1. Zygote

2. • Zygote is the cell resulting from the
fertilization of a secondary oocyte by a sperm.
• The secondary oocyte finishes its second
meiotic division immediately after the entry of
the sperm and becomes ovum.

4. • Fertilization occurs in the ampullary part of
fallopian tube so zygote is formed in the
ampullary part of fallopian tube.
•
• Immediately after its formation it starts:
• 1. Moving towards uterus
• 2. Multiplying mitotically

5. • Cleavage is a mitotic type of cell division
where the daughter cells are smaller than the
parent cells.
• The number of cells increases but the size of
the daughter cells and the cell-mass does not
increase.

7. • 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
as cleavage.

8. • 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.

9. • 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
fertilization.
• Subsequent divisions follow one another,
forming progressively smaller blastomeres.

10. • After three or four divisions, the zygote, looks
like a mulberry and is known as morula (L.
Morus, mulberry).
• It is a solid mass of 12 to 16 cells
(blastomeres).
• This stage is reached about three days after
fertilization and the mass of cells is about to
enter the uterus.

12. • 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.

13. • 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
other membranes.
• Note also the degeneration process of zona
pellucida.

14. • 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
mass.

15. • The fluid-filled spaces soon fuse to form a
single large space or cavity, pushing the whole
inner cell mass to one side.
• The conceptus is now called blastocyst and
the cavity is known as blastocyst cavity or
blastocele.

16. • 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.

17. • The cells of the inner cell mass are attached to
one side of trophoblast and project into the
blastocyst cavity.
• They are now referred to as embryoblast,
which will form the embryo.

19. • The side where the embryoblast is present is
called embryonic pole of the blastocyst.
• The other side obviously will be abembryonic
pole.

20. • 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.

21. • At about 6th day after fertilization the
embryonic pole of the blastocyst becomes
attached to the endometrium and start
penetrating into it.

22. Zona pellucida
• 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.

23. • In each menstrual cycle 12 to 15 primordial
follicles are selected for maturation yet only
one is guided to full maturity.
• As any primordial follicle starts maturing its
follicular cells and perhaps primary oocyte,
secrete a layer of glycoproteins on the surface
of oocyte, thus forming zona pellucida.

24. • This Zona Pellucida is present in all follicles
(primary follicles, secondary follicles, tertiary
follicles and Gra’afian follicles) except
primordial follicles.
• Zona Pellucida surrounds primary oocyte and
itself Zona Pellucida is surrounded by
granulosa cells.

25. • 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.

26. • After ovulation the granulosa cells
surrounding the secondary oocyte and zona
pellucida are given the name corona radiata
cells.

27. • 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
Zona Pellucida.

28. • Once the sperm penetrates Zona Pellucida a
change in the properties of Zona Pellucida occurs
that makes it impermeable to other sperms.
• This is called zona reaction.
• This zona reaction is believed to result from the
action of lysosomal enzymes released by cortical
granules near the plasma membrane of oocyte.

29. • After fertilization Zona Pellucida contains
ovum with two polar bodies.
• Zona pellucida remains present around zygote
and morula at different stages of
development.

30. • Shortly after the morula enters the uterus
(about 4 days after fertilization), fluid starts
appearing within the morula between the
blastomeres.
• This fluid passes from the uterine cavity
through the zona pellucida to form spaces.

31. • As the spaces coalesce and enlarge the morula
is converted into blastocyst.
• These uterine secretions start destroying the
zona pellucida.

33. • After the blastocyst has floated in the uterine
secretions for about two days, the zona
pellucida gradually degenerates, ruptures and
disappears.
• The blastocyst hatches out and this is called
hatching of blastocyst.

34. • It is important to note that the life of zona
pellucida is 15 days if there is no fertilization.
•
• The life of zona pellucida is 20 days if wife
becomes pregnant.