Introduction
Chapter 1 : The reproductive
organs and the formation of
the sex cells
oviduct
ampulla
uterus
ovarium
omentum latum
Fallopian Funnel
cervix
vulva
vagina





The entire system lies in the
pelvic region of the
abdominal cavity
The uterus is thick walled and
consist of two layers the inner
endometrium and the outer
muscular myometrium
The embryo imbeds in the
endometrium by means of the
placenta
The ligament, the omentum
latum hold all the different
structures together
The ovarium consists of a
germinal epithelium
surrounding the connective
tissue the stroma



The vagina acts as the
copulatory organ and the
birth canal
During pregnancy, the sphincter
muscles of the cervix remains
contracted. The remaining
canal is blocked by means of a
gelatinous plug
The embryo implants in the
uterine wall and connection
with the mother is established
by the placenta (foetal &
maternal) tissue
vagina
•placenta performs all the
necessary functions;
•provides nutrition;
•supplies oxygen and
•removes carbondioxide;
•removes waste products




Position of the testes is
outside the body cavity
The spermatozoa (sperm) die
off at normal body
temperature.
Ligaments are relaxed to
hang the testes away from
the body but will contract at
temperatures below the
optimal, to bring the testes
close to the body. Exceptions
are the elephant and the
springhare
Attached to each testis is the
epididymis, which is
expanded into the deferent
duct










The two deferent ducts enter the body
cavity, loop around the bladder and fuse
together and with the urethra, while
passing through the following glands:
Seminal vesicles (2x)
Prostrate gland (1 consisting of
several lobes)
Cowper’s gland (= bulbo-urethral
gland)
Secretions of the glands are added
during ejaculation to produce the semen
Functions of the secretions are:
Maintain correct pH
Provide volume/liquid medium
Provide nutrients
The urethra extends trough the penis







Testis is divided into compartments by
septula testis
Each compartment is packed with
seminiferous tubuli and interstitial
cells
Interstitial cells secrete testosterone –
carried through the bloodstream
Responsible for development of male
characteristics
Immature sex cells develop in the walls
of seminiferous tubuli
Mature sperm come free; move along
efferent ducts into the epididymis
(caput, corpus & cauda epididymis)
Efferent ducts fuse to form deferent
duct that leaves epididymis and enters
the body cavity




Interstitial cells secrete
testosterone – carried
through the bloodstream
Responsible for
development of male
characteristics
Immature sex cells
develop in the walls of
seminiferous tubuli
Mature sperm come
free; move along efferent
ducts





= process of the formation and development of
gametes
Number of chromosomes are halved during
meiosis to form spermatozoa and ova
Spermatozoa and ova are haploid
The development of the male gamete is
spermatogenesis
The development of the female gamete is
oogenesis






1 Male primordial generative cell (spermatogonium)
results in 4 spermatozoa
1 Female primordial generative cell (oogonium) results in
one ovum + 3 polar bodies
New spermatogonia are formed throughout the life of the
male from puberty on
Spermatogenesis commences at puberty and continues
into old age
All the stages of spermatogenesis are represented in the
seminiferous tubuli at any given time
The maturation process from spermatogonium to
spermatozoon takes about two months





Spermatogonia lie dormant
in seminiferous tubuli against
basement membrane
Continual mitotic division
since puberty
Some cells start growth
process – 1 primary
spermatocyte
Meiosis I – 2x secondary
spermatocytes = ½ size of
primary spermatocyte
Meiosis II – 4x spermatids
= ½ size of secondary
spermatocyte





Spermatogonia lie dormant
in seminiferous tubuli against
basement membrane
The cells move away from
basement membrane while
growth and meiosis progress
& lie among Sertoli cells
Spermatids lie against the
lumen of seminiferous tubule
Process of differentiation
follows
Normal spherical spermatid
becomes a free-swimming
sperm






Granules become visible in Golgi body
Vacuole + granules lie close to nucleus
Vacuole of Golgi body enlarges
Granule becomes visible and enlarges to form the proacrosomal body
Liquid moves away and form the Golgi rest
Membrane of vacuole spreads out over nucleus = cap






Nucleoplasm with organelles flows to one side
Just a thin layer of cytoplasm remain covering the spermatozoon
Golgi Rest + excess cytoplasm is discarded
Pronucleus + acrosonal granule = head of the sperm
Pronucleus elongates
Spermatid elongates








Centrosome contains 2
centrioles
Centrioles move to opposite side
of pronucleus
Become proximal & distal
centrioles
Proximal centriole lies in
depression in nucleus (forms 1 of
the asters during 1st division)
Axial filament grows out of distal
centriole (forms the basal granule)
Axial filament consist of 1 pair of
central & 9 pairs of fibres situated
around the circumference
Mitochondrion forms a spiral
around the axial filament (provides
energy)
At the end of the mitochondria is
the ring centriole (function
unknown)






From proximal centriole to
ring centriole = middle piece
Axial filament elongates form flagellum
Flagellum is covered by
a thin layer of cytoplasm and
a sheath in mammals
The sheath extends from the
ring centriole to almost the
tip of flagellum
The tip of the flagellum is
naked.
Sheath consists of 9 fibres
twisted around flagellum




Process start before birth of baby girl = Prenatal maturation
Cells of germinal epithelium divide through mitosis to form oogonia
Some grow to form primary oocytes that separate from epithelium and
move into stroma
Cells from the stroma known as follicle cells surround each of the
primary oocytes to form a number of primordial follicles



Meiosis I starts but process is arrested in Prophase I before birth of girl
baby, due to the secretion of a hormone, Oocyte Maturation Inhibitor
(OMI) by the surrounding follicle cells
The girl baby is born with all the primary follicles she will ever have
All the primary follicles remain suspended in Prophase I until puberty,
when Postnatal maturation commences.





Process is stimulated by Follicle Stimulating Hormone (FSH) secreted by the
pituitary gland at the onset of puberty
The pituitary is situated ventral and attached to the brain by a stalk
From puberty on, one primary follicle will enlarge and move deeper into the
stroma, approximately once every 28 days in one of the ovaria
The primary oocyte now proceeds with Meiosis I while the follicle develops
Follicles in different stages of maturation can be found in the ovaries of the
reproductive female at any time




The follicle cells around the primary oocyte become collumnar and start secreting
estrogen = primary follicle
The influence of estrogen is:
(i) development of female characteristics & (ii) build up of uterine lining
The follicle layer becomes multi-layered = secondary follicle
An eccentric cavity, the follicular antrum, filled with fluid probably secreted by the
follicle cells, appears within the follicle cells = Graaffian/ tertiary follicle




The young Graaffian follicle moves further into the stroma, and the stroma
lays down a tough capsule, the theca folliculi around it
The young Graaffian follicle grows as more fluid collects in the antrum,
pushing the dividing primary oocyte surrounded by some follicle cells
against the side of the Graaffian follicle
Gradually the follicle moves back towards the surface of the ovarium
A membrane, the zona pellucida is secreted by the follicle cells around
the dividing primary oocyte



Meiosis I is completed just before the Graaffian follicle opens to release
the secondary oocyte surrounded by the zona pellucida + follicle cells into
the body cavity = ovulation
The division of the cytoplasm is unequal. One daughter cell receives most
of the cytoplasm = secondary oocyte. The other receives mostly nuclear
material = polar body
Polar body is non-functional & might divide again (2 polar bodies) before
degenerating



Ovulation is regulated by 2 hormones secreted by the pituitary, FSH and
the luteinizing hormone (LH)
Meiosis II commences just after ovulation, but stops in metaphase II. It is
only completed after a sperm has penetrated the cell membrane of the
secondary oocyte
The Meiosis II division also results in a cell that retains most of the
cytoplasm = ovum + a polar body




The ovum that contains a large quantity of cytoplasm is larger than any
other somatic cell in the body
Oogenesis is completed by the completion of Meiosis II, while the sperm
is already contained in the cytoplasm of the ovum.
The follicle cells of the ruptured Graaffian follicle secretes a large quantity
of estrogen into the bloodstream. The blood supply to the uterine wall is
increased under the influence of the large quantity of estrogen
The ruptured Graaffian follicle proceeds to grow, becoming a temporary
endocrine gland, the corpus lutheum




The corpus luteum remains throughout pregnancy, secreting the hormone
progesterone.
Progesterone inhibits ovulation during pregnancy and for sometime
thereafter
No pregnancy – the corpus luteum will degenerate after about 10-12 days
= corpus albicans
Towards the end of pregnancy, the secretion of progesterone decreases,
a birth process commences = corpus albicans





No of primary oocytes at birth of female infant =
about 2 million
30 – 40,000 at puberty
Only approx 400 become secondary oocytes
and are released by the ovaria
Female reaches menopause (end of
reproductive life) after last ovulation
Age – 45 to 50 years






The ovum (>somatic cells)
contains haploid no of
chromosomes (pronucleus) +
usual cytoplasmic organelles
In mammals it is surrounded by
the zona pellucida
In addition it containes:
Superficially situated cortical
granules
Yolk only in animals that lay
eggs – absent in mammals
Brown or black pigment
granules
NB. The size of the ovum is
relative to the amount of yolk it
carries i.e. Bird/reptile egg is
large compared to the small
mammalian egg.
1.
Primary egg membranes = membranes formed by the
female cell
1.1 Plasmalemma = membrane of female sex cell
1.2 Vitteline membrane – found in most animals; absent in
mammals = thin & transparent lying close to the
plasmalemma,
Forms the fertilization membrane during the zona
reaction, where it is present after penetration.
2.
Secondary egg membranes = membranes secreted by
the follicle cells (only found in mammals)
2.1 Zona pellucida – clear, thick membrane secreted by the
follicle cells. Transformed into the fertilisation membrane
during the zona reaction
2.2 Corona radiata – consists of the follicle cells that
surround the secondary oocyte after ovulation. Follicle
cells are held together through hyaluronic acid. These
cells gradualy disperse after fertilization and before
implantation
3. Tertiary egg membranes =
secreted by the
oviduct/glands of the oviduct.
Absent in placental mammals
3.1 Jelly – around amphibian
eggs
3.2 albumin & leathery or hard
shells of reptile and birds
Chapter 2 : Fertilization
Stages:
1. Fertilisation
2. Cleavage
3. Gastrulation
4. Neurulation & Formation of the primary
organ rudiments
5. Organogenesis
6. Growth




Ovulation places sec oocyte in the body
cavity. The movement of the fimbriae of
the Fallopian funnel creates a current in
the peritoneal fluid in the body cavity and
the sec oocyte is swept into the
infundibulum of the oviduct. The epithelial
cells of the oviduct are ciliated, and these
cilia move the sec oocyte into the ampulla
part of the oviduct
About 200 – 500 million sperm is deposited
in the area around the opening of the cervix
The sperm is stored in the mucus and
crevices of the cervix & released over a
period of 3 days
The liquid contents of the vagina breaks
down the plasmalemma of the head of the
sperm = capacitation (about 7 hours)





The acrosomal granules are now released,
become dissolved, forming enzymes called
sperm lysins
Sperm lysins have the ability to dissolve the egg
membranes
In some animals a long rigid filament form
(centre of the acrosome) known as the
acrosomal filament
Acrosomal filament is absent in man –
plasmalemma becomes perforated to release
the spem lysins
While capacitation takes place the sperm is
carried through the cervix, lumen of the uterus
and up the oviducts to the ampulla on both
sides
The sperm collide with the
sec oocyte by chance:
• Sperm can move, but
the direction is random.
•Many sperm are present
•The egg is very large



Large quantity of sperm reach
the ampulla.
The sperm become ‘sticky’ –
adheres to the corona radiata of
the sec oocyte & to each other.
Excess sperm is neutralized in
this way
Corona radiata releases fertilizin
& surface cytoplasm of sperm
releases antifertilizin
•Fertilizin + antifertilizin = chemical bond
•Fertilizin = glycoprotein/mucopolysaccharide
•Antifertilizin = small acid protein
•Fertilizin & antifertilizin is species specific i.e. will only
combine with the other of its own species




Chemical process
Hyaluronic acid (mucopolysaccharide)
hold follicle cells together = corona
radiata
Perforated acrosome of agglutinated
sperm secretes enzyme hyaluronidase
Hyaluronidase dissolves hyaluronic acid
& follicle cells falls apart to allow sperm
to reach zona pellucida



Perforated acrosome of
agglutinated sperm secretes
enzyme acrocin that dissolves
zona pellucida – sperm head
reaches plasmalemma
Plasmalemma of sperm head &
plasmalemma of sec oocyte
fuse, breakdown – contact
between cytoplasm of gametes
Head of sperm = male
pronucleus + flagellum swims
free, while rest of sperm i.e.
plasmalemma, sheath of tail &
thin layer of cytoplam remain
attached to plasmalemma of sec
oocyte






Secondary oocyte completes
meiosis II
Flagellum of sperm degenerates
and disappears
Head of the sperm recovers fluid
from the female cytoplasm – male
pronucleus
Male pronucleus rotates 180
degrees and approaches with prox
centriole
Nuclear membranes break down to
form the zygote (2N)
Chromosomes become visible on
equator = 1st mitotic division =
cleavage





Triggered by the passage of the sperm
through the zona pellucida and starts
at the point of penetration, gradually
spreading over the surface of the sec
oocyte
Cortical granules take fluid from the
cytoplasm and explode. Membrane of
the cortical granule ruptures to release
its contents = lysosomal enzymes
into the space between the
plasmalemma and the zona pellucida.
Space created is perivitteline space
Lysosomal enzymes act on zona
pellucida – becomes impereable to
other sperm = fertilization membrane
Process takes 20 minutes








Second temporary change involves a
change in electrical potential of the
inside surface of the plasmalemma
The electrical charge is normally –
Upon penetration the electrical charge
changes to +
The electrical potential of the sperm
head is +
I.e. any other sperm head will be
repelled by the ovum plasmalemma
The process takes 2 seconds
The ovum plasmalemma’s electrical
charge changes back to – as soon as
the fertilization membrane is in place
The receptivity of the egg to penetration
is reduced to 1/120 by the zona reaction



1. Monospermia – 1 sperm penetrates the sec oocyte
in most groups due to zona reaction
2. Pathological polyspermia – 2 sperms penetrate
due to high density of sperm – triploid zygote that
cannot divide and dies off
3. Physiological polyspermia – Found in birds. It is
normal to find more than one sperm in the cytoplasm of
the ovum, but only 1 male pronucleus fuse with the
female pronucleus, while the others die off.
Chapter 3 : Morphogenesis or
Ontogeny
Stages:
 1. Fertilisation
 2. Cleavage
 3. Gastrulation
 4. Neurulation & Formation of the primary
organ rudiments
 5. Organogenesis
 6. Growth


•
•
•
•
•
•

Single celled zygote is transformed into a multicellular body through rapid
mitotic divisions
The characteristics of the cleavage divisions are:
No growth occurs
Shape remains the same (sphere = blastula)
A cavity originates inside the blastula = blastocoele
No qualitative changes in the cytoplasm just transformation of food
reserves into active cytoplasm & cytoplasmic substances into nuclear
substances
Posistion of parts of cytoplasm remains in the same positions
Ratio nucleus:cytoplasm is normalised
Different cleavage types are determined by:
1. Presence and amount of yolk in the cytoplasm
2. The phylogenetic position of the group












Eg. Branchiostoma / the lancelet / Amphioxus
The zygote contains very little yolk
Divisions are complete
Daughter cells are of equal size
1st division is vertical – 2 identical
blastomeres
2nd division is vertical - 4 identical
blastomeres
3rd division is horizontal – 8 blastomeres;
4 micromeres & 4 macromeres
4th division is vertical – 8 micromeres & 8
macromeres = 16 cell stage
5th division is horizontal – 32 cells = morula
Further divisions – blastula: blastoderm
surrounding a blastocoele
Blastoderm is a single layer of cells
Thinner in the animal hemisphere than in the
vegetal hemisphere









Eg the amphibian egg
The zygote contains a moderate
amount of yolk
Divisions are complete, but it takes
time to complete the division
The initial four daughter cells are
similar in size, but daughter cells of
following division differ in size
The amount of yolk included in the
cells slow down the division processes
1st division is vertical
2nd division is vertical
3rd division is horizontal
Blastula consists of a multilayered
blastoderm and an eccentric
blastocoele in the animal hemisphere
Blastoderm cells in the vegetal
hemisphere are larger and
fewer that the cells in the
animal hemisphere




Eg bony fish, birds and reptiles
The egg contains a large amount
of yolk
Divisions are incomplete i.e. the
cell membrane that separates the
two daughter cells are never
completed
The incomplete cleavage divisions
only takes place in the animal pole
where the yolk load is very heavy to
form a blastodisc






Eg. placental mammals
The egg contains no yolk
Cleavage is a series of rapid,
complete, non–synchronised mitotic
divisions resulting in progressively
smaller blastomeres
i.e. stages like 5 and 7-cell stages
can be seen
Cleavage results in a blastocyst
The blastocyst consists of a
trophoblast, a blastocyst cavity
and some formative cells, called the
inner cell mass
Stages:
 1. Fertilisation
 2. Cleavage
 3. Gastrulation
 4. Neurulation & Formation of the primary
organ rudiments
 5. Organogenesis
 6. Growth





Gastrulation is the process whereby the blastula/ blastodisc/ blastocyst is
transformed into the three germ layers; the ectodem; mesoderm;
endoderm
All the tissues of all chordate animals originate in these three germ layers:
Ectoderm – neural tissue i.e. nervous system and epidermis i.e. skin
Mesoderm – support system i.e. (skeleton and muscles; cardiovascular system (heart, blood vessel & blood; urogenital system
(kidneys & reproductive systems)
Endoderm – alimentary canal and associated organs (lungs, liver,
pancreas)
 The process of gastrulation like cleavage differs between the different classes
of the vertebrates again according to:
 The amount of yolk contained in the cytoplasm of the zygote and
 The phylogenetic position of the class
Chapter 4 : The Development of
Branchiostoma
Cleavage = Equal holoblastic cleavage




The future of the blastoderm can be
plotted
The animal hemisphere cells are mostly
yolk free and is destined to develop into
ectoderm
Most of the vegetal hemisphere cells
are contain yolk, the cytoplasm is
granular and is destined to develop into
endoderm
A small crescent of loosely packed cells
in the vegetal hemisphere is also yolkfree like the future ectodermal cells, but
stains with basic stains. These cells are
destined to become mesoderm which
includes the future notochord material






The single walled blastula needs to become an embryo consisting of 3 layers
Gastrulation takes place through the invagination of the future endodermal
and mesodermal cells
The 1st sign of gastrulation is the flattening of the vegetal pole (endoderm)
The endoderm invaginates into the blastocoele followed by the mesoderm
The mesoderm includes both the future muscle tissue and the rudiment of the
notochord
The hollow sphere now becomes a double walled cup; the meso & endoderm
forming the inner layer, and the ectoderm situated in the outer layer
•The continuation of invagination obliterates the blastocoele and a new cavity
the archenteron is formed.
•The embryo is now called a gastrula
•The rim of the double walled cup is at first wide, but it contracts to form the
blastopore, enclosing the endoderm and the mesoderm.
•The mesoderm includes the muscle tissue and the rudiment of the notochord
•The notochordal tissue converge on the mid-dorsal line with the remaining
mesodermal tissue on either side
•A pear-shaped part of the ectoderm on the mid-dorsal line flattens under the
influence of the underlying notochord to become the neural plate. The
appearance of the neural plate shows that the process of neurulation has
started. The neural plate separates from the surrounding ectoderm, sinks in
and becomes covered by the remaining ectoderm, now called the epiderm.
•The epiderm will become the integument or skin
•The archenteron is lined dorsally by the notochord that forms a longitudinal
band on the mid-dorsal line with the mesodermal tissue on either side.
•The lateral and ventral walls of the archenteron are formed by the endoderm
• The presumptive tissues of the different germ layers separate from each
other through the formation of crevices
• The notochord band becomes a solid rod
• The future muscle tissue separates from the endoderm through the
formation of a groove in contact with the archenteron. This groove closes off
from the archenteron and the tube thus formed breaks up into segments
• Each of these segments surrounds a cavity and is called somites and the
process is metamerisation. The cavities in the somites will become the
coloem
•The neural plate rolls itself into a tube, the neural tube.
•All the primary organ rudiments have been laid dowm.
•From here on the body of the lancelet will elongate further, the
blastopore becomes the anus, while the mouth breaks through just
anterior to the tip of the notochord.
Chapter 5 : Early development
of the avain embryo and the
extra-embryonic membranes






Cleavage is typical Meroblastic,
discoidal or Incomplete
The egg contains a large amount of yolk
Divisions take place in the animal pole
and are restricted to the cytoplasmic cap
where a little of cytoplasm surrounds the
nucleus of the zygote
Divisions are incomplete i.e. the cell
membrane that separates the two
daughter cells are never completed
The first divisions are all vertical, the
daughter cells are separated by the
cleavage furrows but remain open to the
yolk
Later on horizontal divisions take place
and a slit-like cavity (comparable with the
blastocoele)is formed between the
blastomeres and the yolk to form the
blasotodisc
• The entire embryo develops out of the blastodisc. As nutrients re accumulated
and need to be provided, waste products needs to be stored, gaseous
exchange need to be facilitated and the embryo need to be protected against
desiccation and mechanical shock extra-embryonic membranes also develop
out of the blastodisc
•There are 4 extraembryonic membranes:
1. The amnion – envelopes the
embryo and filled with amniotic
fluid
2. The yolk sac that envelops the
yolk with fingerlike pojections that
penetrate deep into the yolk
3. The allantois - an outgrowth of
the hind-gut that spreads out
between the amnion, yolk sac en the overall enveloping chorion (4)
• The amnion, allantois and yolk sac is connected to the embryo by means of
the umbilical chord
• Embryonic bloodvessels run through the umbilical chord to the amnion, yolk
sac and allantois
1. Amnion – (i) prevents desiccation and (ii) protects the embryo against
mechanical shock
2. Yolk sac – (i) Absorbs the yolk into the bloodstream and (ii) Transports the
nutrients to the embryo
3. Allantois – (i) stores the waste products inside the allantoic cavity and (ii)
absorbs the oxygen from the air that enters through the shell, shell
membranes and chorion and stransports it to the embryo and transports the
CO2 from the embryo away from the embryo so that is can move out
4. Chorion – (i) protects the
embryo and the other extraembryonic membranes from
friction against the hard shell
and (ii) from adhesion to the
shell
Chapter 6 : The development of
the mammalian embryo
•Contains
NO yolk
•Embryo is retained in the uterus
•Attached to the wall of the uterus by
means of a new structure the placenta
•The placenta facilitates gaseous
exchange, provides nutrition and
removes waste products
• Fertilization takes place in the ampulla of the oviduct
• Cleavage takes place as the zygote moves down the oviduct towards
the uterus
•The morula enteres the uterus 3 days after fertilization
•The morula is 12 – 16
cells forming a solid ball of
cells and is still
surrounded by the zona
pellucida
• As cleavage divisions
continue cavities appear in
the interior of the morula





The cavities become filled with fluid from
the uterine cavity to become the
blastocyst cavity
The embryo is now a blastocyst,
consisting of the trophoblast,
surrounding the blastocyst cavity with
the inner cell mass/embryoblast/
formative cells at the embryonic pole
The inner cell mass is basophilic-stains
bright when stained with basic dyes.
The trophoblast gives rise to the
extra-embryonic membranes, while the
embryo proper develops out of the inner
cell mass.
The zona pellucida (fertilization
membrane) is still intact around the
blastocyst and the blastocyst lies free in
the uterine cavity.




The zona pellucida starts to
degenerate and eventually
disappears.
This enables the blastocyst to
become attached to the
endometrial epithelium.
The embryo is now referred to as
a conceptus.
The conceptus orientates itself in
such a way that the embryonic
pole lies against the endometrium
of the uterus. The trophoblast
attaches itself to the endometrial
epithelium in the following way.





The trophoblast cells in contact
with the endometrial epithelium
undergo multiple divisions
Differentiates into two layers:
inner cellular layer =
cytotrophoblast and an outer
= syncytotrophoblast.
Cell membranes of the outer
layer disappear to form a
multi-nucleated protoplasmic
mass = syncytium
Forms finger-like processes
which grow into the
endometrium.
The embryo is superficially
implanted by the end of the first
week.
• A new layer becomes visible during
attachment and superficial implantation
• A flat layer of cells possibly originating
from the trophoblast arranges, through
a process of migration, into a thin layer
of flat cells lining the inner surface of the
inner cell mass to form the hypoblast.
• The hypoblast therefore forms a
ceiling to the blastocyst cavity,
separating the inner cell mass from the
blastocyst cavity.




The inner cell mass +
hypoblast arrange into a
double layered disc = the
embryonic disc, consisting of
a top layer of columnar cells =
the epiblast and a second
layer of cuboidal cells, the
hypoblast.
Also known as the bilaminar
embryonic disc.
The epiblast develops into all
three germ layers of the
embryoproper.
The cells of the hypoblast
develop into endoderm or part
of it. It is believed that its
contribution is mostly extra
embryonic in that it mainly
contributes to the endoderm of
the extra embryonic
membranes.



The syncytotrophoblast
invades deeper into the
endometrial stroma. The
cells of the stroma
degenerate in the area of
the penetrating
syncytotrophoblast.
The trophoblast continue to
divide producing cells that
migrate into the increasing
mass of the
syncytotrophoblast where
they soon loose their cell
membranes.
The formation of the syncyto
trophoblast expands around
the cytotrophoblast of the
conceptus as implantation
progresses.
Cavities form between the
inner cell mass and the
trophoblast. These spaces join
each other to form the
amniotic cavity.

The part of the cytotrophoblast
overlying the epiblast splits
into 2 layers.

The inner layer consists of
amnioblast cells

The amnioblasts &
cytotrophoblast together
becomes the the amnion.

The cells of the epiblast form
the floor of the amniotic cavity
and are continuous
peripherally with the amnion.
 The cells of the hypoblast form the roof of the blastocyst cavity and are
peripherally continuous with the cytotrophoblast.





Delamination of the
cytotrophoblast
surrounding the blastocyst
cavityalso takes place
The inner layer of the
cytotrophoblast around the
blastocyst cavity is
continuous with the
hypoblast
It is a thin membranous
layer called the
exocoelomic membrane
The blastocyst cavity is now
referred to as the
exocoelomic cavity.
Cavities appear in the syncytotrophoblast called lacunae which soon
become filled with maternal blood from the ruptured capillaries and
secretions from the eroded endometrial glands.
This nutritive fluid is known as embryotroph and reaches the
embryonic disc by diffusion.
 The lacunae become connected with the maternal blood vessels and
in this way the uteroplacental circulation is established.
Oxygenated blood reach the lacunae through the arteries, and the
deoxygenated blood is removed from the lacunae by the uterine veins,
creating a primitive uteroplacental circulation.
A lacunar network is established giving the syncytotrophoblast a
sponge-like appearance.
The lacunar network is initially formed in the area of the embryonic
pole. This network increases and spreads throughout the whole
endometrium to surround the conceptus completely.


At this stage cavities appear in the
syncytotrophoblast called lacunae which soon
become filled with maternal blood from the
ruptured capillaries and secretions from the
eroded endometrial glands. This nutritive fluid
is known as embryotroph and reaches the
embryonic disc by diffusion.


The lacunae become connected with the
maternal blood vessels and in this way the
uteroplacental circulation is established.
Oxygenated blood reach the lacunae through


The conceptus continues to penetrate deeper
into the stroma of the endometrium until it is
completely embedded. A closing plug
consisting of a blood clot and cellular debris
remain on the surface of the epithelium for a
short while until the regenerating epithelium
covers over the conceptus.
This type of implantation where the entire
conceptus is embedded into the endometrium
of the uterus is called interstitial implantation.