In this article we will discuss about:- 1. Definition of Fertilization 2. Prerequisite of Fertilization 3. State of Gametes 4. Events 5. Activation 6. Significance 7. Parthenogenesis.
Contents:
- Definition of Fertilization
- Prerequisite of Fertilization
- State of Gametes before Fertilization
- Events in Fertilization
- Activation of Fertilization
- Significance of Fertilization
- Parthenogenesis and Fertilization
1. Definition of Fertilization:
Fertilization is the process which marks the beginning of an individual. It involves the union of two gametes—sperm and ovum.
The union involves a number of physical and chemical events which result into:
(i) The activation of the egg and
(ii) Amphimixis, i.e., intermingling of parental hereditary traits in the offspring.
2. Prerequisite of Fertilization:
The essential prerequisites for fertilization are sperm from male and ovum from female. The ensurement of fertilization demands—
(i) Maturity of sperm and ovum and
(ii) Coming together of these cells.
The first demand is fulfilled during gametogenesis and second one is facilitated by the act of sexual reproduction. In many animals, i.e., Amphioxus, fishes and amphibians, the fertilization is external, in others like, reptiles, birds and mammals, the process is internal. In animals where internal fertilization occurs, number of devices are seen for the transfer of sperm cells within the gonoduct of the female.
3. State of Gametes before Fertilization:
Ovum:
After maturity, the egg is fully prepared with organised membranes, cytoplasmic particles and reserve materials. Physiologically the egg lies with suspended metabolic state and reduced glycogen content. Revival of metabolic activity occurs only after fertilization, which must happen within a specific period of time. If no fertilization occurs within that period, the egg undergoes degeneration.
Sperm:
Sperm cells are adapted for locomotion. They are metabolically active and obtain their energy from both exogenous and endogenous pathways. Recently, it has been shown that synthesis of some protein takes place within the sperm.
4. Events in Fertilization:
The entire events in fertilization may be divided into three phases: A. Penetration, B. Activation and C. Fusion of gametic nuclei.
Penetration:
During this event the sperm and ovum come in contact. The act involves a number of physical and chemical processes.
Physical process:
Leeuwenhoek, for the first time, described the structure of sperm and inferred that development begins with the impregnation of ovum by sperm. F. R. Lillie (1919) advanced the view that chemical events accompany the physical events of sperm and egg contact.
He showed that in sea-urchin, penetration is possible due to the interaction, of two chemical substances—fertilizin by the egg and antifertilizin by the sperm.
The fertilizin is a glycoprotein with a molecular weight of 82,000 or more. It contains a number of amino acids and a polysaccharide. The antifertilizin is composed of acid proteins. The fertilizins and anti- fertilizins combine in a specific way to establish an initial bond to facilitate penetration of the spermatozoon into an egg.
The fertilizin-antifertilizin reaction is species specific and resembles antigen and antibody reaction. Tyler (1959) forwarded that during fertilization, sperm head is engulfed by the surface of the egg. He termed this phenomenon as specific pinocytosis. Colwin and Colwin (1961) made detailed study of events during penetration in the Enteropneust, Saccoglossus and a polychaete worm, Hydroides, under electron microscope.
According to their observations (Fig. 5.8):
(i) Sperm head comes in contact with the vitelline membrane.
(ii) The plasmalemma of the sperm head
and the membrane of acrosome vesicle unite and split in the middle.
(iii) The acrosome granules come in contact with the vitelline membrane and produce lesion to it.
(iv) The acrosomal plasma membrane everts into one (in Saccoglossus) and many (in Hydroides) tube-like projections which approach the plasma membrane of egg through the gap of vitelline membranes.
(v) The plasma membrane of the egg sends one or several finger-like villi.
(vi) The opposing tubes—from egg and sperm unite and the acrosomal plasma membranes of sperm and egg become continuous, thus a connection is established between sperm and egg.
(vii) Sperm nucleus and centriole pass through these tubes inside the egg cytoplasm.
(viii) Later, plasma membrane of egg and acrosomal plasma membrane of sperm become a continuous membrane of zygote.
Similar events have also been recorded in toad, in cat, in chick and in man.
It is to be noted that these studies have given a new insight to the meaning of fertilization. Formerly, it was believed that fertilization means only the fusion of nuclei, but now it is understood that it involves fusion of two cells.
Chemical process:
Penetration is largely a chemical mechanism. Since Lillie forwarded the concept of fertilizin-antifertilizin, many workers provided more information about these chemical substances. Tyler regarded that the spermatozoon contains an enzymatic substance (sperm lysin) which causes local dissolution of egg membrane to make the path clear for sperm entry.
But other workers like Hart- man, Rothschild and Runnstrom have collectively called the substances as gamones; these gamones are regarded as the active principles involved in the process of fertilization. The gamones from sperm are called androgamones and those from the egg are known as gynogamones.
Precursor of the active principles:
According to Brachet, RNA compound of low specific gravity which is present in high concentration at the outer side of the gametes are the precursors of the active principles.
Chemical nature of active principles:
(a) Gynogamone (I):
It contains a bluish- red pigment called “echinochrome—A”, which remains in association with proteins. It is responsible for liberating Gynogamone (II).
(b) Gynogamone (II):
This is a non- dialisable lipase containing 5% nitrogen but without protein. It can be precipitated by ammonium sulphate, and if heated for two hours at 95°C., its property is lost.
(c) Androgamone (I):
It is a colourless, nondialisable colloid, which is resistant to boiling and cannot be precipitated by ammonium sulphate. It gives a colour reaction with protein and is quickly inactivated by trypsin.
(d) Androgamone (II):
This substance is yellowish in colour and soluble in sea- water. It is also resistant to boiling.
(e) Androgamone (III):
It is dialisable, trypsin resistant and soluble in methanol.
Mechanism of actions of active principles:
The success of fertilization depends upon the equilibrium of two sets of gamones. In addition to gamones, certain other substances are necessary for the success of fertilization.
In case of mammals, these are:
(i) Hyaluronidase and
(ii) Glutathione. Hyaluronidase, produced by the plasma membrane of the spermatozoa, helps to break the cells of enveloping layers and Glutathione maintains the metabolic stability of sperm cells.
Successful fertilization thus depends upon the presence of specific quantity of sperm cells which provide the requisite quantity of these chemicals.
The sperm cells immediately after liberation, release androgamone (I), the production of it continues till the arrival of sperm to the attraction zone of the egg. It is responsible for retarding the excess activity of the sperm and thereby results into the conservation of energy. Almost at the same time the egg liberates Gynogamone (I).
It excites the sperm movement and attracts them to the egg, by positive chemotaxis, the result is the oriented movement of sperms to the egg. Immediately after the touch of the first sperm with the egg the latter produces Gynogamone (II). It binds or agglutinates the sperm in contact and at the same time provokes the production of Androgamone (II) in other sperms which make them non-agglutinated.
The Gynogamone (II) also initiates the restoration of metabolic activity in egg itself which is marked by the production of second polocyte. The Androgamone (II) produced in sperm helps it in agglutination with egg to penetrate by dissolving the mucous jelly, in others it neutralises the Gynogamone (II) and thus negates agglutination.
Another gamone from the sperm, called Androgamone (III), obtained only from the sperms of sea-urchin, is believed to have the power of liquefying the egg membranes.
5. Activation of Fertilization:
It means the awakening of a sleeping egg to a state of activity. Though many changes in the penetrated egg are well known, yet clear understanding of specific biochemical events which give the necessary stimuli of activation is a begging.
The first event of activation involves the cortical reaction and formation of fertilization membrane (Fig. 5.9). The vitelline membrane lifts itself from the cortical surface of the egg to form the perivitelline space. Subsequently dense lamellar materials (derivatives of cortical granules) are accumulated in the inner side of the vitelline membrane which is now called the fertilization membrane.
Structural change of penetrated egg:
(a) Cytoplasm moves in streamlined fashion towards the point of sperm attachment.
(b) Different components of the cytoplasm are reshuffled and reorganised.
(c) Some irregular amoeboid movements result into the final return to spherical shape.
(d) The refractive index of the cortex alters.
(e) Thickness of the membrane increases.
(f) The cytoplasmic part or vitellus reduces in volume.
(g) There is redistribution of calcium in cytoplasm which results into the change of viscosity in the cytoplasmic part of the cortex.
(h) Near the region of sperm attachment, a small pseudopodium is formed which engulfs the sperm head. This is fertilization cone. It was formerly thought, that it is only the property of echinoderm eggs, but recent electron microscopic studies have shown that it happens in most of the eggs including mammals.
Metabolic changes of penetrated egg:
(a) Within an un-penetrated egg, there is a cytochrome oxidase inhibitor, which is inactivated after fertilization, this results into the increase of oxygen consumption.
(b) A mature un-penetrated egg has little ability to incorporate amino acids to protein. It has been found that in spite of this disability, the enzymes isolated from such egg do synthesize protein in vitro. This in vivo incapaciation of protein synthesis is believed to be due to the suppression of functional state of messenger RNA or ribosomes.
(c) DNA synthesis resumes after penetration.
Resumption or beginning of meiosis:
During oogenesis, for necessary organisation of ovum, meiosis remains suspended (excepting sea-urchin and coelenterates, where meiosis is independent of fertilization).
In different organism the phase of arrest may differ:
(a) No meiosis before fertilization, e.g., Ascaris, Molluscs and Crustaceans.
(b) Arrested at the metaphase of first meiotic division, e.g., Nemer tines, Insects.
(c) Arrested at the end of first meiotic division, e.g., amphibians. Whatever may be the cause of arrest, it continues and meiosis ends during the period of activation.
Block to polyspermy:
In most eggs considerable changes take place during activation by which entry of more than one sperm is prevented. In the eggs of annelid, sea-urchin, staifish, Amphioxus and frog, it is done by the formation of a fertilization membrane.
Formation of fertilization membrane:
In unpenetrated sea-urchin eggs, the vitelline membrane is thin and closely set with plasma membrane. Within the cortex the cortical granules remain enclosed in vesicles. Immediately after penetration, the cortical granules are released from the vesicles and the contents adhere to the inner side of the vitelline membrane.
The vitelline membrane lifts up from the plasma membrane and this lifting is accompanied by the accumulation of fluid and a hyaline layer between vitelline membrane and plasma membrane. The vitelline membrane becomes thicker and is known as fertilization membrane.
Runnstrom (1952) has shown that fertilization membrane is formed by the separation of cortical and pigment granules.
The eggs of teleostean fishes have hard covering called chorion. An opening called micropyje serves as the passage through which only one sperm can enter at a time. Immediately after the entry of one sperm the cytoplasm of egg invades to close the micropyle.
In mammalian eggs, the prevention of the entry of more than one sperm is done by progressive changes in the zona pellucida, which is known as zona reaction.
In the eggs of many organisms, i.e., insects, elasmobranchs, urodeles, reptiles and birds, more than one sperm may enter within the egg but only one male pronucleus unites with the female pronucleus. The other sperm pronuclei help in the breakdown of yolk.
Fusion of the pronuclei:
The process of fusion varies in different organisms—in some there is complete fusion, in others the chromosomal elements arrange themselves along the spindle fibres.
The process which happens in frog will be described here:
(a) Sperm head and middle piece enter within the egg cytoplasm. Immediately after the entry, the egg completes its meiosis and loses its centriole.
(b) After the entrance, the sperm rotates 180° and thus the middle part comes in the front end. At this stage the sperm moves within the egg at random. This path is called penetration path.
(c) The egg pronucleus moves to the periphery and the direction of the sperm becomes oriented to it. The second path is called copulation path. Both the paths are trailed by marks of pigments.
(d) Considerable changes occur within the sperm pronucleus which swells up and the chromosomes become reticular in appearance. In the inidpiece, the centron some and centriole form the asters around them.
(e) The centriole of the sperm divides and moves to the poles forming fibres in between them, where the chromosomes of male and female pronuclei arrange themselves in the equator after the disappearance of nuclear membranes. It resembles metaphase plates of mitosis and soon results into cleavage.
6. Significance of Fertilization:
The process of fertilization has three important significances:
Ensures propagation:
The union of haploid chrcmoscmes and the attainment of diploid state helps the organism to propagate its own species. In lower organisms, the entire individual is involved, but in higher forms this is achieved through sexual reproduction and by the fusion of two specialized cells.
Restoration of normal chromosome number occurs in a variety of ways. In some earthworms and planarians the chromosome number becomes doubled before meiosis. The meiotic division causes the adjustment of normal chromosome number.
Rejuvenates:
The union of two cells provides some stimuli which are necessary for the vitality of the individual.
Produces vanations:
The process of gametogenesis results into the production of gametes which are dissimilar among themselves and to their parents. The recombination of these gametes, therefore, produces large number of variations which face the ordeal of natural selection. Fertilization thus ensures the production of variations.
7. Parthenogenesis and Fertilization:
Large number of evidences are available which show that egg may start to develop without being impregnated by sperm. Such, development is called parthenogenesis. Several agents (heat, cold, ultraviolet-ray and X-ray, hyper- and hypotonic salt solutions, mechanical agitation, electric currents) can stimulate development.
The development of an unfertilized egg initiated by such artificial agencies is called artificial parthenogenesis. But in many invertebrates, viz., rotifers, aphids, bees, wasps, ants and some vertebrate species especially the domestic turkey, parthenogenetic development is a normal process. This phenomenon is called natural parthenogenesis.
The parthenogenetic eggs develop mostly into males while the fertilized eggs develop into females. Parthenogenetic individuals are haploid in the initial stage but in number of cases it has been recorded that these haploid individuals become diploid by duplication of their chromosomes. Thus a diploid homozygous individual is formed.
Parthenogenesis reveals the fact that all the factors in development are present in the egg but it requires some external stimuli which are provided by the penetration of the spermatozoon during fertilization.