Question

Polyspermy is normally prevented by

• A. The fertilizin and antifertilizin reaction.
• B. Repulsion of excess number of sperm by ova.
• C. Inability of some sperm to penetrate ova.
• D. Formation of fertilization membrane.

Answer 1

The correct option is D Formation of fertilization membrane.

Fertilization of the egg by more than one sperm, a condition known as polyspermy, results in an offspring with extra sets of chromosomes, which is usually lethal. Two reactions, known as the "fast" and "slow blocks" to polyspermy, prevent this occurrence. In the "fast block" to polyspermy, the egg plasma membrane depolarizes, which prevents its fusion with additional sperm. An unfertilized egg is polarized, that is, the cytoplasm is negatively charged relative to the outside. However, within a few seconds after sperm fusion, ion channels in the plasma membrane open, permitting positively charged calcium ions to diffuse across the membrane and depolarize the egg.
The "slow block" to polyspermy, the cortical reaction, requires up to several minutes to complete, but it is a complete block. Binding of the sperm to receptors on the vitelline envelope activates one or more signal transduction pathways in the egg. These events cause calcium ions stored in the egg endoplasmic reticulum to be released into the cytosol. This rise in the level of intracellular calcium causes thousands of cortical granules, membrane-enclosed vesicles in the egg cortex, to release enzymes, various proteins, and other substances by exocytosis into the space between the plasma membrane and the vitelline envelope. Some of the enzymes dissolve the protein linking the two membranes; thus, the space enlarges as additional substances released by the cortical granules raise the osmotic pressure, which causes an influx of water from the surroundings. Thus, the vitelline envelope becomes elevated away from the plasma membrane and forms the fertilization envelope, a hardened covering that prevents entry of additional sperm.