Fertilization starts just after the transfer of gametes takes place and ends with the formation of zygote, which then differentiates to form your physical self. It is not just the fusion of the nucleus of a sperm with that of an ovum, but a sequence of events that takes place even before the sperms reach the ovum. In humans, it usually occurrs in the ampulla of the fallopian tube of the ♀ reproductive tract.

The Gametes

The Sperm

In testis, the immature male germ cells (spermatogonia) produce sperms by the process of spermatogenesis that begins at puberty. The structure of a typical sperm is given as a diagram.

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Image courtesy: NCERT

A plasma membrane envelops the whole body of sperm. The sperm head contains an elongated haploid nucleus, the anterior portion of which is covered by a cap-like structure, acrosome. The acrosome is filled with hydrolytic enzymes that help fertilization of the ovum. The middle piece possesses numerous mitochondria, which produce energy for the movement of tail that facilitate sperm motility in the  essential for fertilization.

The Ovum

Ovum refers to secondary oocyte stage of oogenesis, where the second meiotic division is yet to occur. In this condition the matured ovum is released from ovary and enters into uterus in mammals for fertilization. In mammals including man the ovum is discharged from the Graafian follicle (during ovulation) with one polar body.

The cytoplasm of the egg is called ooplasm. It contains a very little amount of yolk in man and therefore it is alecithal. The peripheral layer of ooplasm is known as cortex and it contains many microvilli and cortical granules. The nucleus is large and centrally placed in mammals but towards the animal pole in yolky eggs.

In humans, the ovum is covered over by a thin vitelline membrane which is further covered over by another primary membrane known as zona pellucida. There is a narrow space between these two membranes known as perivitelline space. During discharge of ovum from the ovary, several layers of epithelial cells adhere to the outer surface of zona pellucida and are arranged radially forming corona radiata.

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The Events

Capacitation

Capacitation is a process of physiological maturation or conditioning of the sperms to hyperactivate them, or make them able to fertilize the ovum. This event takes place in the ♀ reproductive tract and can take time, usually ranging from six to seven hours. It involves many changes in the physiology of the sperm.

  1. Cholesterol is removed from the sperm body to reduce the stability of the plasma membrane enclosing the sperm.
  2. As a result of removal of cholesterol, the membrane becomes more permeable for the passage of Ca2+ ions.
  3. This is followed by the dilution of decapacitation factors, which ensure the fertilizing ability of the sperms before they enter the ♀ reproductive tract.
  4. The undulating i.e. smooth wave-like motions of the sperm tail are converted into rapid, whiplashing movements.

Agglutination

Agglutination is referred to a condition in which the head or tail of one sperm sticks to another sperm restricting the motility of the sperm. This mechanism allows species-specific fusion of gametes, which means that sperms of a only a specific species would be able to fuse with the ovum.

This is facilitated by the Fertilizin-antifertilizin reaction which is chemo-attractive in nature. Fertilizin is a glycoprotein secreted by the membrane of the ♀ gamete. On the other hand, anti-fertilizin is an acidic protein that is secreted by the ♂ gamete. This reaction checks for compatibility of the gametes.

Acrosome Reaction

This process helps the sperm to penetrate the egg membrane by the action of hydrolytic enzymes, present on the sperm head. Different enzymes are accountable for specific reactions.

  1. Hyalouronidase: This dissolves the extracellular matrix of the membrane.
  2. Corona penetrating enzymes: These help the sperm penetrate the corona radiata of the egg.
  3. Zonalysin/Acrolysin: These help the sperm penetrate the zona pellucida of the ♀gametophyte.

It is a fact to be noted that in case of fertilization in Homo sapiens, the whole sperm enters the egg, while in other species the head is able to enter the same.

Block to Polyspermy & Activation of 2° Oocyte

Polyspermy describes an egg that has been fertilized by more than one sperm. Diploid organisms normally contain two copies of each chromosome, one from each parent. But a cell resulting from polyspermy, on the other hand, will contain three or more copies of each chromosome. Usually, the result is an inviable zygote. This may occur because sperm are too efficient at reaching and fertilizing eggs due to the selective pressures of sperm competition. Such a situation is often detrimental to the ♀ gametophyte.

To avoid such a situation from arising, we have developed a set of reactions that ensure the entry of a single sperm into the ♀ gamete. This comprises the fast block and the slow block to polyspermy.

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  1. Fast Block to Polyspermy: As the sperm head comes in contact with the egg membrane, it stimulates the influx of Na+ ions (sperm head carries +ve charge and egg membrane carries -ve charge). This leads to the depolarization of the egg membrane. This event is referred to as the ‘fast block to polyspermy.
  2. Slow Block to Polyspermy:
    • As the sperm enter the ooplasm, it triggers the release of  Ca2+ ions from the endoplasmic reticulum. This in turn, stimulates the exocytosis of cortical granules into the perivitelline space. This is referred to as the cortical reaction.
    • Secretion of cortical granules block the sperm receptors ZP2 and ZP3 on the zona pellucida, making it hard. The membrane at this stage is known as the fertilization membrane. This is the zona reaction.
    • The cortical reaction and zona reaction comprise the mechanism of ‘slow block to polyspermy.

When the sperm enters the ooplasm, it also releases fertilin which triggers the secondary oocyte to complete the second meiotic divison by breaking down the metaphase promoting factors (MPF) and turning on the anaphase promoting complex (APC). This is because the secondary oocytes get arrested in metaphase before completing Meiosis II and can complete it only when the sperm enters the oocyte.

Syngamy

Syngamy or amphimixis is the last event in fertilization and refers to the process of fusion or union of the pronuclei of the haploid ♂ and ♀ gametes. This results in a diploid structure known as zygote or synkaryon

Syngamy is a significant event due to the following reasons:

  1. It restores diploidy in the life cycle of the organism (as gametes were haploid).
  2. It is responsible for the introduction of variations in a population.bio2
  3. It is the sex-determining process of reproduction. Why ? A few things to keep in mind are:
    • Sex chromosome Y is the sex-determining factor to form a male foetus.
    • Barr-bodies or inactivated X chromosomes are seen if two X chromosomes are present, that is, in a female foetus.

Syngamy initiates the mitotic cleavage, which continues till implantation on the wall of uterus. This implanted blastocyst (zygote after a few divisions) forms connections with the mother and you slowly, come into being.

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