The zona pellucida is a glycoprotein layer surrounding the mammalian egg. It is composed of three glycoproteins - ZP1, ZP2, ZP3. ZP3 binds to sperm receptors, triggering the acrosome reaction which allows enzymes to degrade the zona pellucida. This forms a tunnel through which the sperm penetrates. ZP2 then maintains contact between the sperm and egg. Once through the zona pellucida, the sperm moves through the perivitelline space until contacting the egg's plasma membrane. The zona pellucida thus functions to induce the acrosome reaction, prevent polyspermy, and block heterospecific fertilization

1. Zona
Pellucida
Tasmina Ferdous Susmi

2. Index
Ovum
Zona Pellucida
Glycoproteins of Zona pellucida
Function of Glycoproteins
Mechanism of Zona Pellucida

3. Ovum
Single cell released from either of the female reproductive organs,
the ovaries which is capable of developing into new organism when
fertilized with a sperm cell.
Has a central nucleus that contains the female’s genetic material.
Enclosed by a cell wall consisting of two layers, an outer one is
zona pellucida and inner one is Vitelline membrane.

5. Zona Pellucida
A glycoprotein layer surrounding the plasma membrane of
mammalian oocytes.
A vital constitutive part of the oocyte.
First appears in unilaminar primary oocyte.
Is secreted by both the oocyte and the ovarian follicles.

6. Zona Pellucida
An extracellular matrix composed of three glycoproteins .
This structure binds with spermatozoa.
Triggers the sperm head to undergo the acrosome reaction.
Once the sperm has penetrated the zona pellucida, it moves
through a narrow, oblique path for fertilization.

7. Glycoproteinsof Zona Pellucida
Three glycoproteins are- ZP1, ZP2, ZP3.
ZP3 mediates sperm-specific egg binding.
ZP2 mediates subsequent sperm binding.
ZP1 cross-links ZP2 and ZP3 as protein meshwork.

10. Function of ZP
As a receptor.
Inducer of acrosomal reaction.
Prevention of polyspermy.

11. Function of ZP
Protection of the embryo before implantation.
Blockade of heterospecific fertilization.
Species-specific barrier to sperm binding and
penetration.

12. Mechanism of Zona Pellucida
Receptors on the sperm plasma membrane attach to ZP3.
Binding to ZP3 allows the sperm to adhere to the zona pellucida
and is a critical step in fertilization.
It triggers the sperm head to undergo the acrosome reaction.

13. Mechanism of Zona Pellucida
An influx of calcium and a rise in pH and cAMP levels within the sperm
head cause exocytosis of the acrosomal vesicle. Exocytosis occurs when
the plasma membrane of the sperm fuses with the outer acrosomal
membrane, forming many small openings to the acrosome.
Contents of the acrosome, which are hydrolytic enzymes, spill out and
degrade the zona pellucida near the sperm head. This forms a tunnel in
the zona, through which the sperm begins to move.
Degradation of the sperm plasma membrane during the acrosome
reaction causes the loss of ZP3 receptors.

14. Mechanism of Zona Pellucida
However, now the inner acrosomal membrane is exposed, and it
appears to have receptors for another zona pellucida glycoprotein
called ZP2.
This ZP2 binding maintains the contact between egg and sperm.
The sperm tail continues to beat vigorously, helping the sperm
penetrate through the zona pellucida and make contact with the
plasma membrane of the egg.

15. Mechanism of Zona Pellucida
Once the sperm has penetrated the zona pellucida, it moves
through a narrow, oblique path into the perivitelline space (the area
between the zona pellucida and the vitelline membrane).
Penetration of the human zona pellucida by a sperm takes less
than 10 min under experimental conditions.

16. Zona Pellucida Reaction

17. Thank you