Question

1.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

a.) 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 :

Capacitation-

1.The sperm cell membrane is altered by the removal of cholesterol by albumin proteins this changes the position of lipid rafts hich now cluster over the anterior sperm head. These lipid microdomains contain proteins that can bind the zona pellucida.

2. Protein phosphorylation occurs- two chaperone (heat-shock) proteins migrate to the surface of the sperm head when they are phosphorylated. Here, they may play an essential role in forming the receptor that binds to the zona pellucida.

3. The membrane potential of the sperm cell membrane becomes more negative as potassium ions leave the sperm (hyperpolarization). This change in membrane potential allows calcium ion influx into the sperm The calcium ions are necessary for acrosome reaction.

4. The outer acrosomal membrane comes in contact with the sperm cell membrane for fusion.

Hyperactivation-

5. The opening of calcium channels increase the motility of the sperm. The sperm now beat their flagella with more power in a synchronous manner leading to directional movement.

6.Chemotactic substances secretes by the cumulus cells provide direction to the sperm. The sperms also do thermostat is and move towards warmer regions to approach the zona pellucida of the egg.

7. An oligosaccharide present on the outer membrane of the sperm binds to ZP3 present on zona pellucida.

8. The outer membrane and inner membrane of the sperm fuses at multiple sites releasing acrosomal contents. These acrosomal vesicles contain pre-acrosin. Pre-acrosin binds to ZP2 and gets activated to form acrosonin enzyme. It digests Ona pellucida and the sperm bind and fuses to the oocyte membrane.

b.) The human sperm enters the egg almost tangentially. As the sperm binds to the oocyte membrane, actin polymerization and microvilli extension in initiated. These microvilli from the egg membrane extend to form a cone like structure known as fertilization cone. The centriole contributed by the sperm polymerises microtubules and pushes the sperm pronucleus. This places the oocyte nucleus is at 180 degrees with respect to the entry of the sperm nucleus. The microtubule organizing centre (centrosome) is placed in between the two nuclei along the same axis. The microtubules from cetrosome bind to the two nuclei and pull them together for fusion. In this way juxtaposition of the two nuclei is facilitated.

c.) The orientation of pronuclear DNA allows the two nuclei to be pulled towards each other by microtubules. In this way the two nuclei come in contact with each other resulting in membrane breakdown and fusion.

The orientation of pronuclear DNA is critical in embryogenesis as it decides the plane of the first mitotic cleavage in the embryo. It also marks the level of lateral symmetry in the embryo.

D.) Defects that lead to fertilization or developmental failure:

1. Defects in the expression of receptors in the oocyte cell membrane (CD9) or ZP2 lead to failure of sperm entry.

2. Defects in Calcium ion channels or calcium pumps inhibit hyperpolarization and acrosomal reaction leasileto failure of susion of sperm and egg.

3. Improper pronuclear orientation cause aneuploid embryos.