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The process of union of two gametes is known as fertilisation. During fertilisation, sperm and egg fuse and give rise to a diploid zygote. The fertilisation process is most studied in marine invertebrates sea urchins. They undergo external fertilisation in water, which contains many sperm and eggs.
External fertilisation is more easily learned than the internal fertilisation of mammals. In plants, pollination is known as the process of sexual reproduction. The flowers have all the necessary structures needed for sexual reproduction. Here, we review fertilisation biology, with topics involving the attachment and fusion of the gametes.
Gametes
Gametes are the sex cells or reproductive cells which are utilised during the process of fertilisation to form a new individual. These are haploid, and each gamete carries one copy of the chromosome.
The male gamete is known as sperm which has either X or Y chromosome, while the female gamete is known as ova or egg with X chromosome as the sex chromosome. The process of meiosis forms the gametes.
Process of gamete formation
Meiosis is the two-step process of forming gametes in the male and female reproductive organs. Two meiotic division results in the shape of four haploid daughter cells, each of which carries only a single set of chromosomes. All gametes are found to be genetically different from each other.
Examples of gametes
Sperm and ova are examples of gametes. Both male and female gametes are produced from germ cells due to “gametogenesis” within their respective reproductive organs. The gametogenesis for both the gametes are different. The process of formation of sperm is known as “spermatogenesis”, whereas the formation of female ova or egg is known as “oogenesis”.
Sperm cells
Sperm is the male reproductive cell. It is relatively more minor in size than the female gamete and is motile. The sperm cell is unable to divide and has a limited life span. It is divided into head and tail, surrounded by a single membrane.
Head: head contains the haploid nucleus with packed DNA. The director also includes a flattened structure known as an acrosome which contains necessary enzymes.
Middle piece: the portion which connects the tail and head is the central piece with mitochondria. It provides the energy necessary for sperm movement.
Tail: the tail is the longest part, which helps in sperm penetration. It has a length of 50µm.
The sperm is served to reach and fertilise the egg to form a zygote. It has two types of chromosomes, X and Y chromosomes, which are responsible for the sex of the progeny.
Ovum (egg cells)
The egg cells are oval or spherical in shape, formed in the female reproductive organs, ovaries. The egg cells are more significant and non-motile than the sperm cells. The egg of the human has an average diameter of about 0.1 mm. Ostrich egg is a giant egg with 170×135 mm diameter. The egg is protected into a thick membrane, zona pellucida.
The primary function of the ovum is to carry the female genetic material and serve the environment for the fusion of male and female gamete when fertilisation occurs. It contains only one type of sex chromosome, the X chromosome.
Fertilisation
The cell recognition process, which involves the fusion of male and female gametes to form a zygote, is called fertilisation.
It involves four stages-
- Sperm activation and acrosome reaction: Ejaculated sperm undergoes various changes or capacitation, which leads to the activation of the sperm. The spermatic motility is also enhanced in this stage.
- Sperm egg recognition and binding: The acrosomal contents cause Na+ influx and H+ efflux, increasing pH. It causes the penetration of egg coating and results in the binding of egg plasma membrane and sperm. The acrosome fuses with egg plasma, and the contents get released in this stage.
- Fusion: The sperm is drawn into the egg with the help of the egg’s microvilli. The nucleus and other sperm organelles get incorporated into the egg. The sperm nucleus is followed by various changes which produce the male pronucleus. This male pronucleus with functionalized microtubules reaches the female pronucleus to form a diploid nucleus.
- Activation of the zygote: The next egg’s effort is to protect polyspermy through cortical reactions. Polyspermy is when the egg gets fused with more than one sperm. After uniting gametes, the zygote is formed and undergoes various development stages.
Conclusion
Fertilisation produces a cell (zygote) capable of cell division and forming a new person. The egg undergoes various reactions as a result of the fusing of two gametes. One of them produces a change in the egg membrane(s), preventing many spermatozoids from attaching and penetrating the egg.
