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| − | [[Image:Fertilization histology.jpg|right|thumb|250px|<small><center> Electron Scanning Micrograph of Spermatozoa in the Uterine Horn. Copyright RVC 2008 (Courtesy of John Bredl (RVC))</center></small>]]
| + | == Fusion with the Oocyte == |
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| − | == Fusion with the Oocyte ==
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| − | * 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.
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| − | * Oocyte plasma membrane fuses with the '''equitorial segment''' and the fertilizing spermatozoon is engulfed.
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| − | **Brought about by a fusion protein that is inactive prior to the [[Sperm in the Female Tract - Anatomy & Physiology|acrosome reaction]].
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| − | * Nucleus of the spermatozoon is within the oocyte cytoplasm.
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| − | * Sperm nuclear membrane disappears.
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| − | * Sperm '''nucleus decondenses'''.
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| | == 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.
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| − | * Cortical granules consist of:
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| − | ** Mucopolysaccharides
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| − | ** Proteases
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| − | ** Peroxidase
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| − | * Cortical granules undergo '''exocytosis''', releasing their contents into the '''perivitteline space'''.
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| − | * Contents of cortical granules '''cross-links zona proteins''' to make them impenetrable to further spermatozoa.
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| − | * This is known as the '''zona block''', it prevents polyspermy (fertilization by more than one sperm)which would result in embryo death.
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| − | * The cortical reaction also reduces the ability of the oocyte plasma membrane to fuse with additional spermatozoa.
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| − | * This is the '''vitelline block to polyspermy'''.
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| − | * 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.
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| − | * Thus, the block depends on limiting the number of Spermatozoa in the vicinity of the Oocyte.
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| − | * 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.
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| | == Formation of Pronuclei == | | == Formation of Pronuclei == |
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| − | After the sperm nucleus enters the Oocyte it becomes the '''male pronucleus'''. Before this can occur, the sperm nucleus must undergo marked changes within the Oocyte cytoplasm.
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| − | * Disulphide cross-links render the nucleus almost inert. This is important when exposed to the hostile environment of the female tract during transport and also on penetration through the Zona Pellucida.
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| − | * In the Oocyte cytoplasm, the nucleus must '''decondense''' in order for male and female chromosomes to pair.
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| − | * Disulphide cross-links are rapidly reduced in the Oocyte cytoplasm primarily by '''Glutathione'''.
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| − | == Syngamy ==
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| − | * Male and female pronuclei fuse.
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| − | * Sperm delivers a haploid complement of chromosomes, but also activates the Oocyte by:
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| − | ** 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.
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| − | ** Provides the '''centriole''' contribution to initial cell division, allowing formation of the two-celled embryo.
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| − | ''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.''
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| − | [[Category:Pregnancy and Parturition]]
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| − | [[Category:Bullet Points]]
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