1. The document provides an overview of key topics in general embryology, including fertilization and cleavage, formation of the blastocyst and bilaminar germ disc, implantation and early placentation, and gastrulation. 2. During the second week, the trophoblast differentiates into the cytotrophoblast and syncytiotrophoblast layers. The embryoblast also differentiates into the epiblast and hypoblast layers, forming the bilaminar germ disc. The extraembryonic mesoderm divides into the somatopleuric and splanchnopleuric layers. 3. In the third week, gastrulation occurs as migrating epiblast cells form

1. General Embryology
Part 2
Embryology SCT for 1st year General Medicine.
Jón Kolbeinn Guðmundsson

2. Topic list
1. Erythropoesis.
2. Thrombopoeisis, monocytopoeisis and lymphocytopoeisis.
3. Granulocytopoeisis.
4. Spermatogenesis.
5. Oogenesis.
6. Fertilization and cleavage. (continued)
7. Formation of the blastocyst and bilaminar germ disc.
8. Formation and differentiation of the extraembryonic mesoderm.
9. Implantation. Formation and differentiation of the trophoblast. Early phases of
placentation.
10. Gastrulation, early differentiation of the intraembryonic mesoderm.
11. Differentiation of the intraembryonic mesoderm.
12. Differentiation of the ectoderm
13. Differentiation of the endoderm, folding of the embryo.
14. Fetal membranes. Umbilical cord. Amniotic fluid.
15. Development of the external features of the fetus. External features of a matured
newborn. Twin pregnancy. Fetal membranes in twins.
16. Development of the skull and the vertebral column.

3. Fertilization and Cleavage
(continued)
Topic 6

4. A little review of
Follicular growth
Pre-antral stage
Primordial follicle Growing Follicle Primary Follicle Secondary Follicle
FSH stimulates follicles to grow.
Estrogen is secreted from thecal cells  causing endometrium to grow and the
Anterior Pituitary to release LH

5. BBOOM Ovulation
Secondary follicle
Secondary oocyte
stuck in
metaphase of
meiosis 2. Sharp LH increase
LH causes stromal cells to undergo luteinization and secrete
progesterone.

7. Fertilization
Now the oocyte travels up the uterine tube where it meets the spermatozoa in a part of the tube called ampula.
A couple of spermatozoa (fastest swimmers) pass through the corona radiata of the oocyte and in the meantime are
stripped of a glycoprotein coat covering the acrosome.
This process is called capacitation (not decapacitation, but close). This allows the sperm to move through the corona
radiata and onto the zona pellucida.

8. Acrosome reaction
Once the sperm reach the zona pellucida the proteolytic enzymes
(trypsin, acrosin) contained in the acrosome start digesting the
zona pellucida.
This is the acrosome reaction, and it continues until 1
spermatozoa manages to pass through.

9. Analogy:
Imagine that Dr. Eggmann is the
spermatozoa and the drill on his brilliant
car is the acrosome.
The egg is the oocyte and the eggshell is
the zona pellucida.
Dr. Eggmann wants to use his drill to get
through the eggshell of the egg, but he
must first pass through the corona
radiata, who is Sonic in this case.
Kill sonic  then drill through the egg

10. After one of the sperm has passed through the zona
pellucida, it’s cell membrane fuses with the cell membrane
of the oocyte and the sperm nucleus is released into the
oocyte cytoplasm
When this occurs, 3 major things happen
1. Once the sperm cell membrane touches the oocyte’s
membrane, the oocyte releases cortical granules
containing lysosomal enymes making:
a) The cell oocyte cell membrane
impermeable to other spermatozoa
- Cortical reaction
b) The zona pellucida alters it’s structure
making it impermeable.
- Zona reaction
2. The oocyte will complete it’s 2nd meiotic division
producing a 2nd polar body and the definite oocyte.
2. The oocyte will start becoming metabolically active.

11. The 1st polar body divides also into two smaller
ones, however these two die eventually along
with the 2nd polar body. What remains is the
definite oocyte, which is the actual oocyte.
The chromosomes of the oocyte and the
sperm form a female pronucleus and a male
pronucleus.
The two nuclei replicate their DNA and
eventually loose their nuclear envelopes to
prepare for mitosis, the cell is now diploid and
is called a zygote.
The cell undergoes mitosis and yields two
identical cells with 46 chromosomes each
(diploid)
The corona radiata starts
disappearing

12. The zygote is now moved further along the uterine tube by the sweeping movement of cilia.
During this time in the uterine tube, the zygote continuously divides by mitosis where the daughter cells become
increasingly smaller and are now called blastomeres. This form of mitosis, where cells divide by mitosis but don’t grow
in size intermittently, is called cleavage.
2 cell stage, 4 cell stage, 8 cell stage, 16 cell stage (Morula)
The cells get increasingly smaller since they are still contained within the zona pellucida which hasn’t disappeared yet.
2 cell stage 4 cell stage 8 cell stage Morula
(16 cell stage)
Day 1 Day 4

13. After the 8 cell stage the cells start to become more compact
due to tight junctions between them.
Once the morula (16 cells) is formed there is a distinction
between the outer and inner cells.
The outer cells are held together by tight junctions and the
inner cells are held together by gap junctions.

14. Formation of the blastocyst
and bilaminar germ disc
+
Implantation. Formation and
differentiation of the
trophoblast. Early phases of
placentation.
Topic 7 and 9

15. Blastocyst formation
At around the 4th day the zona pellucida surrounding the morula becomes permeable to water which flows into its center
and creates a cavity (blastocyst cavity).
The inner cells and outer cells now form:
1. Embryoblast – becomes the embryo
2. Trophoblast – becomes a large part of the placenta.
Once in the uterus the blastocyst releases proteases which dissolve the zona pellucida, now it is able to implant in the
uterine endometrium.

16. Implantation – 6th day
The blastocyst sticks to the endometrium, L-selectins present on
the trophoblasts facilitate initial adhesion of the trophoblast to
the uterine epithelium on the endometrium.
The trophoblast layer of the blastocyst starts to proliferate and
penetrate into the uterine endometrium now referred to as
decidua. This process is called implantation.
The uterus is in a secretory stage at the time of implantation,
where its glands are active and blood vessels in the
endometrium grow in preparation for further development of
the embryo.

17. Start of the 2nd week of pregnancy.
In the very early development of the embryo it is important to know the week or days in which
things occur, since certain important structures from in a particular week.
The definition of pregnancy is the day that implantation occurred in the uterus.
The 2nd week of pregnancy is conveniently called “The week of twos” since a lot of structures
differentiate into 2 separate things:
Trophoblast  cytotrophoblast & syncytiotrophoblast
Embryoblast  epiblast & hypoblast
Extraembryonic  somatepleuric & splanchnopleuric extraembryonic mesoderm
Mesoderm
Two cavities from  amniotic & yolk sac cavities

18. The trophoblast differentiates into 2 layers:
• Cytotrophoblast – inner mononucleated cell layer
which keeps proliferating and has distinct cell
borders
• Syncytiotrophoblast – an outer multinucleated cell
layer which has no distinct cell borders.
The embryoblast now differentiates into 2 cell layers:
• Epiblast – a columnar cell layer which will give rise
to the embryo
• Hypoblast – a cuboidal cell layer which will give rise
to the yolk sac
*This is the so called bilaminar germ disc*
Formation of the bilaminar germ disc
8th day
The trophoblast penetrates deeper into the
uterine stroma of the endometrium and is now
partially embedded in it.
1. Blastocyst cavity
2. Amniotic cavity
The Epiblast layer creates a cavity – Amniotic cavity. This
cavity is lined by the epiblast cells and amnioblasts (epiblast
derived) adjacent to the cytotrophoblast.

19. Analogy:
An oversized western tourist
(cytotrophoblast) wants to set up
camp in a bamboo forest
(decidua).
He pays a Mexican
(syncytiotrophoblast) to cut it
down for him.
The role of the syncytioblast is to
erode the endometrial epithelium and
the endometrial stroma in order to
allow the blastocyst to implant in the
endometrium. It does this by releasing
proteolytic enzymes.
The cytotrophoblast keeps producing
more and more syncytiotrophoblast.

20. Lacunar stage
Day 9 & 10
At this point there is complete
embedding of the blastocyst in the
endometrial stroma.
The syncytiotrophoblast enter a
lacunar stage where lacunae form.
A membrane forms originating
from the hypoblast layer.
It is called the Exocoelomic
membrane (Hauser’s) and lines
the cytotrophoblasts forming a
cavity called primitive yolk sac (1)
1
2

21. Formation and differentiation of
the extraembryonic mesoderm.
Topic 8

22. Day 11 and 12
Now the lacunae become continuous
with the sinusoids of the
endometrium, this causes maternal
blood to enter into the lacunae
system.
This creates the uteroplacental
circulation which is important for
providing nutrition for the developing
embryo.
A connective tissue structure called
extraembryonic mesoderm is formed
from the cytotrophoblast and
surrounds the primitive yolk sac and
amniotic cavity

23. Soon large cavities start forming within the
extraembryonic mesoderm creating a new
cavity called the extraembryonic coelum or
chorionic cavity (see next slide)
The extraembryonic mesoderm starts
dividing into two:
• Somatopleuric extraembryonic
mesoderm
 close to cytotrophoblast cells
• Splanchnopleuric extraembryonic
mesoderm
 close to primitive yolk sac
and amnioblast cells

24. Day 13
Now the chorionic cavity has formed
and it covers the primitive yolk sac and
amniotic sac except where the embryo
is anchored to the cytotrophoblast by a
connecting stalk (will later become the
umbilical cord)
The primitive yolk sac will now be
replaced by a secondary yolk sac.
Chorionic
cavity

25. The secondary yolk sac is formed and what
remains of the primitive yolk sac is now a small
cyst called exocoelomic cyst.
The somatopleuric extraembryonic mesoderm
lining the cytotrophoblast starts making villous
elongations into it.
These structures are called primary villi and
are involved in creating the separation
between fetal and maternal circulation.
Exocoelomic
cyst
Primary
villi

26. Gastrulation, early differentiation
of the intraembryonic mesoderm.
Topic 10

27. Start of the 3rd week of development.
The main things that happen in the 3rd week is:
1. The establishment of 3 germ layers (3rd week = 3 germ layers) known as gastrulation.
2. The establishment of body axes (i.e. left and right side)

28. Gastrulation – 3rd week
The first thing that happens to the epiblast during
the beginning of 3rd week is the formation of a
primitive streak which is formed on the surface of
the epiblast cells.
This creates a narrow groove where other epiblast
cells can migrate through to reach the middle
between the epiblast and hypoblast layers.
Amnion
Prechordal plate
Primitive streak

29. Endoderm
Ectoderm
Mesoderm
Epiblast
Hypoblast
Migrating
epiblast cells
The migrating epiblast cells slowly replace the
hypoblast cells and create the endoderm.
Mesoderm is formed by epiblast cells that occupy
the space between the epiblast and hypoblast.
Finally what was called epiblast before is now called
ectoderm.

30. Questions
1. Define acrosome reaction. PP
2. Define cortical and zona reaction. PP
3. Define capacitation. PP
4. Define cleavage. PP

31. 4. Name the distinct parts of the extraembryonic mesoderm at the end of the 2nd week. PP
5. What is the role of the syncytioblast during implantation? PP
4. What is the difference between cytotrophoblast and syncytiotrophoblast? PP

32. 7. Name the structures indicated by the arrows. PP
a.
b.

33. 8. Name the structures indicated by the arrows. PP

34. 9. Name the 3 germ layers which form during gastrulation.
10. What are the different origins of intraembryonic mesoderm and extraembryonic mesoderm.