells inthe pre-embryo, but the pre-embryoremains the same size.
During each succeeding division, thecells are smaller and smaller.
8/3/2019 Ch03 Embryo
41/68
41
8/3/2019 Ch03 Embryo
42/68
3-42
Cleavage Before the 8-cell stage, cells are not
tightly bound together
after the third cleavage division, thecells become tightly compacted into aball called a morula (16 cells).
8/3/2019 Ch03 Embryo
43/68
3-43
Blastocyst formation Zona pellucida begin to disintegrate as
morula enters the uterus.
Blastocyst cavity develops.
Pre-embryo now a blastocyst:
Trophoblast
Inner cell mass or embryoblast
8/3/2019 Ch03 Embryo
44/68
3-44
Implantation
Implantation is the process by whichthe blastocyst burrows into and embeds
within the endometrium. Begun about day 7; done by day 9
Trophoblast cells invade
Trophoblast subdivides Cytotrophoblast
Syncytiotrophoblast
8/3/2019 Ch03 Embryo
45/68
45
8/3/2019 Ch03 Embryo
46/68
46
F ti f Bil i G i l
8/3/2019 Ch03 Embryo
47/68
47
Formation of Bilaminar GerminalDisc
By day 8, embroblast begins todifferentiate
Hypoblast layer: adjacent to blastocyst cavity
Epiblast layer: adjacent to amniotic cavity
Together called bilaminal germinal disc
8/3/2019 Ch03 Embryo
48/68
3-48
Formation of Extraembryonicmembranes
8/3/2019 Ch03 Embryo
49/68
3-49
Amnion Eventually encloses the entire embryo
in a fluid-filled sac called the amniotic
cavity to prevent desiccation. The amniotic membrane is specialized
to secrete the amniotic fluid that bathes
the embryo.
8/3/2019 Ch03 Embryo
50/68
3-50
Chorion The outermost extraembryonic
membrane, is formed from rapidly
growing cells. These cells blend with the functional
layer of the endometrium and
eventually form the placenta.
8/3/2019 Ch03 Embryo
51/68
3-51
The Placenta Functions in exchange of nutrients, waste
products, and respiratory gases between the
maternal and fetal bloodstreams. Transmission of maternal antibodies to the
developing embryo or fetus.
Production of hormones to maintain and build
the uterine lining.
8/3/2019 Ch03 Embryo
52/68
3-52
Embryonic Period Begins with establishment of the three
germ layer
By process of gastrulation
Ends at about week 8
Main organ systems laid in
8/3/2019 Ch03 Embryo
53/68
3-53
Gastrulation Occurs during the third week of development
immediately after implantation. One of the most critical periods in the
development of the embryo. Cells of the epiblast migrate and form the
three primary germ layers: Ectoderm
Mesoderm endoderm.
Trilaminar structure now called an embryo
8/3/2019 Ch03 Embryo
54/68
3-54
Primitive streak formation Dorsal surface of the bilaminar germinal
disc
Depression on surface of epiblast
Cephalic end: primitive node
Primitive pit
8/3/2019 Ch03 Embryo
55/68
3-55
8/3/2019 Ch03 Embryo
56/68
3-56
Invagination Inward movement of cells
Cells from epiblast detach, move from
primitive streak to area between epiblast andhypoblast.
Forms mesoderm
Other migrating cells replace the hypoblast:form endoderm
Remaining cells in epiblast become ectoderm
8/3/2019 Ch03 Embryo
57/68
3-57
8/3/2019 Ch03 Embryo
58/68
3-58
Folding of the Embryonic Disc
Begins late third and fouth weeks
Some areas grow faster than others.
Cephalocaudal folding:
Helps form head and buttocks
Transverse (or lateral) folding
Helps form trunk
8/3/2019 Ch03 Embryo
59/68
3-59
8/3/2019 Ch03 Embryo
60/68
3-60
Neurulation: differentiation of ectoderm
Notochord forms in area of primitivestreak
This induces neurulation Neural plate
Neural folds
Neural Groove Neural tube
8/3/2019 Ch03 Embryo
61/68
3-61
8/3/2019 Ch03 Embryo
62/68
3-62
Differentiation of Mesoderm
Five categories:
Notochord
Paraxial mesoderm Somites: most bone, muscle, cartilage, dermis, CT
Intermediate mesoderm
Urinary and reproductive systems
Lateral Plate Mesoderm Cardiovascular, lining of body cavities, CT of limbs
Head Mesenchyme
CT and musculature of face
8/3/2019 Ch03 Embryo
63/68
3-63
8/3/2019 Ch03 Embryo
64/68
3-64
Differentiation of Endoderm
Linings of digestive, respiratory andurinary tracts.
Thyroid,parathyroid, thymus, most ofliver, pancreas and gallbladder.
8/3/2019 Ch03 Embryo
65/68
3-65
8/3/2019 Ch03 Embryo
66/68
3-66
8/3/2019 Ch03 Embryo
67/68
3-67
8/3/2019 Ch03 Embryo
68/68
Organogenesis
Once the three primary germ layers haveformed, and the embryo has undergonefolding, organogenesis begins.
The upper and lower limbs attain their adultshapes, and the rudimentary forms of mostorgan systems have developed by week 8.
By the end of the embryonic period, theembryo is slightly longer than 2.5 centimeters(1 inch), and yet it already has the outwardappearance of a human.