Fertilisation

As a result of copulation, semen containing sperms is ejaculated into the vagina. Once the semen (mixture of spermatozoa and accessory fluids) is deposited into the vaginal passage of the female, the spermatozoa start their journey through the uterus to reach the oviducts. On an average, the spermatozoa can swim several millimetres each second. Apart from this speed, they need to increase their mobility.

Spermatozoan mobility is aided by:

a) Ejaculation of the semen into the vagina itself triggers mobility

b) Muscular contractions of the wall of the uterus and the oviducts

c) Secretion of a viscous liquid from the secretory cells of the epithelial lining of oviductal mucosa

This phenomenon of sperm activation in mammals is known as capacitation, which takes about 5 - 6 hours.

The sperms migrate up through the uterus in to the fallopian tube. The fusion of the sperm nucleus with egg nucleus is called fertilisation and a zygote is formed in the fallopian tube.

In all mammals fertilisation is internal. In fishes and amphibians fertilisation is external as the egg is fertilised outside the body.

Physical and Chemical Events of Fertilisation

Approach of Sperm to Ovum

During copulation, the male inserts its erectile penis into the vagina of the female and releases about 3.5 ml of seminal fluid. The semen contains millions of sperms. This process is called insemination.

Site of Fertilization of Egg and Implantation of Embryo in a Mammal

The sperms travel a great distance through the female genital tract beset with chemical hazards in the form of strong acid secretions from the female tract. They also meet with mechanical obstacles while passing through crooked and compressed tract which often gets narrowed or occluded.

In an ejaculate of semen, the number of sperms is about 200,000,00. This high number ensures that at least a few sperms reach the fallopian tube which is the site of fertilisation. Actively motile sperms swim like tadpoles in the fluid medium at the rate of 1.5 - 3mm per minute to reach the site.

The fertilisability of human sperm in the female genital tract is of 12 - 24 hrs while they survive upto 3 days. Since the ovum is non motile and the energy content of the sperm is low, the sperm must approach the egg as quickly as possible. So it is only about 24 hours.

Penetration of Sperm

The sperm generally comes in contact with the ovum at the animal pole. A number of sperms surround the ovum, but only one of them fuses with the egg.

Fertilisation of the egg with only one sperm is known as monospermy.

Fertilisation occurs in the ampulla of the oviduct.

Process and Events during Fertilization

The acrosome of the sperm produces an enzyme hyaluronidase which acts as the sperm lysin. It dissolves the mucus and egg membranes, making the passage for the sperm nucleus into the egg cytoplasm.

Activation of Ovum

The penetration of the sperm activates the egg and initiates a series of changes in the egg cortex and in the egg cytoplasm. Ovulation in the human female occurs at the secondary oocyte stage in which meiosis - I has been completed and the first polar body has been released but second meiotic division has yet to be completed. When the sperm lysin dissolves the egg membrane, the ovum is activated to release the second polar body and begin cleavage.

Amphimixis

The fusion of the male and female nuclei is called amphimixis. This restores the original diploid condition in the zygote nucleus. Moreover it bring about the intermingling of the paternal and maternal hereditary material so that the resulting offspring receives both paternal and maternal characteristics.

The entry of the sperm invokes a chemical signal in the egg cell. This makes the egg imperviou for the entry of any more sperms, thus preventing polyspermy. The signal is transmitted to the egg surface in capacitating hundred of sperms in the vicinity of the egg cell.