Implantation,conception,
development of placenta and
establishment of fetomaternal
circulation
Gametogenesis
• Maturation of two highly specialized cells
spermetozoon in males and ovum in
females
Oogenesis- Development of mature
ovum
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In female gonad , the germ cells undergo rapid mitotic division and differentiate
into oogonia
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Oogonia enter into prophase of first meiotic division - Primary oocytes
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Primary oocytes do not finish the first meiotic division until puberty is reached
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Primary oocyte undergoes first meiotic division – secondary oocyte and first
polar body
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Ovulation occurs soon after the formation of the secondary oocyte
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Secondary oocyte completes the second meoitic division after fertilization by
the sperm in falliopian tube- ovum and second polar body. In the absence of
fertilization the secondary oocyte does not complete the second meiotic
divison and degenerate
Ovulation
• Is the process whereby secondary oocyte is
released from the ovary following rupture of
mature graaafian follicle and become available
for conception
• Only one secondary oocyte is likely to rupture in
each ovarian cycle which starts at puberty and
ends in menopause
• In relation to the menstrual period event occurs
about 14 days prior to the expected period
Changes in the follicle before
ovulation
• Graafian follicle becomes enlarged ( 20mm )
• Follicular wall near the ovarian surface becomes
thinner
• Stigma develops as conical projection which
penetrates the outer surface of the ovary and
persist as thin layer
• Cumulus escapes out of the follicle as a slow
oozing, process taking about 1-2 min
• Stigma is closed by a plug of plasma
Changes in oocyte before ovulation
• Increase in cytoplasmic volume
• Changes in the number and distribution of
mitochondria and in the Golgi apparatus
• Completion of arrested first meiotic division
occurs with formation of secondary oocyte and
first polar body each containing haploid number
of chromosomes(23X )
Cause of ovulation
Combined FSH/LH midcycle surge is responsible for the final stage of
maturation, rupture of the follicle and expulsion of the oocyte
LH surge• Sustained peak levels of oestrogen for 24-36 hours in the late
follicular phase cause LH surge from anterior pitutary.
• Ovulation occurs apprx 16-24 hours after LHsurge
• LH stimulates completion of reduction division of the oocyte,initiates
leutinisation of the granulosa cells, synthesis of progestrone and
prostaglandins
FSH rise
• Preovulatory rise of progestrone facilitates the
positive feed back action of estrogen to induce
FSH surge
• FSH surge causes increase in plasminogen
activator which converts plasminogen into
plasmin , which in turn causes lysis of the wall of
the follicle
Structure of a mature ovum
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Largest cell in the body
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Consists of cytoplasm and a nucleus with its nucleolus in eccentric position
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Contains 23 chromosomes (23 X )
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Sorrounded by a cell membrane called a vitelline membrane
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Outer transparent mucoprotein envelope is called zona pellucida
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Tiny channels in zona pellucida are for the transport of the materials from the
granulosa cells to the oocyte
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Space between the vitelline membrane and zona pellucida is called perivitelline
space which accomodates the polar bodies
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Oocyte after its escape from the follicle, retains a covering of granulosa cells
known as corona radiata derived from the cumulus oophorus
Spermatogenesis
• Spermatogenesis-Development of spermatids
from the primodial male germ cells and their
differenciation into spermatozoa
• In man time required for a spermatogonium to
develop into a mature spermatozoon is about
61 days
Spermatogenesis
• Primodial germ cells undergo mitosis in seminiferous tubules to
develop into spermatogonia
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• Spermatogonia differenciates into primary spermatocytes (46 XY)
which remain in the stage of prophase of the first meiotic division
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• With completion of first meiotic division – two secondary
spermatocytes are formed( 23 X or 23 Y )
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• Immediately follows the second meiotic division- 4 spermatids are
formed, containing haploid number of chromosomes
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• Spermatids undergo extensive morphological changes to convert
them into spermatozoa
Structure of a mature
spermatozoon
• It has head and a tail
• Head consist of nucleus and a acrosomal
cap,rich in enzymes
• The tail is divided into four zonesneck,middle piece,the principal piece and
the end piece.
Sperm capacitation and acrosome
reaction
• Capacitation is the physiochemical change in
the sperm by which sperm become hypermotile
and is able to bind and fertilize a secondary
oocyte
• Activation of acrosomal enzyme causes release
of hyaluronidase, hydrolytic enzymes,
proacrosin,acrosin that help the sperm to digest
the zona pellucida and to enter the oocyte
Fertilization
• Is the process of fusion of spermatzoon with the mature
ovum
• Fertilization occurs in the ampullary part of the uterine
tube
• Ovum , following ovulation is picked by tubal fimbriae
and is transported to the ampullary part
• Out of the hundreds of millions sperm deposited in the
vagina at single ejaculation only thousand capacitated
spermatozoa enter the tube while only 300-500 reach
the ovum
• Fertilisable life span of oocyte ranges from 12-24 hours
where as that of sperm is 48-72 hours
• Complete disolution of the cells of the corona radiata
occurs by the chemical action of the hyaluronidase
liberated from the acrosomal cap of the sperms
• Penetration of the zona pellucida is facilitated by the
release of hyaluronidase from the acrosomal cap
• After the one spermatozoon has entered the ovum ,
others are prevented from entering by zona reaction
• Completion of the second meiotic division of the
oocyte immediately follows resulting in the
female pronucleus (23 X) and 2nd polar body
• At the same time the head of the spermetazoa
separates from middle piece and tail and
transforms into male pronucleus (23Xor 23Y)
• Male and female pronucleus unite at the
center resulting in formation of zygote (46
XX or 46XY)
• Sex of the child will depend on the pattern
of sex chromosome supplied by the sperm
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
Cleavage cont’d
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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
• Outer cell layer later give rise
to trophoblast and inner cell
mass give rise to embryo
proper
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 embryo
Cleavage cont’d
• At this stage, the
conceptus is called
Blastocyst. It has two
poles: embryonic &
abembryonic
• Zona pellucida gradually
degenerates and
disappears soon after the
morula reaches the uterine
lumen
• Blastocyst takes its
nourishment from uterine
secretions and enlarges in
size. It is ready to get
attached and implanted to
the uterine wall
Embryonic pole
Abembryonic pole
Formation of germ layers , chorion
and amnion
• Some cells of the inner cell mass become flattened and
come to lie on its free surface and constitute the
endoderm
• Remaining cells of inner cell mass become columnar
and constitute the ectoderm
• A space appears between the ectoderm and the
trophoblast . This is the amniotic cavity filled by amniotic
fluid
• The roof of the cavity is formed by amniogenic cells
derived from the trophoblast, while its floor is formed by
the ectoderm
• Flattened cells arising from the endoderm
spread and line the inside of the blastocystic
cavity
• In this way , a cavity lined on all sides by cells of
endodermal origin is formed. This cavity is called
primary yolk sac
• Cells of the trophoblast give origin to a mass of
cells called extra-embryonic mesoderm or
primary mesoderm
• These cells come to lie between the trophoblast
and the flattened endodermal cells lining the
yolk sac
The process by which the
developing mass gets
embedded within the uterine
wall
Implantation
• Begins 6 days after
fertilization:
 The blastocyst attaches to
the endometrial epithelium,
usually adjacent to the
embryonic pole
 The blastocyst goes deeper
and deeper into the uterine
mucosa till whole of it comes
to lie with in the thickness of
the endometrium. This is
called interstitial implantation
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
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
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
• After the implantation of the embryo, the uterine
endometrium is called the decidua
• When the morula reaches the endometrium , it is
in the secretory phase
• After implantation , features of the endometrium
in secretory phase are intensified- stromal cells
enlarged , become vacuolated and store
glycogen and lipids. This change in stromal cells
is called decidual reaction
• The portion of the decidua where the placenta is
to be formed ( deep to the developing blastocyst
) is called decidua basalis
• Part of the decidua that separates the embryo
from the uterine lumen is called decidua
capsularis
• Part of the decidua lining the rest of the uterine
cavity is called decidua parietalis
Formation of the chorionic villi
• The essential functional elements of the placenta are
very small finger like processes or villi
• These villi are surrounded by maternal blood
• In the subustance of the villi, there are capillaries
through which fetal blood circulates
• Exchanges between maternal and fetal circulations take
place through the tissues forming the walls of the villi
• The villi are formed as offshoots from the surface of the
trophoblast
• As the trophoblast along with the underlying extraembryonic mesoderm constitutes chorion, the villi are
known as chorionic villi
• Chorionic villi are first formed all over the trophoblast and
grow into the surrounding decidua
• Those related to decidua capsularis are transitory and
degenrate and this part of the chorion becomes smooth
and is called chorion laevae
• The villi that grow into the decidua basalis
undergo considerable development
• Along with the tissues of the decidua basalis
these villi form a disc shaped mass which is
called the placenta
• The part of the chorion that helps to form the
placenta is called the chorion frondosum
Formation of the chorionic villi
 Trophoblast proliferates rapidly
and differentiates into two layers:
 inner cellular cytotrophoblast
or Langhan’s layer
 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
Implantation cont’d
• Small cavities, the
lacunae appear in
syncytiotrophoblast,
and get filled with
maternal blood,
establishing primitive
uteroplacental
circulation
• The syncitotrophoblast grows into the endometrium
• As the endometrium is eroded , some of its blood
vessels are opened up and blood from them fills the
lacunar space
• Each trabeculus is , initially, made up entirely of
sycytiotrophoblast
• Later on, cells of cytotrophoblast grow into the
trabeculus , followed by extra embryonic mesoderm and
blood vessels giving rise to primary villus, secondary
villus and tertiary villus respectively
• Blood vessels of the villus establish connections
with the circulatory system of the embryo
• Fetal blood now circulates through the villi, while
the maternal blood circulates through the
intervillous space
• Intially cytotrophoblast that that grows into the
trabeculus does not penetrate the entire
thickness of syncytium
• At a later stage ,cells of the cytotrophoblast emerge
through the syncytium and spread out to form a layer
that completely cuts off the syncytium from the decidua
and is called cytotrophoblastic cells
• The villi that are first formed are attached on the fetal
side to the embryonic mesoderm and on the maternal
side to the cytotrophoblastic shell and are called
anchoring villi
• Each anchoring villus consists of a stem (truncus
chorii); this divides into a number of branches (
rami chorii ) which in turn divide into finer
branches (ramuli chorii )
• Anchoring villi give off numerous branches which
grow into the intervillous space as free villi
• As a result , the surface area available for
exchanges between maternal and fetal
circulation becomes enormous
Circulation of blood through
placenta
• Maternal blood in the intervillous space is
constantly in circulation
• Both arteries and the vein open into the
roof of the cotyledon and that the pressure
of the blood in the artery is sufficient to
drive blood to the fetal end of the
intervillious space