• Fertilization occurs in the ampullary part of fallopian
tube so zygote is formed in the ampullary part of
• Immediately after its formation it starts:
• 1. Moving towards uterus
• 2. Multiplying mitotically
• Zygote immediately after its formation, in the
ampullary part, is guided medially through the
uterine tube toward the uterine cavity.
• As the zygote, passes through the uterine
tube, it undergoes mitotic cell divisions known
• This phase of development begins with the
first mitotic division of the zygote and ends
with formation of blastocyst.
• It extends for 6 to 7 days or a week.
• The 1st division of zygote gives rise to two
daughter cells. They are called blastomeres.
• Each blastomere is half the size of parent cell.
They develop about 30 hours after
• Subsequent divisions follow one another,
forming progressively smaller blastomeres.
• After three or four divisions, the zygote, looks
like a mulberry and is known as morula (L.
• It is a solid mass of 12 to 16 cells
• This stage is reached about three days after
fertilization and the mass of cells is about to
enter the uterus.
• At this time, the morula consists of a group of
centrally located cells, the inner cell mass, and
a surrounding layer, the outer cell mass.
• The inner cell mass will give rise to the tissues
of the embryo proper, while the outer cell
mass forms the trophoblast which later
contributes in the formation of placenta and
• Note also the degeneration process of zona
• Morula enters uterine cavity about 4th day
after fertilization. As the morula enters the
uterine cavity fluid begins to penetrate
through the zona pellucida. Spaces appear
between the inner cell mass and outer cell
• During the formation of blastocele the cells of
outer cell mass becomes flattened.
• So the outer cell mass is now a single layer of
flattened cells forming the wall of blastocyst
and it is now given the name trophoblast
(Gr. Trophe, nutrition), which will form
placenta and associated membranes.
• The cells of the inner cell mass are attached to
one side of trophoblast and project into the
• They are now referred to as embryoblast,
which will form the embryo.
• The zygote lies free in the uterine
secretions for about two days.
• During this time the zona pellucida
degenerates and disappears. The
blastocyst hatches out and this is called
hatching of blastocyst.
• At about 6th day after fertilization the
embryonic pole of the blastocyst becomes
attached to the endometrium and start
penetrating into it.
• Zona pellucida is created around primary
oocyte when primordial follicle is transformed
into primary follicle.
• So Zona Pellucida is not present in primordial
follicle while primary follicle is characterized
by Zona Pellucida.
• This Zona Pellucida is present in all follicles
(primary follicles, secondary follicles, tertiary
follicles and Gra’afian follicles) except
• Zona Pellucida surrounds primary oocyte and
itself Zona Pellucida is surrounded by
• Just before ovulation the primary oocyte is
converted into secondary oocyte and it is
always the secondary oocyte, which ovulates.
• Now the Zona Pellucida contains secondary
oocyte and first polar body.
• After ovulation the granulosa cells
surrounding the secondary oocyte and zona
pellucida are given the name corona radiata
• During fertilization the sperms after passing
through corona radiata, become attached to
Zona Pellucida. The enzymes released from
the acrosome (esterases, acrosin, and
neuraminidase) causes lysis of Zona Pellucida,
thereby forming a path for the sperm through
• After fertilization Zona Pellucida contains
ovum with two polar bodies.
• Zona pellucida remains present around zygote
and morula at different stages of
• Shortly after the morula enters the uterus
(about 4 days after fertilization), fluid starts
appearing within the morula between the
• This fluid passes from the uterine cavity
through the zona pellucida to form spaces.