Question

How is the body of a sperm suited for fertilisation with an egg?

Answer 1

Hint: Sperm is a uni-flagellated male gamete consisting of the main four parts- head, neck, middle piece and tail. Fertilisation is the process of fusion of male and female gametes to form a diploid zygote.

Complete answer:
Human sperm has four parts. The head of sperm is broad and flattened consisting of two parts: nuclear and a cap-like acrosome. Acrosome is derived from the Golgi complex of the spermatid and contains proteolytic and lysosomal enzymes. Neck possesses two centrioles. Cytoplasm is present. Middle piece is a long cylindrical part of sperm which lies between the neck and tail. A sheath of cytoplasm and plasmalemma lie on the outside. Mitochondrial activity helps in providing energy. Tail is the longest and vibratile part of sperm. When a sperm comes in contact with an egg, following events occur.

Acrosomal reaction- When the head of the sperm comes in contact with the surface of egg, the acrosome releases its hydrolytic enzymes, also called sperm lysins. Hyaluronidase dissolves the cementing substance of corona cells of egg, resulting in their separation. As the sperm head reaches the zona pellucida, a compatibility reaction occurs which stimulates development of an outgrowth by the oocyte called fertilisation cone or cone of reception.

• Sperm Entry - Sperm head comes in contact with the fertilisation cone. The membranes of sperm and egg dissolve in the region of contact. Components of the head (nucleus), neck and middle piece of sperm enter the cytoplasm of the egg. Tail is left out.
• Cortical Reaction - Fertilisation membrane does not allow entry of second sperm.
• Zona Reaction - Chemicals extruded from the egg act on zona pellucida and destroy the sperm receptors preventing attachment to any other sperm.

Note: The phenomenon by which a single sperm fertilises an egg is called monospermy. Fertilisation of egg by more than one sperm or polyspermy is prevented by cortical reaction and zona reaction.