The fertilization is a process of fusion of two types of sex cells (male and female) or gametes is known as fertilization. In other words fusion of sperm with ovum is known as fertilization.
Fusion of sex cells in multicellular animals:
In multicellular animals, as a result of this fusion of sex cells, the fertilized egg is said to have become active to start embryonic development. Further, as a result of fusion of the two haploid nuclei of sperm and ovum required to form diploid zygote, the diploid zygote receives hereditary characters of both the parent’s for the next generation. We can study the process of fertilization by dividing into the following three phases;
- Approach and penetration of spermatozoon to ovum or egg.
- Activation of egg
- Fate of spermatozoon-inside the egg.
Approach and Penetration of Spermatozoon into Ovum:
The approach and penetration of spermatozoa into ovum is the first step in the fertilization process the meeting of the spermatozoon with the egg or ovum.
In both external and internal types of fertilization this union is achieved by the capacity of the spermatozoa to swim actively.
In other words, this is possible mainly due to the motility of sperm. In this process millions of spermatozoa actively swim about in a liquid medium Without any particular direction, and ultimately contact with the inactive ovum. It is generally believed that to make this sudden and accidental process relatively more successful, the ovum size is very large and the number of spermatozoa is enormous so this makes the possibility in the fertilization.
The second step in fertilization is accomplished by an extremely complex chemical mechanism. In the presence of mature eggs, the spermatozoa become ‘sticky’ and they either begin to stick to the surface of the egg itself or agglutinate together to form clumps.
Agglutination in spermatozoa:
Agglutination in a spermatozoa is brought about by a substance called fertilizin, which is secreted either by the mature ovum itself or by the follicle cells (surrounding it).
Fertilizin is a glycoprotein whose protein part is made up of many amino acids and the carbohydrate part is composed of one or more monosaccharide sugars. The amino acids and sugars of particular Fertilizin vary in specific species.
Another chemical occurs on the cytoplasmic surface of a spermatozoon called antifertilizin. In a particular animal species, the chemical fertilizin of ovum is able to generally link with only antifertilizin of its own spermatozoa.
Mechanism of penetration of sperm into ovum:
The mechanism of penetration of sperm into the ovum is a chemical process, the spermatozoa attach to the surface of the ovum. The spermatozoon produces enzyme-like substances generally known as sperm lysins. Sperm lysins are secreted by the acrosome of spermatozoon.
Penetration in mammals:
In mammals, penetration of spermatozoon into the egg is a relatively complex process. Besides the zona pellucida, the ovum is also surrounded by the cellular corona radiata. To remove this cellular layer the mammalian spermatozoon secretes an enzyme called hyaluronidase. Which, which dissolves the mucopolysaccharide called hyaluronic acid. As a result of the enzymatic action the corona radiata layer disintegrates from the ovum.
Acrosome Reaction in Fertilization:
The acrosome reaction starts due to the secretions produced by the ovum, the structure of the sperm also begins to change. In which primary modification occurs in the acrosome. The anterior acrosomal membrane breaks and the acrosomal granule comes out into water and begins to dissolve. At the same time the posterior acrosomal membrane elongates into a relatively rigid tube and appears as a process protruding from the anterior tip of the spermatozoon heads. This tube is known as the acrosomal filament.
Capacitation in Fertilization:
Capacitation is the ability of the mammalian sperm to fertilize an ovum is known as capacitation. In the capacitation process in which the acrosomal membrane of the spermatozoon breaks and the acrosome secretes sperm lysin-containing substances with the help of which the spermatozoon dissolves the cellular corona radiata and zona pellucida and then comes in contact with the egg in a lateral direction.
At the contact point, the plasmalemma of both sperm and ovum dissolve and the spermatozoon enters into the ovum. As soon as the sperm comes in contact with the surface of the ovum, the cytoplasm of the ovum projects out as a transparent process termed as the fertilization cone.
The fertilization cone envelopes the sperm and begins to retract again into the egg cytoplasm with the result the sperm enters the ovum.
Monospermy and polyspermy:
Monospermy and polyspermy found in most animal phyla e. g. frog, mammals, etc. Monospermy– when only one spermatozoon enters into the ovum. This state of the fertilization process is known as monospermy.
Polyspermy– under abnormal circumstances when more than one spermatozoa becomes successful in entering the egg. This state is referred to as polyspermy.
The zygotes produced by polyspermic fertilization, embryonic development is short-lived and abnormal and the embryo itself if not viable. Such type of fertilization is known as pathological polyspermy.
To ensure monospermic fertilization, the exact method adopted by the ova (or sperm) of animals is not clearly known.
In pathological polyspermy, only one spermatozoon participates in the formation of a zygote and embryonic development, the rest of the spermatozoa are absorbed and destroyed in the ovum cytoplasm.
Activation of Ovum (Reaction of the Egg) :
Even before the sperm enters fully into ovum, the ovum begins to show signs of significantly important changes, that is, activation of ovum is initiated. In other words, the ovum acquires the capacity for embryogenesis by initiating the development process.
Cortical Reaction in Fertilization:
Cortical Reaction is the first signs of activation of the egg appear in the peripheral cytoplasm and is followed by a series of reactions which are jointly referred to as the cortical reaction. The cortical reaction varies in different animals. A deep study of the cortical reaction has been made in the eggs of the sea-urchin (phylum Echinodermata).
The sea urchin egg at the time of fertilization changes its colour from yellow to white. Although, the significance of colour change is not known but immediately following it sequential structural alterations take place by which the egg-cortex is profoundly affected, especially the unique structures called cortical granules, located near the plasmalemma.
The plasmalemma of the mature egg of sea urchin is relatively thick and double layered. Immediately following fertilization, the two layers of the plasmalemma separate from each other. and the outer layer delimits from the egg surface as the fertilization membrane. Simultaneously at this time the cortical granules begin to swell and burst and the contained mucopolysaccharides granules dissolve in the water and become in the fluid state.
This fluid then fills up the perivitelline space between the fertilization membrane and egg-cytoplasm. Along with this, the fluid coming out from these cortical granules further fills the perivitelline space resulting in making the fertilization membrane more away and distinct from the egg-surface. Besides, from the cortical granules, a sticky layer called the hyaline layer is also formed.
The study of the fertilization process has revealed that the cortical reaction is variable in different animals. Among the various animal species, the nearest resemblance to the fertilization process of sea-urchin is shown by bony fishes and frog. In bony fishes and frog, on attachment of the sperm to the egg, the cortical granules also burst and the liquefied granules are released on the surface of the egg, however, in contrast to the sea-urchin egg, the eggs of these animals are already lined by distinct membranes which are performed on the egg in the ovary itself for example;
The vitelline membrane of the frog’s egg and the chorion of the bony fish egg). In those mammals, where cortical granules are present (eggs of human, hamster, rabbit), at fertilization these granules are released into the perivitelline space present between the egg cytoplasm and the pre-existing membrane, the zona pellucida. The granules soon disappear probably as a result of dissolving. In these mammalian types also, a de novo fertilization membrane is not formed as a consequence of fertilization.
Essence of Activation:
When a mature ovum is compared before and after fertilization then it becomes apparent that as a result of fertilization from a semi-inactive state the egg enters into an active dynamic state; If fertilization does not occur, then after sometimes the ovum begins to show degenerative changes leading to its ultimate loss. In contrast, if fertilization does take place then activation of the egg occurs in which are included significantly important changes.
For example, if the meiotic divisional process is still incomplete then it is completed, fusion of the male and female pro-nuclei takes place, the relocation and rearrangement of different cytoplasmic substances of the ovum occurs and the ovum enters into the rapid divisional phase, that is, commencement of cleavage occurs, etc.
The physiological factors which govern the important alterations in the egg in fact are referred to as the activation of egg. Briefly, activation of the egg can be described as under;
In this context, one of the generally agreed fact is that at the time of fertilization the sperm releases some important substance into the egg which causes the stimulation of all the types of above described processes in the egg In the process of the penetration of the Sperm into the egg, keeping in view the importance of the fertilizin-antifertilizin reaction it was argued, that it is these substances which are also the probable factors responsible for the activation of the egg.
It was considered that the primary action of the sperm is the activation of the oxidative enzymes of the ovum and as a result the extra energy which is made available is utilized for regulating all the basic changes which take place in the ovum. However, other investigations have revealed that in the animal eggs (for example, Nereis) the consumption of oxygen does not increase after fertilization.
Some other metabolic changes which have been observed in the egg after fertilization are excessive secretion of acid immediately of few minutes after fertilization, increase in the permeability of the egg plasma membrane, and increase in the activity of the proteolytic enzymes.
To gain a better understanding of the essence of activation the role of protein-synthesis was also examined in great detail and it was revealed that before fertilization the ribosomes are individually distributed in the egg cytoplasm, but after fertilization they join together into groups which is indicative of active protein-synthesis.
Changes in the Organization of the Egg Cytoplasm:
The process of penetration of sperm into the egg besides establishing the process of activation of the egg and amphimixis also causes several rearrangements in the components of the egg-cytoplasm in many animals. As a result, compared to the situation in the unfertilized ovum, in the fertilized egg. fundamental alterations in the distribution of various constituents and organelles of the egg-cytoplasm occur.
In some animal eggs, these changes are so profound that in the cytoplasm new areas are demarcated. As a consequence of fertilization, the organizational alternations in the fertilized egg are essential and highly important for embryonic development.
One of the important alterations is the removal of the cortical granules from the ovum and replacement of the original outer egg surface layer by inner layers. Since in morphogenetic processes the cell surface layers have an essential role, therefore, the change In the surface layer of the egg must have its own significance.
Fertilization in Frogs egg:
In the frogs egg, the surface layer of the egg cytoplasm begins to shrink in the direction of the animal pole and as a result this pigmented cytoplasm covers that colourless region of the animal pole from where the first polar body is removed. As the cortical cytoplasm spreads towards the animal pole, it also disperses over the vegetal pole. The vegetal pole is the future ventral end of the embryo. As soon as the edge of the spreading pigmented cortical above the equator of the egg, then the relatively deeply situated band-like layer of peripheral cytoplasm becomes visible.
In comparison to the cortical cytoplasm the peripheral cytoplasm is lighter in colour because it has a relatively lesser amount of pigment. As a result of these arrangements, the egg of the frog acquires a distinct bilateral symmetry whose median sagittal section passes through the grey crescent. It is in this cytoplasmic state, cleavage in the egg commences.
After the formation of grey crescent, the permeability of the egg reduces the animal pole and ventral surface and thus it becomes relatively more at the grey crescent surface. At the time of gastrulation it is this grey crescent which plays a specific role in the formation of blastopore.
3. Fate of Spermatozoon inside the egg:
In some animal groups, particularly in mammals, the entire spermatozoon, that is, head, middle piece and tail, enters into the egg cytoplasm. However, in most other animals (e.g. echinoderms) only the head and middle piece enter the egg while the tail is left behind outside the vitelline membrane.
In other animals, (e.g. Nereis) only the head of the Sperm is able to enter the ovum while the middle piece and tail are left behind. Since in most animal groups, the tail of spermatozoon is left behind at fertilization, it is believed that the major function of the tail is locomotory. The role of the middle piece of spermatozoon during the fertilization process is not fully known.
In vertebrate eggs, the first meiotic division and the metaphase of the second meiotic division is completed in the ovary itself. At this phase, the divisional process stops. Ovulation takes place and fertilization might occur. The second meiotic division is completed only if the ovum is fertilized by a spermatozoon.
Therefore, in these animals reduction division (meiosis) in the egg is completed only after the sperm enters the egg and then only the second polar bodies are formed.
At the time of the entry of the sperm into the egg cytoplasm, the acrosome is anterior most and behind it are present the nucleus and centrosome, respectively. Very soon the nucleus and centrosome rotate by 180° and as a result the centrosome containing portion of the sperm-head becomes anterior-most. The rest of sperm now severes its connection with the nucleus and the centrosome. Both nucleus and centrosome undergo structural changes.
The nucleus is now known as male pronucleus. The pronucleus swells and its chromatin material becomes finely granular. By absorbing fluid from the egg cytoplasm it becomes vesicular in appearance. Sometimes, the centrosome becomes surrounded by asters. Along with these changes, the male pronucleus and centrosome also move towards the egg nucleus or female pronucleus for fusion.
The exact region in the egg cytoplasm where the male and female pronuclei fuse depends upon the amount of yolk present in the cytoplasm of the egg. In oligolecithal eggs (mammals) containing very little yolk, this region is present somewhere the centre of the ovum.In mesolecithal eggs (frog), this region lies in the middle of the animal half of the egg. The egg of frog contains pigment granules.
When the sperm enters and travels in the egg for fusion it displace the pigment Granules and as a result a lighter path becomes apparent in the egg cytoplasm. This travel path of sperm is known as the penetration path. The male and female pronuclei fuse in different ways in different animals.
In vertebrates, the nuclear membranes of both male and female pronuclei disappear and as in ordinary mitotic division their chromosomes first arrange themselves in the midline into a metaphase plate and ultimately convert into a diploid nucleus. The fusion of the male and female pronuclei is known as amphimixis.
Significance of Fertilization:
During fertilization, the inactive ovum is activated. Activation of ovum is extremely important and essential since all future embryonic development depends upon the activation of the egg.
As a result of fertilization in many animals, maturation or reduction division in the egg is completed.
Through fertilization the haploid male and female gametes fuse to form a normal diploid offspring.
By fertilization the hereditary characters of male and female parents intermix and as a result the offspring inherits characteristic features of both the parents.
As a result of fusion of male and female pronuclei during fertilization probably rejuvenescence of the cytoplasm of the ovum occurs.
Rejuvenescence of the ovum-cytoplasm is considered essential since it has been experimentally seen that in the absence of this process the dividing power of the developing ovum slows down or sometimes even completely stops.
The point of entry of spermatozoa into the egg establishes the future plane of cleavage. Thus, the axis which passes through the point of entry and the grey crescent, is the dorso-ventral axis of the developing egg.
If you are interested in biology also read this notes – Structure of a Sperm Cell, Essay on Spermatogenesis