Generally only one sperm enters into a
egg, this phenomenon is known as Monospermy. Rarely more than one sperm
enters in one egg, this phenomenon is known as polyspermy. Podyspermy is
of two types —
(a) Pathological Polyspermy— All the nucleus of sperms fuse with nucleus of egg. Egg degenerate and embryo do not develop.
(b) Physiological polyspermy— Nucleus of only one sperm fuse with nucleus of the egg and all the other sperms dies in cytoplasm of egg. The dead sperms in cytoplasm of egg are known as "merocyte". It is commonly found in polylecithal eggs of Reptiles, Aves, Prototheria etc.
Just after entry sperm rotates at 180o. Thus middle part of sperm comes in anterior position and head becomes posterior in position.
There are often two blocks to polyspermy. The fast block is electrical and is mediated by
sodium ions: the egg membrane resting potential rises, and sperm can no longer fuse with the egg.
The slow block is physical and is mediated by calcium ions. A wave of calcium ions propagates
from the point of sperm entry, causing the cortical granules to fuse with the egg cell membrane.
The released contents of the granules cause the vitelline membrane to rise and to harden into the
fertilization envelope. In mammals, blocks to polyspermy include the modification of the zona proteins by the
contents of the cortical granules. Sperm can no longer bind to the zona. Inositol 1,4,5-triphosphate (IP3) is believed to be responsible for releasing calcium ions from
storage in the endoplasmic reticulum. DAG (diacylglycerol) is thought to initiate the rise in egg
pH. The free calcium ions, supported by the alkalization of the egg, activate egg metabolism,
protein synthesis, and DNA synthesis.
Sperm and egg nuclei moves in egg cytoplasm, these are called as path.
(i) Sperm penetration path : The movement of sperm nucleus in a straight line from entry site.
(ii) Sperm copulation path - The movement of sperm nucleus towards egg nucleus.
(iii) Egg copulation path -The movement of egg nucleus towards sperm nucleus.
(iv) Cleavage path - The movement of both pronucleus towards animal pole.
Proximal centriole is given to egg cell. Sperms diastal centriole aquires opposite pole position in egg and start to develop spindle fibres, as the mammalian egg lacks centrioles of its own. The centrioles of egg degenerates during second maturation division. The nucleus of sperm is enlarged by absorbing water from cytoplasm of egg. Now this nucleus of sperm is known as male pronucleus.
Egg nucleus also slightly enlarges and now called as female pronucleus, both pronuclei fuses & moves by cleavage path.
The nuclear membranes of both pronucleus degenerate and chromosomes form pairs with each others. With this fusion the amphimixis is completed.
(a) Pathological Polyspermy— All the nucleus of sperms fuse with nucleus of egg. Egg degenerate and embryo do not develop.
(b) Physiological polyspermy— Nucleus of only one sperm fuse with nucleus of the egg and all the other sperms dies in cytoplasm of egg. The dead sperms in cytoplasm of egg are known as "merocyte". It is commonly found in polylecithal eggs of Reptiles, Aves, Prototheria etc.
Just after entry sperm rotates at 180o. Thus middle part of sperm comes in anterior position and head becomes posterior in position.
sodium ions: the egg membrane resting potential rises, and sperm can no longer fuse with the egg.
The slow block is physical and is mediated by calcium ions. A wave of calcium ions propagates
from the point of sperm entry, causing the cortical granules to fuse with the egg cell membrane.
The released contents of the granules cause the vitelline membrane to rise and to harden into the
fertilization envelope. In mammals, blocks to polyspermy include the modification of the zona proteins by the
contents of the cortical granules. Sperm can no longer bind to the zona. Inositol 1,4,5-triphosphate (IP3) is believed to be responsible for releasing calcium ions from
storage in the endoplasmic reticulum. DAG (diacylglycerol) is thought to initiate the rise in egg
pH. The free calcium ions, supported by the alkalization of the egg, activate egg metabolism,
protein synthesis, and DNA synthesis.
Sperm and egg nuclei moves in egg cytoplasm, these are called as path.
(i) Sperm penetration path : The movement of sperm nucleus in a straight line from entry site.
(ii) Sperm copulation path - The movement of sperm nucleus towards egg nucleus.
(iii) Egg copulation path -The movement of egg nucleus towards sperm nucleus.
(iv) Cleavage path - The movement of both pronucleus towards animal pole.
Proximal centriole is given to egg cell. Sperms diastal centriole aquires opposite pole position in egg and start to develop spindle fibres, as the mammalian egg lacks centrioles of its own. The centrioles of egg degenerates during second maturation division. The nucleus of sperm is enlarged by absorbing water from cytoplasm of egg. Now this nucleus of sperm is known as male pronucleus.
Egg nucleus also slightly enlarges and now called as female pronucleus, both pronuclei fuses & moves by cleavage path.
The nuclear membranes of both pronucleus degenerate and chromosomes form pairs with each others. With this fusion the amphimixis is completed.
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