Question

Describe the physicochemical events that take place during fertilization in humans.

Answer 1

Physical and Chemical Events of Fertilization :

These events include the following ‘ processes.

(i) Acrosomal Reaction: After ovulation, the secondary oocyte reaches the Fallopian tube (oviduct). The capacitated sperms undergo an acrosomal reaction and release various chemicals contained in the acrosome. These chemicals are collectively called sperm Iysins. Important sperm lysins are

(i) hyaluronidase that acts on the ground substances of follicle cells,
(ii) corona penetrating enzyme that dissolves corona radiata and
(iii) zona lysine or acrosin that helps to digest the zona pellucida.
optimumpH, Ca⁺⁺, Mg⁺⁺ ions concentration and temperature are essential for acrosomal reaction. Ca⁺⁺ plays a major role in the acrosomal reaction. In the absence of Ca⁺⁺, fertilization does not occur.

Due to the acrosomal reaction, the plasma membrane of the sperm fuses with the plasma membrane of the secondary oocyte so that the sperm contents enter the oocyte. Binding of the sperm to the secondary oocyte induces depolarization of the oocyte plasma membrane. Depolarization prevents polyspermy (entry of more than one sperm into the oocyte). It ensures monospermy (entry of one sperm into the oocyte).

(ii) Cortical Reaction: Just after the fusion of sperm and plasma membranes of an oocyte, the secondary oocyte shows a cortical reaction. The cortical granules are present beneath the plasma membrane of an oocyte, and release their contents including cortical enzymes between the plasma membrane and the zona pellucida. These enzymes harden the zona pellucida which also prevents entry of additional sperms (polyspermy).

(iii) Sperm Entry: At the point of contact with the sperms, the secondary oocyte forms a projection termed the cone of reception or fertilization cone which receives the sperm. The distal centriole of the sperm divides and forms two centrioles to generate the mitotic spindle formation f pr cell division. The mammalian secondary oocyte (egg) does not have centrioles of its own.

(iv) Karyogamy (Amphimixis): Sperm entry stimulates the secondary oocyte to complete the suspended second meiotic division. This produces a haploid mature ovum and a second polar body. The head of the sperm which contains the nucleus separates from the middle piece and the tail and becomes the male pronucleus. The second polar body and the sperm tail degenerate. The nucleus of the ovum is now called the female pronucleus. The male and female pronuclei move towards each other. Their nuclear membranes disintegrate. Mixing up of the chromosomes of a sperm and an ovum is known as karyogamy or amphimixis. The fertilized ovum (egg) is now called a zygote. The zygote is a diploid unicellular cell that has 46 chromosomes in humans. The mother is now said to be pregnant.

(v) Activation of Egg: Sperm entry stimulates metabolism in the zygote. As a result, the rates of cellular respiration and protein synthesis increase greatly. Besides activating the egg another role of sperm is to carry DNA to egg.