Embryology

1. CLEAVAGE, IMPLANTATION OF THE
EMBRYO AND BILAMINAR DISC
FORMATION
KEY WORDS IN THIS LECTURE WILL BE
ZYGOTE, CLEAVAGE, IMPLANTATION,
BILAMINAR DISC FORMATION

2. MAIN OBJECTIVES OF LECTURE
• Describe the initial phase of cell division of the
zygote---cleavage
• Implantation of the fertilised egg-key steps
• Changes in the endometrium following
ovulation and fertilisation and implantation
• Changes in the inner and outer cell masses
• Development of the blastocyst
• Development of the bilaminar disc

3. The first Trimester
weeks 1-12; fetus size ~ 3 in.; weight ~ 14 g
Cleavage
Implantation
Placentation
Embryogenesis
Basic organ plan and tissues laid out –
most susceptible to damage or disorganization at this
time

4. Results of Fertilization
• Stimulates the secondary oocyte to complete
its second meiotic division and expel the 2nd
polar body
• Restores normal diploid number (46) of
chromosomes in zygote
• Results in variation in human beings due to
biparental inheritance.
• Determines the chromosomal sex of the
embryo
• Causes metabolic activation of zygote &
initiates cleavage (cell division of zygote)

5. Cleavage is the first phase of embryonic development

6. What is cleavage?
Cleavage is a rapid series of mitotic divisions that occur just after fertilization.
There are two critical reasons why cleavage is so important:
1. Generation of a large number of cells that can undergo differentiation and
gastrulation to form organs.
2. Increase in the nucleus / cytoplasmic ratio. Eggs need a lot of cytoplasm
to support embryogenesis. It is difficult or impossible for one nucleus to
support a huge cytoplasm, and oocytes are one of the largest cells that
exist. One small nucleus just cannot transcribe enough RNA to meet the
needs of the huge cytoplasm.
A larger nucleus to cytoplasmic ratio is optimal for cell function. Cell
division occurs rapidly after fertilization to correct this problem.

7. Early Embryonic Development
• Cleavage is the first major phase
of embryonic development
– It is the rapid succession of
cell divisions (Mitotic)
– It creates a multicellular
embryo from the zygote
– NO growth
– Stages:
• 1. Morula-solid ball of cells
• 2.Blastula- single layer of
cells surrounding a fluid-
filled cavity called the
blastocoel
ZYGOTE 2 cells
4 cells
8 cells
Many cells
(solid ball)
BLASTULA
(hollow ball)
Cross section
of blastula
Blastocoel

8. CLEAVAGE
1. Begins - 12 hours post-fertilization
2. Zygote divides into 2 cells (mitosis)
3. 46 chromosomes in zygote = 46
chromosomes in both daughter cells
4. 2 cell into 4 cell stage (24 – 36 hours)
5. 4 cell into 8 cell stage (36 – 72 hours)
6. 16 cell stage -- Morula

9. Blastocyst
with blastocoele cavity
Morula
solid ball of cells
Cleavage
Early division of zygote into multiple cells without increase in
size, partitions contents
Zygote

10. Cleavage
• Repeated mitotic division of
zygote
• Begins about 30 hours after
fertilization
• There is rapid increase in number
of cells. The cells, blastomeres,
become smaller with each
division
• Normally occurs as the zygote
passes along the uterine tube to
the uterus
• During cleavage, zygote lies
within the zona pellucida

11. Cleavage cont’d
• After nine-cell stage, the
cells become compactly
arranged..compaction
• 12-16 cell stage is called
morula. It is formed about 3
days after fertilization and
enters the uterus
• Internal cells of the morula,
inner cell mass, are
surrounded by a layer of
cells that form the outer cell
mass

12. Cleavage cont’d
• At this stage, the conceptus is
called Blastocyst. It has two
poles: embryonic &
abembryonic
• Zona pellucida gradually
degenerates and disappears
• Blastocyst takes its
• nourishment from uterine
secretions and enlarges in
size. It is ready to get
attached and implanted to
the uterine wall
Abembryonic pole
Embryonic pole

13. Cleavage cont’d
• Fluid filled space called the
blastocyst cavity (blastocele)
appears inside morula
• Blastomeres are separated into:
Outer cell layer, the
trophoblast, which gives rise to
embryonic part of placenta
Centrally located, inner cell
mass (embryoblasts) which
gives rise to the embryo

14. MORULA(ctd)
• Morula enters the uterus - after 3 days in
oviduct

15. THE MORULA
Morula

16. The Morula-entering the uterus
72 hours post-
fertilization
entering uterus

17. THE BLASTOCYST
• Morula, once entering the uterine cavity,
floats freely
• Morula begins to accumulate fluid and forms
a cavity between its cells
• Once cavity appears, it is now called a
blastocyst.

18. Blastocyst(ctd)
• Blastocyst has fluid-filled inner cavity
• Evolves from morula on day 5

19. Late Blastocyst

20. Blastocyst

21. The process by which the
developing mass-the blastocyst gets
embedded within the uterine wall

22. Blastocyst - with blastocoele cavity
Trophoblast - outer layer of cells
Inner cell mass - will form embryo
Lacunae and primary villi formed by
trophoblast
All of these form placental tissues
Implantation - embedding of blastocyst into
uterine lining begins at day 7
Fig 28-3
Trophoblast forms syncytial trophoblast- erodes
into endometrium
Cellular trophoblast - carries nutrients to inner cell
mass

23. Implantation
• Begins 6 days after
fertilization:
The blastocyst attaches
to the endometrial
epithelium, usually
adjacent to the
embryonic pole
Note that the inner cell
mass is also called the
embryoblast

24. Implantation cont’d
 Trophoblast proliferates rapidly and
differentiates into two layers:
inner cellular cytotrophoblast,
outer mass of
syncytiotrophoblast
(multinucleated protoplasm with
no cell boundaries)
 Finger like processes of
syncytiotrophoblast extend through
the endometrium and invade the
endometrial connective tissue

25. Implantation cont’d
• By the end of 7th day, the
blastocyst gets implanted
in the superficial
compact layer of
endometrium and
derives its nourishment
from the eroded
endometrium

26. Implantation cont’d
• The blastocyst gradually
embeds deeper in the
endometrium
• By 10th day it is completely
buried within the ‘Functional
layer’ (stratum compactum +
stratum spongiosum) of the
endometrium

27. Implantation cont’d
• The defect in the
endometrial epithelium is
filled by closing plug (day
10)
• The defect gradually
disappear as the
endometrial epithelium is
repaired (day 12 & 13) by
the proliferation of the
surrounding cells

28. Implantation cont’d
• Small cavities, the lacunae
appear in
syncytiotrophoblast, and
get filled with maternal
blood, establishing
primitive uteroplacental
circulation

29. Implantation continued
• After the implantation of the blastocyst a
bilaminar embryonic disc commonly called the
BILAMINAR DISC forms
• This is composed of the epiblast and the
hypoblast cell layers
• This disc gives rise to the GERM LAYERS that
finally forms all the tissues and organs of the
embryo-see the slide below

31. Principal events in the
second week of
development

32. Day 10
Embryo completely
embedded in
endometrium
Amnion and yolk sac visible
Blasto-disc formation (2 cell
layers) also called
Bilaminar disc formation
– Epiblast cell layer
– Hypoblast cell layer

34. 3. Bilaminar Germ Disc ( 2nd Week )
1) Epiblast -- layer of columnar cells adjacent to
the trophoblast.
2) Hypoblast -- layer of cuboidal cells facing the
blastocele.
3.1 inner cell mass → bilaminar germ disc

35. Week 2
• Implantation continues
• Erosion of maternal blood
vessels
• Complete emersion into
endometrium of uterus

36. Week 2: Implantation and Extra-embryonic
Membrane Formation.
4
5
1
2
3
-- Bilaminar embryonic disc forms (4)
-- Syntrophoblast and extraembryonic mesoderm appear
-- Mesoderm cavitates to form extraembryonic coelom (5)
-- Amnion (1), yolk sac (2) and chorion (3) completed with
a coating of extraembryonic mesoderm.
Dizygotic twins: two of every membrane
Monozygotic twins: one chorion, two amnions

37. Normal Implantation Sites
The implantation site
determines the site of
formation of the placenta
Normally it occurs in the
upper part of the body of
uterus, more often on the
posterior wall

38. Abnormal Implantation Sites
• Uterine:
Implantation in the lower
segment leads to placenta
praevia
• Extrauterine implantation-:
leading to ectopic pregnancies
Fallopian tube-tubal
pregnancy
Ovary
Abdominal cavity-in the
recto-uterine pouch or over
the

39. Ectopic pregnancy:
•Implantation outside the uterus: the uterine tube,
abdominal cavity, or ovary.
•Abortion and severe hemorrhage during the 2nd
month of gestation which may lead to death
•This is a gynaecological emergency

40. LITHOPEDION
• Sometimes the blastocyst that implants in the
ampulla or on the fimbriae may at times be
expelled into the peritoneal cavity and lodge in
the rectouterine pouch formed by the reflection
of the peritoneum from the rectum to the uterus.
• Sometimes this pregnancy may go to term
• More commonly, the conceptus dies and is not
detected and may get calcified and forming a
stone-foetus commonly referred to as a
Lithopedion( lithon=stone, paidon=child in
Greek)

41. PICTURE OF A LITHOPEDION

43. hCG is produced
• hCG is human chorionic gonadotropin
• It is produced by the trophoblasts starting on day 6
• hCG is a hormone
• hCG causes endometrium of uterus to grow and
proliferate
• hCG prevents the menstrual cycle from occurring
• This is why a female misses her periods when she is
pregnant

44. PLACENTA PRAEVIA-implantation
in the lower uterine segment

45. Embryonic Development
• Gastrulation is the second
major phase of embryonic
development
• The cells at one end of the
blastula move inward,
– It adds more cells to the
embryo
– It sorts all cells into three
distinct cell layers
– The embryo is
transformed from the
blastula into the gastrula

46. Next lecture
• GASTRULATION
Gastrulation is a formative
process by which the three
germ layers are established and
the bilaminar disc is converted
to trilaminar disc ectoderm
,mesoderm

48. Embryonic Development
• The three layers produced
in gastrulation
– Ectoderm, the outer
layer
– Endoderm, an
embryonic digestive tract
– Mesoderm, which partly
fills the space between
the ectoderm and
endoderm

49. 1) Notochord (in midline) induces neurulation
2) Paraxial mesoderm (future somites for bone, muscle, c.t.)
3) Intermediate mesoderm (future UG system)
4) Lateral plate (future body wall and cavities; gut wall)
23
4
1
Amnion
Gastrulation Is In Week 3
Section shown
at right
Before We Are Born, 5th Ed.,
W.B. Saunders, 1998

50. Gastrulation: 3 Germ Layers Formed
Ectoderm (forms from epiblast)
Nervous system
Epidermis
Endoderm (forms from
hypoblast)
Mucosae (eg: GI-tract
Associated glands
Mesoderm
Everything else
day 12:

51. Formation of Extra-embryonic Membranes
visible after day 10:
Amnion – Protection of
embryo/fetus
Yolk sac –
Early site of blood cell
formation

52. Early placental membrane consists of:
Syncytiotrophoblast
Cytotrophoblast
Connective tissue
Endothelium of blood vessels
Late placental membrane:
Syncytiotrophoblast
Endothelium
(They share a basal lamina)

53. Amnion
Uterine cavity Villous
chorion
Decidua
basalis
Smooth
chorion
Mucous
plug Vagina
Decidua
basalis
Decidua
capsularis
Decidua
parietalis
Embryo is within the mucosa (= endometrium = decidua)
-- Decidua basalis is the maternal component of the placenta
-- Decidua capsularis is the “bag” with chorion & amnion
-- Decidua parietalis is the uninvolved uterine mucosa
Before We Are Born, 5th Ed.,
W.B. Saunders, 1998