Give a brief account of the mechanism of fertilization in mammals (PDF).
Ans: Fertilization is the process by which male and female gametes unite to form a zygote. Fertilization in mammals can be described as follows:
(i) Encounter of spermatozoa with ovum:
In terrestrial mammals the male deposits sperms directly into the reproductive tract of the females. In some cases the ovum secretes chemical substances which attract the sperms of similar species.
(ii) Contact:
For successful fertilization a firm contact must be established between the sperm and the ovum. This is explained by the formation of fertilizirs antifertilizer complex formed by chemical substances produced by egg and sperm respectively.
(iii) Acrosomal reaction:
A series of changes called the acrosomal reaction takes place when the egg comes in contact with the sperm. They are —
(a) The acrosomal gramules come in contact with the egg’s vitelline memberance when the plasmalemma of both egg and sperm head fuse together and the membrane of acrosome fuses to release the acrosomal granules.
(b) The lytic enzyme in acrosomal gsanules lyse the plasma membrame of egg.
(c) The acrosomal membrance produces acrosomal tubules in the form of projections which penetrate the egg envelop.
(d) These acrosomal tubules are then everted and joined to form the single tubule that penetralo the egg envelop.
(e) The plasma membrane of egg produces several figure like ville.
(f) The membranc of the aerosomal tubules and the egg membrate fuse at the point of contact and a continuous connection is formed between sperm and egg cytoplasms. The sperm nucleus and centrole pass into the egg cytoplaim.
(iv) Activation of Ovum:
(a) When the acrosomal tubules penetrate the egg membrane the egg cytoplasm bulges to form a fertilization cone which gradually engulfs the sperm.
(b) The cortical granules after didintegration of egg memberane, liquify and migrate to the space between vitellion and egg membrane. The cortical granules form fertilization membrane to present polyspermy.
(v) Migration of pro-nuclei and amphimixis:
The nuclie of the sperm and egg before fusion is called the pro-nuclei. They migrate forwards each other and fusion takes place. This process, called amphimixis marks the end of fertilization. In mammals fertilization is completed in the fallopian tubule.