Changes

== Fusion with the Oocyte ==

== Fusion with the Oocyte ==

* When the Spermatozoan completely penetrates the Zona Pellucida and reaches the Perivitelline Space, it settles into a bed of '''microvilli''' formed by the Oocyte plasma membrane.

* When the spermatozoon completely penetrates the zona pellucida and reaches the perivitelline space, it settles into a bed of '''microvilli''' formed by the oocyte plasma membrane.

* Oocyte plasma membrane fuses with the '''equitorial segment''' and the fertilizing Spermatozoon is engulfed.  

* Oocyte plasma membrane fuses with the '''equitorial segment''' and the fertilizing spermatozoon is engulfed.  

**Brought about by a fusion protein that is inactive prior to the [[Copulation_-Sperm_in_the_Female_Tract_-_Anatomy_%26_Physiology|acrosome reaction]].

**Brought about by a fusion protein that is inactive prior to the [[Sperm in the Female Tract - Anatomy & Physiology|acrosome reaction]].

* Nucleus of the Spermatozoon is within the Oocyte cytoplasm.

* Nucleus of the spermatozoon is within the oocyte cytoplasm.

* Sperm nuclear membrane disappears.

* Sperm nuclear membrane disappears.

* Sperm '''nucleus decondenses'''.

* Sperm '''nucleus decondenses'''.

== 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.

* Male and female pronuclei fuse.

* Male and female pronuclei fuse.

* Sperm delivers a haploid complement of chromosomes, but also activates the Oocyte by:  

* Sperm delivers a haploid complement of chromosomes, but also activates the Oocyte by:  

** A unique '''phospholipase Cζ''' causes release of calcium from the endoplasmic reticulem to the cytosol.  This is then re-sequestered and bound in the endoplasmic reticulem to give regular 'spikes' or oscillations of calcium that signal rapidly across the Oocyte.  The calcium signal activates the Oocyte from meiotic arrest.

** A unique '''phospholipase Cζ (zeta)''' causes release of calcium from the endoplasmic reticulem to the cytosol.  This is then re-sequestered and bound in the endoplasmic reticulem to give regular 'spikes' or oscillations of calcium that signal rapidly across the Oocyte.  The calcium signal activates the Oocyte from meiotic arrest.

** Provides the '''centriole''' contribution to initial cell division, allowing formation of the two-celled embryo.

** Provides the '''centriole''' contribution to initial cell division, allowing formation of the two-celled embryo.

''N.B: Although sperm mitochondria enters the Ooctye cytoplasm, they carry a ubiquitin signal.  They are subsequently destroyed by Ubiquiitinase and do NOT contribute to mitochondrial DNA.  Therefore mitochondial DNA is entirely from the maternal side.''

''N.B: Although sperm mitochondria enters the Ooctye cytoplasm, they carry a ubiquitin signal.  They are subsequently destroyed by Ubiquiitinase and do NOT contribute to mitochondrial DNA.  Therefore mitochondial DNA is entirely from the maternal side.''