Introduction:

Fertilization is a fusion of gametes to generate a new organism. It is also called as conception, fecundation and syngamy. In animals, fertilization is a fusion of the sperm cell with egg cell. Penetration of egg cell by chromosome-containing part of sperm cell causes the reaction that prevents extra sperm cells from entering egg. Egg and sperm each contribute half of the new organism's genetic material. The fertilized egg cell is called as a zygote. Zygote goes through continuous cell division that ultimately generates a new multicellular organism. Fertilization in humans takes place in oviducts (fallopian tubes) of female reproductive tract and occurs within hours following sexual intercourse. Only one of the about 300 million sperm released in female's vagina during intercourse can fertilize single female egg cell. Successful sperm cell should enter uterus and swim up fallopian tube to meet the egg cell, where it passes by thick coating surrounding the egg. This coating, comprising of sugars and proteins, is called as the zona pellucida. Tip of the head of sperm cell has enzymes that break through zona pellucida and help penetration of sperm in egg. Once the head of the sperm is inside egg, tail of the sperm falls off, and perimeter of egg thickens to prevent another sperm from entering. Sperm and egg each have only half the normal number of chromosomes, a situation called as haploid. When genetic material of two cells fuses, fertilization is complete.

In humans, numerous variables influence whether or not fertilization takes place following intercourse. One factor is the woman's ovulatory cycle. Human eggs can only be fertilized a few days after ovulation that generally takes place only once every 28 days. In other species, fertilization takes place either internally or externally, depending on species involved.

Types of Fertilization:

There two methods of fertilization; external and internal fertilization.

1) External Fertilization:

External fertilization takes place generally in wet environments and needs both male and female to release their gametes in their surroundings (generally water). The benefit of external fertilization is that it results in production of the large number of offspring. One drawback is that environmental hazards like predators greatly decrease possibility of surviving in adulthood.

Whole procedure of development of new individuals is known as procreation, act of species reproduction. Consideration as to whether the animal (more particularly a vertebrate) utilizes internal or external fertilization is frequently dependent on method of birth. Fertilization outside of animal's body takes place in aquatic animals like sea urchins, fish, and frogs. In sea urchins, numerous billion sperm are released in water and swim towards eggs released in same area, in most of fish and in several amphibians, eggs and sperms are released in water around animals and fertilization occurs there. When fish are about to reproduce, males and females swim close together.

Oviparous animals laying eggs having thick calcium shells, like chickens, or thick leathery shells generally reproduce through internal fertilization so that sperm fertilize the egg without having to pass through thick, protective, tertiary layer of egg. Ovoviviparous and euviviparous animals also utilize internal fertilization. It is significant to note that though some organisms reproduce via amplexus, they may still use internal fertilization, as with some salamanders. Benefits to internal fertilization comprise: least waste of gametes; greater possibility of individual egg fertilization, comparatively longer time period of egg protection, and selective fertilization; several females have skill to store sperm for extended periods of time and can fertilize the eggs at their own wish, negligible contact and transmission of bodily fluids; decreasing risk of disease transmission, and greater genetic variation (particularly during broadcast spawning external fertilisation methods).

Aquatic Animals with External fertilization:

i) The sea urchin:

Like most aquatic animals, sea urchin sheds the gametes in water and fertilization is external. Gametes are generated by meiosis. Production of sperm by male (spermatogenesis) involves usual meiotic divisions to generate haploid daughter cells. Though, these cells aren't sperm until they go through morphological changes.

Sperm are attained using same procedure, except that milky sperm suspension is gathered from urchin's body surface and diluted in the small amount of sea water. Before fertilization, vitelline membrane sticks to egg surface and is not visible as the separate structure. Fertilization membrane quickly hardens to give protective covering for developing embryo. Rising fertilization membrane also pushes other sperm away from egg and forms the permanent block to polyspermy. The sperm of one species cannot generally fertilize eggs of another species. To complete fertilization procedure, nucleus of sperm and egg should fuse. Compacted DNA inside sperm nucleus expands and can be observed inside egg. Fusing nuclei are known as pronuclei and resulting diploid cell is known as zygote.

ii) Frog and Zebrafish eggs:

Several aquatic species produce eggs containing more yolk than the sea urchin. Thus these eggs are much larger, comparative to size of the adult animal. The yolk is asymmetrically distributed inside these eggs, while in sea urchin egg the relative small amount of yolk is present as small granules all through the egg cytoplasm.

Frog eggs are fairly large, approx 1 mm in diameter. The brown part of egg is mostly cytoplasm and yellow part is chiefly yolk. Frog eggs have comparatively more yolk than sea urchin eggs, and frog embryo develops longer before hatching. Therefore newly hatched larva of frog embryo is more mature than that of the sea urchin. Frog egg is enclosed by vitelline membrane that lies so close to egg surface that it is not visible as separate structure. This zebrafish egg is physically smaller than frog egg and should be observed under microscope. Though, adult fish is smaller than frog, so egg is really larger than frog egg comparative to animal size. Like all fish eggs, it has an enormous amount of yolk. With so much yolk, embryo develops in mature-looking larva before hatching. Covering around the egg is known as a chorion but is analogous to vitelline membrane of other species.

2) Internal Fertilization:

Animals which use internal fertilization specialize in protection of developing egg. For instance, reptiles and birds secrete eggs which are covered by the protective shell which is resistant to water loss and damage. Mammals, with exception of monotremes, take thought of protection step further by permitting embryo to develop inside mother. This additional protection increases probability of survival as mom supplies everything that embryo requires. In fact, most mammalian mothers carry on to care for their young for numerous years after birth.

Mammals and internal fertilization:

Gametogenesis in mammals is exclusive in that resulting eggs don't have yolk. Therefore, they are smaller than eggs of most other animals. As mammals live on land, gametes can't be shed in water, so fertilization is internal. As compared to external fertilization, comparatively small number of sperm arrives at location of egg, so block to polyspermy is less robust and slower to take effect.

Gametes:

While all sperm have similar basic structure, there are frequently differentiating features in different species. This light micrograph illustrates human egg. It is typical of mammalian eggs and has no yolk. The initial meiotic division has been finished and first polar body is visible. As in several mammalian species, second meiotic division will not take place until egg is fertilized. Covering which surrounds egg and polar body is known as zona pellucida. It bears sperm binding sites and is analogous to vitelline membrane of non-mammalian eggs.

Fertilization in mammals takes place internally, as is typical of terrestrial animals. Sperm uses acrosomal reaction to penetrate through zona pellucida and is then pulled in egg cytoplasm. After second meiotic division is complete, sperm and egg pronuclei fuse as in all animal species.

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