Fertilization in Humans - Practice Questions & MCQ

• The process of union of a haploid male gamete or sperm with a haploid female gamete or ovum to form a diploid cell, the zygote, is called fertilization.
• Site of Fertilization:
  • In humans, fertilization is internal as in other mammals. 
  • It takes place usually in the ampullary-isthmic junction of the fallopian tube.
• Process of Fertilization:
  • Male discharges the semen into the vagina of the female during copulation (coitus). 
  • From the vagina, the sperm reach the ampulla partly by the movement of their tails and partly by the action of the uterus. 
  • The sperm can survive in the female’s reproductive tract for 1 to 3 days and it can fertilize the ovum in 12 to 24 hours following ovulation. 
  • During sexual intercourse, nearly 300 million sperms are introduced into the vagina, but only a few hundreds of them reach near the ovum.
  • The secondary oocyte is released in the process of ovulation. 
  • The second meiotic division in the secondary oocyte is completed with the entry of the sperm.
  • The secondary oocyte now becomes ovum. 
  • The secretions of the female genital tract remove coating substances deposited on the surface of the sperm, particularly on the acrosome.
  • This exposes the receptor sites on the acrosome and sperm become active to penetrate the secondary oocyte. 
  • This process of sperm activation is called capacitation.
     

Acrosomal Reaction:

• The capacitated sperm undergo acrosomal reaction to release various chemicals present in the acrosome. Such chemicals are called sperm lysins.
• Sperm lysins are:
  • Hyaluronidase: It acts on the ground substance of follicles.
  • Corona penetrating enzymes: These dissolve corona radiata.
  • Zona lysin or Acrosin: It digests the zona pellucida.
• The plasma membrane of the sperm now fuses with the plasma membrane of the secondary oocyte.
• This causes depolarization of the plasma membrane of the secondary oocyte.
• It prevents polyspermy and ensures monospermy.

Cortical Reaction:

• The fusion of sperm and oocyte plasma membranes induces a cortical reaction in the oocyte.
• The cortical granules present beneath the plasma membrane of oocyte fuse with the plasma membrane and release their cortical enzymes between the plasma membrane and zona pellucida. 
• This causes hardening of the zona pellucida to prevent the entry of additional sperm.

Entry of Sperm:
A projection formed by the secondary oocyte called the fertilization cone receives the sperm.

Karyogamy or Amphimixis:

• As stated, with the entry of sperm the secondary oocyte completes its second meiotic division to produce a haploid ovum and a second polar body.
• The head of the sperm gets separated from the middle piece. 
• The nucleus of the sperm called the male pronucleus is released into the cytoplasm of the ovum.
• The male and female pronuclei fuse with each other in the process of karyogamy. 
• The fertilized ovum now becomes the zygote. 

Fertilizin-Antifertilizin Reaction:

• The sperm can fertilize an ovum only they are able to secrete the chemical hyaluronidase and possess a surface protein called antifertilizin (composed of acidic amino acid). 
• The ovum secretes a chemical named fertilizin (composed of glycoprotein = monosaccharides + amino acids). 
• The fertilizin interacts with the antifertilizin of the sperm and this interaction makes the sperm to stick to the surface of the ovum.