Blastulation refers to the process in early embryonic development where the zygote undergoes rapid cell divisions through cleavage to form a solid ball of cells called a morula. The morula then develops a fluid-filled cavity, forming a structure called a blastocyst composed of an inner cell mass and outer layer of trophoblast cells. The blastocyst undergoes further differentiation, with the inner cell mass forming the embryo and extraembryonic tissues such as the amnion, yolk sac, and allantois developing to support the growth and development of the embryo.

1. Blastulation
Dr. Komal Parmar

2. Fertilization
 Fertilization normally occurs in
the ampullary region of the
uterine tube, probably within 24
hours of ovulation.
 The two pronuclei grow, move
together and condense in
preparation for syngamy and
cleavage in about 24 hours
after that.
 Division of the zygote into
blastomeres begins
approximately 30 hours after
fertilization.
 Pronuclear fusion as such does
not occur: the two pronuclear
envelopes disappear and the
two chromosome groups move
together to assume positions
on the first cleavage spindle.
No true zygote containing a
membrane-bound nucleus is
formed.

3. Clevage
 Cleavage consists of
repeated mitotic divisions
of the zygote, resulting in
a rapid increase in the
number of cells
(blastomeres).
 These embryonic cells
become smaller with each
successive cleavage
division.
 Cleavage occurs as the
zygote passes along the
uterine tube toward the
uterus.
 During cleavage, the
zygote is within the zona

4.  Division of the zygote
into blastomeres begins
approximately 30 hours
after fertilization.
 Subsequent cleavage
divisions follow one
another, forming
progressively smaller
blastomeres.
 The cell cycle is quite
long, the first two cell
cycles being around 24
hours each, thereafter
reducing to 12–18
hours.

5.  Cell division is asynchronous
 Daughter cells may retain a cytoplasmic link through
much of the immediately subsequent cell cycle via a
midbody, as a result of the delayed completion of
cytokinesis.
 No centrioles are present until 16 to 32 cells are seen,
but amorphous pericentriolar material is present and
serves to organize the mitotic spindles.
 All cleavage divisions after fertilization are dependent
upon continuing protein synthesis which up to the 8 cell
stage is derived from the available maternal mRNA and
thereafter from the embryonic mRNA. Hence the initial
cell divisions are independent of the mRNA synthesis.
 Early cleavage, up to the formation of eight cells, requires
pyruvate or lactate as metabolic substrates, but
thereafter more glucose is metabolized and may be
required.
 Up to the formation of eight cells, cells are essentially
spherical, touch each other loosely, and have no
specialized intercellular junctions or significant

6.  After the nine-cell stage, the blastomeres change their
shape and tightly align themselves against each other to
form a compact ball of cells: Compaction.
 Cells flatten on each other to maximize intercellular
contact, initiate the formation of gap and focal tight
junctions.
 As a result of the process of compaction, the embryo
forms a primitive protoepithelial cyst, which consists of
eight polarized cells, in which the apices face outward
and basolateral surfaces face internally.
 Cell surface and the calcium dependent cell–cell
adhesion glycoprotein, E-cadherin (also called LCAM or
uvomorulin).

7. Morula
 Two-cell populations are formed in
the 16-cell embryo that differ in
phenotype (polar, apolar) and
position (superficial, deep).
 The outer polar cells contribute
largely to the trophectoderm,
whereas the inner apolar cells
contribute almost exclusively to the
inner cell mass in most embryos.
 The generation of cell diversity, to
either trophectoderm or inner cell
mass, occurs at the time of 4th /5th
day in the 16-cell morula
 During the next cell division (16 to 32
cells), a proportion of polar cells
again divide differentiatively as in the
previous cycle, each yielding one
polar and one apolar progeny.

8.  After division to the 32 cells, the
outer polar cells complete their
differentiation into a functional
epithelium, display structurally
complete zonular tight junctions
and begin to form desmosomes.
 The nascent trophectoderm engages
in vectorial fluid transport from
uterine cavity to the inside of embryo
to generate a cavity that expands in
size during the 32- to 64-cell cycles
and converts the ball of cells, the
morula, to a sphere, the
blastocyst.
 Once the blastocyst forms, the
diversification of the
trophectoderm and inner cell
mass lineages is complete, and
trophectoderm differentiative
divisions no longer occur

9. Blastocyst
 The blastocyst ‘hatches’ from its
zona pellucida at 6–7 days, possibly
assisted by an enzyme similar to
trypsin.
 Early pregnancy factor, an
immunosuppressant protein, is
secreted by the trophoblastic cells
and appears in the maternal serum
within 24 to 48 hours after
fertilization. EPF forms the basis of a
pregnancy test during the first 10
days of development.
 The outer cells of the blastocyst, the
trophoblast or trophectoderm, are
flattened polyhedral cells, which
possess ultrastructural features
typical of a transporting epithelium.
 The trophoblast covering the inner
cell mass is the polar trophoblast and
that surrounding the blastocyst cavity
is the mural trophoblast.

10.  Implantation occurs
during a period of 7–12
days postovulation.
 Even at this early stage,
cells of the inner cell
mass are already
arranged into an upper
layer (i.e. closest to the
polar trophoblast), the
epiblast, which will give
rise to the embryonic
cells, and a lower layer,
the hypoblast, which has
an extraembryonic fate.
 Dorsoventral axis of the
developing embryo and a
bilaminar arrangement of
the inner cell mass are
both established at or
before implantation.

11. Attachment to the uterine wall
 Implantation includes the
following stages:
 Dissolution of the zona
pellucida;
 orientation and adhesion of
the blastocyst onto the
endometrium;
 trophoblastic penetration into
the endometrium;
 migration of the blastocyst into
the endometrium;
 spread and proliferation of the
trophoblast, which envelops and
specifically disrupts and invades
the maternal tissues.
 The site of implantation is
normally in the endometrium of
the posterior wall of the uterus,
nearer to the fundus than to the
cervix and may be in the
median plane or to one or other

13. Formation Of Extraembryonic Tissues
 Epiblast and amniotic cavity
 closest to the implanting face of the
trophoblast,
 A basal lamina surrounds a
spherical cluster of epiblast cells,
and isolates them from all other
cells.
 Epiblast cells adjacent to the
hypoblast become taller and more
columnar than those adjacent to
the trophoblast, and this causes
the epiblast sphere to become
flattened and the centre of the
sphere to be shifted towards the
polar trophoblast.
 Amniotic fluid accumulates
and the amniotic cavity is
formed.

14.  Hypoblast and yolk sac
 Hypoblast is the term used to
delineate the lower layer of
cells of the early bilaminar
disc.
 During early implantation,
the hypoblast extends
beyond the edges of the
epiblast and can now be
subdivided into those cells in
contact with the epiblast
basal lamina, the visceral
hypoblast, and those cells in
contact with the mural
trophoblast, the parietal
hypoblast.
 The parietal hypoblast cells
are proliferating and
spreading along the mural
trophoblast.
 The cavity that the layer
initially surrounds is termed
the primary yolk sac, or
alternatively the primary

15.  At the same time, a space
appears between the parietal
hypoblast (Heuser’s
membrane) and the mural
trophoblast that limits the
circumference of the
hypoblastic cavity.
 The resultant smaller cavity
inside lined by hypoblast is
termed the secondary yolk sac.
 The visceral hypoblast cells
are believed to be important in
many aspects of the early
specification of cell lines.
 The cells induce the formation
of the primitive streak, thus
establishing the first axis of the
embryonic disc.
 They are also believed to be
necessary for successful
induction of the head region
and for the successful
specification of the primordial

17.  After the formation
of the secondary
yolk sac, a
diverticulum of the
visceral hypoblast,
the allantois, forms
towards one end of
the embryonic
region and extends
into the local
extraembryonic
mesoblast.
 It passes from the
roof of the
secondary yolk sac
to the same plane

18. Extraembryonic mesoblast
 Extraembryonic tissues
encompass all tissues that do not
contribute directly to the future
body of the definitive embryo.
 Extraembryonic mesoblast:
 It will come to cover the amnion,
secondary yolk sac and the
internal wall of the mural
trophoblast, and will form the
connecting stalk of the embryo
with its contained allanto-enteric
diverticulum.
 The first mesoblastic
extraembryonic layer gives rise to
the layer known as
extraembryonic mesoblast,
arranged as a mesothelium with
underlying extraembryonic
mesenchymal cells.

19.  Later extraembryonic mesoblast populations mushroom
beneath the cytotrophoblastic cells at the embryonic pole,
forming the cores of the developing villus stems, and villi and
the angioblastic cells that will give rise to the capillaries within
them and the earliest blood cells.
 .

20. • Initially, the
extraembryonic mesoblast
connects the amnion to
the chorion over a wide
area.
• Continued development
and expansion of the
extraembryonic coelom
means that this
attachment becomes
increasingly circumvented
to a connecting stalk,
which is a permanent
connection between the
future caudal end of the
embryonic disc and the
chorion.
• The connecting stalk forms
a pathway along which
vascular anastomoses
around the allantois
establish communication
with those of the chorion

21. Clinical Correlates

22. MOSAICISM
 If nondisjunction (failure of a chromosome pair to separate)
occurs during an early cleavage division of a zygote, an
embryo with two or more cell lines with
differentchromosome complements is produced.
 Example: Down’s Syndrome

23. PREIMPLANTATION GENETIC
DIAGNOSIS
 Can be carried out 3 to 5 days after IVF of the
oocyte.
 One or two cells (blastomeres) are removed from
the embryo known to be at risk for a single gene
defect or chromosomal anomaly.

24. ABNORMAL EMBRYOS AND
SPONTANEOUS ABORTIONS
 Many zygotes, morulae, and blastocysts abort
spontaneously.
 Early implantation of the blastocyst is a critical
period of development that may fail to occur owing
to inadequate production of progesterone and
estrogen by the corpus luteum.
 Early spontaneous abortions occur for a variety of
reasons, one being the presence of chromosomal
abnormalities. More than half of all known
spontaneous abortions occur because of these
abnormalities.

25. Ectopic implantation
 The conceptus may be arrested at any point during its migration through
the uterine tube and implant in its wall.
 Previous pelvic inflammation damages the tubal epithelium and may
predispose to such delay in tubal transport.
 The presence of an intrauterine contraceptive device or the use of
progesterone-based oral contraceptives may also predispose to ectopic
pregnancy, probably because of alteration in the normal tubal transport
mechanisms.

26. Abnormal Implantation within Uterus
 Implantation near the internal os results in the
condition of placenta praevia, with its attendant risk
of severe antepartum haemorrhage.

27. Twinning: Identical Twins

29. Staging and Study of Embryo

30. Embryology
 The study of spatial and temporal
developmental processes that take
place within an embryo as it
develops from a single cell into a
recognizable organism is known as
embryology.
 Much of our knowledge of the early
developmental processes is
derived from experimental studies
on amniote embryos, particularly
the chick, mouse and rat.
 A classification of human embryos
into 23 stages occurring during the
first 8 weeks after ovulation was
developed most successfully by
Streeter (1942) and the task was
continued by O’Rahilly & Müller
(1987).
 An embryo was initially staged by
comparing its development with
that of other embryos.

32. Embryonic Staging Fetal Staging
 The embryonic period has
been defined by Streeter
as 8 weeks
postfertilization, or 56
days.
 This timescale is divided
into 23 Carnegie stages, a
term introduced by
O’Rahilly & Müller (1987)
to replace developmental
‘horizons’.
 The designation of stage
is based on external and
internal morphological
criteria and not on length
or age.
 Staging of fetal development and
growth is based on an estimate of the
duration of a pregnancy.
 Whereas development of a human from
fertilization to full term averages 266
days, or 9.5 lunar months (28 day
units), the start of pregnancy is
traditionally determined clinically by
counting days from the last menstrual
period; estimated in this manner,
pregnancy averages 280 days, or 10
lunar months (40 weeks).
 In obstetric practice the duration of the
period of gestation is regarded as nine
calendar months, which is
approximately 270 days.
 The period of pregnancy is divided into
thirds, termed trimesters. The first and
second trimesters each cover a period
of 12 weeks, and the third trimester
covers the period from 24 weeks to