In this article we will discuss about Fertilisation in Animals:- 1. Meaning of Fertilisation 2. Process of Fertilisation 3. Kinds 4. Significance.
Contents
Meaning of Fertilisation:
The union of the cytoplasm and pronuclei of the male and female gametes is known as the fertilisation (L., fertilis = to bear; frevo = to bear). The fertilisation is the most commonly used method for the production of the diploid zygotes in the sexually reproducing organisms of Metazoa and Metaphyta.
In the process of fertilisation, the haploid male gamete (spermatozoon or pollen), which carries the paternal genetic information of the male parent, unites with the haploid female gamete (ovum or egg), which carries the genetic information of female parent, to form a diploid zygote.
The egg carries the maternal hereditary information’s in it. The zygote ultimately produces a diploid multicellular organism by the several repeated and organised mitotic divisions and cellular differentiation.
External and Internal Fertilisation:
The fertilisation always occurs in the aquatic media such as sea water, freshwater or intra-somatic (body) fluid of the maternal individual. If the fertilisation occurs outside the body of the organism, it is known as external fertilisation and if it occurs inside the body of the organism then it is termed as internal fertilizations.
The external fertilisation is common in various invertebrates and chordates, while the internal fertilisation occurs only in those animals which possess specialized sex organs for receiving and transmitting the sperms, e.g., reptiles, birds, mammals and angiosperms, etc.
Fertilisin and Antifertilisin:
The process of fertilisation is very specific. The sperms of one particular species fertilise the ova of the same species. This type of specificity of male and female gametes is of utmost biological importance and is achieved by the help of certain chemical compounds. It is found that the egg and sperm both contain a chemical substance known as fertilisin.
The fertilisin is a glycoprotein which is composed of different types of amino acids and monosaccharide’s (glucose, fucose, fructose and galactose) according to the species. The molecular weight of the fertilisin is 300,000 and it contains large molecules.
The surface layer of sperm contains another proteinaceous substance known as antifertilisin. The antifertilisin is a protein which is composed of acidic amino acids. It has small molecules and the molecular weight is about 10,000.
The fertilisins of the eggs are supposed to attract the sperms which contain a particular type of antifertilisin.
It has been found that egg fertilisin of any species reacts efficiently with the sperm antifertilisin of the same species. It has also been reported that the fertilisin in egg-water attracts the sperms of the same species and many sperms adhere together. This type of mutual adhesion of the sperms is known as the agglutination and is most common in sea urchins.
Process of Fertilisation:
The process of the fertilisation includes two successive steps which are as follows:
1. The activation of the egg;
2. The amphimixis.
1. Activation of the Egg:
The process of activation of eggs is completed in the following stages:
(i) Movement of the sperm towards the egg:
The sperms which occur in the external or internal fluid media around the egg, swim towards the egg at random. They collide with the egg by chance. The chance of colliding the sperms with the egg occurs regularly in the nature and remains fruitful only due to the large number of the sperms and enormously large size of the ovum.
The fertilisins and antifertilisins become active after the chance collision of the sperms with the ova. The egg fertilisin usually occurs in the jelly surrounding the egg. It gradually dissolves in the surrounding water of the egg and forms the so called egg water.
(ii) Activation of the Sperms:
When a sperm with a specific antifertilisin comes in contact with the egg water of its own species, then certain significant changes occur in the acrosome of the sperm. The peripheral portion of the acrosome of sperm collapses and its enzymes, the lysins are extruded and dissolve in the water.
The central portion of the acrosome elongates and forms a 1 to 75µ long, thin tube microscope) view of fertilisation, known as the acrosomal filament. The acrosomal filament is the rigid tube which protrudes out from the sperm head.
When the sperm possesses such an acrosomal filament, it is said to be activated for the ready penetration in the unfertilised egg. When the activated sperms reach to the egg, the acrosomal filaments penetrate into the egg jelly and vitelline membrane by the help of dissolving action of the sperm lysins.
As soon as the tip of the acrosomal filament touches the egg membrane (plasma membrane), various important morphological and physiological changes are started in the egg.
(iii) The Activation of Egg and Insemination:
As soon as the acrosomal filament touches the egg surface, the ooplasm protrudes out at the point of contact into a cone-like process known as the fertilisation cone. The fertilisation cone may be conical, cylindrical or irregular. When the fertilisation cone is irregular in shape, it contains many pseudopodia-like processes of the ooplasm. The fertilisation cone is composed of the plasma membrane and hyaline cytoplasm.
The fertilisation cone engulfs the sperm and the sperm which is surrounded by the hyaline cytoplasm moves inwards. The penetration of the sperm in the egg is known as the insemination. Immediately after the insemination, a thin membrane known as the fertilisation membrane is formed around the plasma membrane of the egg. The fertilisation membrane prevents the entrance of further sperms in the egg.
2. Amphimixis:
During the insemination, the entire sperm may enter in the egg as in the mammals or the tail of the sperm remain outside the egg as in the echinoderms. In certain cases as in Nereis, the tail and middle piece of the sperm remain outside the egg and only the head and centrosome enter in the egg.
The nucleus of the sperm is known as the male pro-nucleus. The male pro-nucleus swells up by absorbing the water from the surrounding ooplasm and it becomes vesicular. The compactly arranged chromatin material of the male pro-nucleus becomes finely granular.
The centriole of the sperm is surrounded by the centrosome and microtubules which form aster rays. The male pro-nucleus and the centriole move towards the egg pro-nucleus. The path is prepared by the enzymatic action of the sperm acrosome and is known as penetration path.
In the case of sea urchins and vertebrates, the two pronuclei (male and female) come close to each other and the close contact takes place between the two. The nuclear envelope is broken at the point of contact and the nuclear contents of both pronuclei are intermingled. The endoplasmic reticulum forms a new common nuclear envelope around the both pronuclei and, thus, forms a zygote nucleus.
In case of Ascaris, annelids and molluscs this type of fusion of two pronuclei does not occur. In these animals the centrioles form the achromatic spindle from the microtubules of the ooplasm and both male and female pronuclei come close to each other, their nuclear envelopes are dissolved.
The paternal and maternal homologous chromosomes get arranged on the equator of the achromatic figure and the first cleavage (mitotic) division of the egg occurs. After this division, the nuclear envelope is formed around the chromosomes of the daughter nuclei.
Post-fertilisation Changes in the Egg:
After the fertilisation following changes occur in the egg:
1. The zygote becomes ready for the cleavage and for the formation of the embryo.
2. The oxygen consumption of the zygote increases enormously.
3. The metabolic rate of the zygote increases greatly. For instance, the amount of amino acids and the permeability of the plasma membrane of the egg increases the volume of the egg, decreases the exchange of phosphate and sodium ions between the zygote and the surrounding media, diffusion of the calcium ions from the egg starts and the hydrolysing activities of the proteolytic enzymes increase.
4. The protein synthesis is started.
Kinds of Fertilisation:
In the organisms, following types of fertilisation occur:
1. Monospermic Fertilization:
In most animals, usually only one sperm enters in the egg, this type of fertilisation is known as monospermic fertilisation. The monospermic fertilisation is common in the coelenterates, annelids, echinoderms, bony fishes, frogs and mammals.
2. Polyspermia Fertilization:
When many sperms enter in the egg, the fertilisation is known as the polyspermic fertilisation.
It may be of two types:
(i) Pathological Polyspermy:
Under certain abnormal conditions, when in a monospermic type of egg many sperms enter into the egg, the condition is known as pathological polyspermy. This type of egg does not develop further and dies soon.
(ii) Physiological Polyspermy:
In the animals with large yolky eggs such as molluscs, selachians, urodels, reptiles and birds, the polyspermic fertilisation usually occurs. Such polyspermic fertilisation is known as physiological polyspermy. In these cases, many sperms enter in the egg but only one unites with the egg pro-nucleus and rest are degenerated soon. Such eggs are viable and develop further.
3. Polyandry:
When two male pronuclei unite with a female pro-nucleus, the union is known as polyandry, e.g., man and rat.
4. Polygamy:
When two egg pronuclei unite with single male pro-nucleus, the phenomenon is known as polygamy, e.g., sea urchins, polychaete worms, urodels and rabbits.
Gynogenesis:
When only sperm activates the egg but its pro-nucleus does not unite with the egg pro-nucleus, the phenomenon is known as gynogenesis, e.g., planarians and nematodes.
Significance of Fertilisation:
1. The fertilisation ensures the usual specific diploidy of the organism by the fusion of the male and female pronuclei.
2. The fertilisation establishes definite polarity in the eggs.
3. The fertilisation provides new genetic constitution to the zygote.
4. The fertilisation activates the egg for the cleavage.
5. The fertilisation increases the metabolic activities and rate of the protein synthesis of the cell.
6. The fertilisation initiates the egg to start cleavage and embryogenesis.
7. Fertilisation combines characters of two parents, thus, introducing variations and making the resulting individuals better equipped for the struggle for existence. This happens only in cross fertilisation.