Changes

== Cortical Reaction - Block to Polyspermy ==

== Cortical Reaction - Block to Polyspermy ==

* During the first and second meiotic divisions of Oogenesis small,dense '''cortical granules''' move to the periphery of the Oocyte cytoplasm.

* During the first and second meiotic divisions of oogenesis small, dense '''cortical granules''' move to the periphery of the Oocyte cytoplasm.

* Cortical granules consist of:

* Cortical granules consist of:

** Mucopolysaccharides

** Mucopolysaccharides

** Peroxidase

** Peroxidase

* Cortical granules undergo '''exocytosis''', releasing their contents into the '''perivitteline space'''.

* Cortical granules undergo '''exocytosis''', releasing their contents into the '''perivitteline space'''.

* Contents of cortical granules '''cross-links zona proteins''' to make them impenetrable to further Spermatozoa.

* Contents of cortical granules '''cross-links zona proteins''' to make them impenetrable to further spermatozoa.

* This is known as the '''zona block''', it prevents polyspermy (fertilization by more than one sperm)which would result in embryo death.

* This is known as the '''zona block''', it prevents polyspermy (fertilization by more than one sperm)which would result in embryo death.

* The cortical reaction also reduces the ability of the Oocyte plasma membrane to fuse with additional spermatozoa.

* The cortical reaction also reduces the ability of the oocyte plasma membrane to fuse with additional spermatozoa.

* This is the '''vitelline block to polyspermy'''.

* This is the '''vitelline block to polyspermy'''.

* If two spermatozoa enter the perivitelline space simultaneously, they both contact the oocyte and proteins are not cross-linked rapidly enough to block penetration of the Zona Pellucida.

* If two spermatozoa enter the perivitelline space simultaneously, they both contact the oocyte and proteins are not cross-linked rapidly enough to block penetration of the zona pellucida.

* Thus, the block depends on limiting the number of Spermatozoa in the vicinity of the Oocyte.

* Thus, the block depends on limiting the number of Spermatozoa in the vicinity of the Oocyte.

* Only a small sub-population are released from the store in the female tract over the period of ovulation so that 7-10 out of the original millions are around the Oocyte at the period of fertilization.

* Only a small sub-population are released from the store in the female tract over the period of ovulation so that 7-10 out of the original millions are around the oocyte at the period of fertilization.