Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college
Page | 1 DSE-1BT: Developmental aspects of embryo
DSE-1BT: Developmental aspects of embryo
Gametogenesis:
Fertilization of mammals
1) Sperm-egg interactions:
a) Fertilization of the primary oocyte occurs in the oviduct. The oocyte is captured by the fimbria (funnel-shaped end) and swept down by the action of cilia in the oviduct lining. Fertility is maintained only for about 24 hours whereas the sperm can live about 2 days in the tract. Sperm must first be capacitated by exposure to the reproductive tract before they can undergo the acrosome reaction.
b) Capacitated sperm must move past the follicle cells and then the zona pellucida contact induces the acrosome reaction. The glycoprotein ZP3 and specifically the N-acetylglucosamine sugar on this protein binds to the sperm in a species-specific manner. ZP3 is linked with ZP2 in a fibrous matrix and cross-links are made with ZP1.
2) Fusion and cortical reaction:
Fusion of the membranes:
In the mouse fertilizin is a protein on the sperm membrane that contains hydrophobic amino acids that may destabilize the egg and sperm membranes so that they can fuse. Fertilizin binds to an integrin of the egg plasma membrane.
The sperm nucleus becomes the pronucleus and the proximal centriole organizes a centrosome in the egg.
A temporary mosaic of sperm and egg plasma membranes exists.
Mitochondria get into the egg cytoplasm but are later lost (by unknown process for unknown reasons).
Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college
Page | 2 DSE-1BT: Developmental aspects of embryo
Block to polyspermy:
1) Polyspermy is a disastrous condition whereby more than one sperm enter.
The embryo will be polyploid and the extra centriole will set up another set of cleavage planes.
2) Slow block to polyspermy through the cortical granule reaction. Within one minute of fertilization the cortical granules are exocytosed to release their contents into the space between the plasma membrane and the vitelline membrane. The cortical granule contents include a glycosidase which clips off the carbohydrate of the ZP3 used for sperm binding. This expels other attached sperm.
Figure: fertilization
