It is the process by which mature male & female gametes fuse to form zygote
Normal site - in ampulla of uterine tube.
200 – 300 million of spermatozoa deposit in female genital tract
Out of them 300 – 500 reach the site of fertilization
Out of them only one is needed for fertilization, others help in penetrating
the corona radiata
(2)Penetration of corona radiata
Capacitation - removal of glycoprotein from acrosome /high motility of
sperm without morphologic change
Corona radiata cells are dispersed by combined action of sperm
(Hyaluronidase ) & tubal mucosal enzymes.
(3)Penetration of Zona pellucida –
It is penetrated by sperm with the aid of enzyme released by the inner
acrosomal membrane, (acrosin)
Zona reaction - Impermeability to other sperm by lysosomal enzymes of 2nd
(4)Fusion of oocyte –sperm cell membrane
Leaving behind plasma membrane, head & tail enter the cytoplasm of oocyte
The Egg responds in 3 ways
Cortical & Zonal reaction
Resumption of 2nd meiotic division
Metabolic activation on the egg
Female – after fusion of cell membrane oocyte completes its 2 nd meitotic
division. One of the 2 daughter cell forms the pronucleus
Male Nucleus from the head of the sperm becomes swollen to form
DNA replication – This occurs in both Pronuclei
Mitosis – Pronuclei meet, their nuclear membrane dissapear & chromosomes
arranged in distinct manner for mitosis
(6)Results of fertilization
restoration of the diploid number of chromosomes
new combination of chromosomes
primary sex determination
initiation of cleavage
2-cell stage(blastomeres) -->
Each of the cells results from
cleavage of zygote is called
The name of the embryo at
the 16 to 32 cell stage.
(8)Formation of Blastocyst
Morula when it is filled with uterine fluid is called Blastocyst
Morula eners the uterine cavity 60 – 72 hours after fertilization
At the same time fluid enters into the intercellular spaces of inner cell
mass through zona pellucida
Intercellular spaces become confluent & finally a single cavity is formed
At this time zona pellucida dissappears & the zygote is then known as
Embryoblast:(Forms Embryo) The cells on the inside.
Trophoblast:(Forms Placenta)(Nutrient providing cells) The cells
surrounding the outside.
Process of placement of blastocyst into the wall of the uterus is called
Normal Site – Posterior wall of the uterus close to the fundus
initiated 6-7 days after fertilization
Uterine mucosa already undergoes several changes (menstrual cycle) to
Duing implantation the mucosa is in secretory phase & poseses 3 layers
Polar trophoblast now adhers to the uterine mucosa & starts penetraion
into it by eroding the surface(by release of proteolytic enzymes )
By erosion ,trophoblast forms a cavity in the mucosa into which the
blastocyst becomes embeded
(13)Uterus after the implantation
After implantation of blastocyst the endometrium is called decidua
After implantation the stromal cells of endometrium enlarge become vacuolated
& store glycogen & lipid
These changes in stromal cells are called decidual reaction
Divisions of decidua
Decidua Basalis – portion where placenta is to be formed
Decidua Capsularis – portion which separates embryo form uterine cavity
Decidua Parietalis – rest part of decidua
(14)Formation of germ layers ( Gastrulation)
By day 8, The trophoblast differentiates into:
An inner cytotrophoblast surrounding the blasocyst
An outer layer of proliferating cells syncitiotrophoblast .
The blastocyst is more deeply embeded in the endometrium by day 9, & the
penetration defect is closed by fibrin coagulum
(15)Embryoblast splits into 2 layers:
The epiblast (primitive ectoderm)
The hypoblast (primitive endoderm), thus forming the bilaminar germ
Within the epiblast, the amniotic cavity develops
Epiblast cells adjacent to cytotrophoblast is called amnioblasts
Cells at the periphery of the hypoblast migrate over the inner surface of the
cytotrophoblast, forming a thin layer called exocoelomic (Heuser’s
membrane) , with the blasocyst cavity hence forth called the primary yolk
sac (Exocoelomic Cavity) .
Formation of germ layers
Exocoelomic membrane prolifates & splits to form 2 extraembryonic mesodermal
Extraembryonic somatopleuric mesoderm, Extraembryonic splanchnopleuric M.
Cavity inbetween – Extraembryonic coelom (chorionic cavity)
Maternal sinusoids have grow into the expanding lacunae to supply the
Hypoblast produce additional cells that migrate along the inside of
exocoelomic membrane .
Cells proliferate & form new cavity is known as secondary or definite yolk sac
Large portions of Primitive yolk sack are pinches off as exocoelomic cysts
The E. mesoderm and 2 two layers of the Trophoblast together form the
chorion. It surrounds the embryo and, later, the fetus.
By end of 2nd week of development, bilaminar embryonic disc becomes
connected to trophoblast by a band of E. mesoderm called the connecting
(body) stalk the future umbilical cord.
Slight thickening in the hypoblast region – Prechordal plate
The embryo is now a bilaminar disc consisting of the:
Epiblast that forms the floor of the amniotic cavity
Hypoblast that forms the roof of the yolk sac
Implantation begins on approximately day 5 and is completed by the end of
the second week
Outgrowths from the syncitiotrophoblast invade the deciduas
They erode the maternal blood vessels.
They become surrounded by trophoblastic lacunae containing maternal
A utero-placental circulation is established by day 13
Is a structure by which fetus is attached to mother & gives O2,nutrition to the
embryo(fetus) & excretes waste products ,CO2 from fetus to mother
Chorion frondosum formed by following stages
consist of syncitiotrophoblast and cytotrophoblast
have a core of extra-embryonic mesoderm
have blood vessels in the mesoderm
Normal implantation: posterior / anterior wall in the body of the uterus.
Ectopic pregnancy/ extra uterine pregnancy
Placenta previa - Atypical placental positioning and attachment within the
lower third of uterus, which may cover the cervix in part or fully.