Question

Short / Long Answer question Type :
What envelops the mammalian ovum preventing the entry of sperm into it easily? How does the sperm gain the entry eventually?

Answer 1

• Fertilization of the ovum occurs in the ischemic-ampullary region of the fallopian tube.
• One spermatozoon pierces the zona pellucida and enters the ovum. As soon as the spermatozoon enters the ovum the second meiotic division is completed and the second polar body is extruded.
• When a spermatozoon comes in contact with the oocyte, plasma membranes of the two cells fuse. This probably occurs at receptor sites. The head of the spermatozoon enters the cytoplasm of the ovum.
• Alterations taking place in the plasma membrane of the oocyte and in the zona pellucida ensure that no other spermatozoon can enter the oocyte.
• The nucleus of the ovum becomes the female pronucleus.
• The head of the spermatozoon separates from the middle piece and tail and transforms itself into the male pronucleus.
• Soon the pronuclei lose their nuclear membranes. The 23 chromosomes of the female pronucleus and 23 of the male pronucleus get mixed up and form 23 pair of chromosomes.
• These 46 chromosomes go through the typical mitotic division and lead to the formation of an embryo having two cells.