Question

Normal fertilization requires changes in the membrane organization of both sperm and oocyte and penetration of the oocyte by a single spermatozoon.

a. What physical, cellular and biochemical molecular processes are necessary for the succesful passage of a spem from the outermost portion of the cumulus oophorus to the oocyte membrane?

b. How does the human sperm nucleus move in the cytoplasm and why does it facilitate the juxtaposition of the material and paternal pronuclei?

c. why is the orientation of pronuclear DNA critical for embryogenesis to proceed and how does it promote pronuclear membrane breakdown (syngamy)

d. What specific defects in the above processes have been shown to result in fertilization or early developmental failure.

Answer 1

Ans a. The cumulus oophorus comprises of granulosa cells which are enriched with hylauronic acid. Cumulus cell supports fertilization. The acrosome contain proteolytic enzymes such as acrosin, trypsin, hyaluronidase and proteases. These are released during the acrosome reaction and facilitates the penetration of sperm. The sperm meets the oocyte and adheres to the oocyte. The acrosome penetrate the egg cover. It starts with the functional coupling of signal transduction pathways to regulate the initiation of acrosome reaction. The bounding is characterised by change in phosphorylation state, elevation in intracellular pH and calcium levels and hyperpolarization of membrane potential.

Ans b. Cytoplasm keeps on moving around the cell, whereas the nucleus is concentrated to a specific location. The sperm cells are located in nucleus and not in cytoplasm. During maturation the nucleus is excreted as polar bodies and the cell grows in maturation as the number of cytoplasm increases. The juxtaposition has been facilitated due to the following reasons:

1) The gametes are released freely into the surrounding medium.

2) The sperm will be easily released and drawn into the respiratory current to the eggs.

3) Naked sperm can be easily transfered to the female genital duct at copulation.

Ans c. The chromatin condensation is characterised by the condensation of H3 and H4 histone proteins and X and Y sperm specific proteins. As the nucleoplasmin has more affinity for X and Y proteins the exchange of histone with X and Y proteins occurs. The vesicles of the nuclear membrane of the sperm bounds to the vesicles of the endoplasmic reticulum of the egg. The membrane of pronucleus is formed. The nucleus completes the meiosis second division and eliminates the second polar body. After entering the nucleus, oocyte and sperm centrosome rotates at 180 degree such that the centrosome is placed between the nucleus of the sperm and egg. In order to maintain this position the pronuclear fusion is facilitated.