(i) fertilization is the fusion of the male sex cell or gamete (sperm) and the female sex cell or gamete (ovum or egg)
(ii) fertilization occurs in the oviduct or fallopian tube
(iii) fertilization finally leads to the formation of zygote
(iv) fertilization may occur when an animal is on heat period
The union of the cytoplasm and pro-nuclei of the male females gametes to form a diploid zygote is known as the fertilization.
External and Internal Fertilization in animals:
Fertilization necessitates discharge of ova and sperms in close proximity. This may be accomplished in water in aquatic animals, or in special cavities of the female, more commonly in land animals.
In most aquatic animals, such as echinoderms, many fish and amphibians (frogs) both ova and sperms are laid directly into water where they fertilize. This is called external fertilization taking place outside the body of the organism. In other aquatic animals (e.g., cephalopods) and in most terrestrial animals, the male deposits sperms, during copulation, either into the oviduct of the female (as in vertebrates) or into special receptacles called spermathecae (e.g., insects, spiders), so that fertilization takes place inside the body of the organism. This is called internal fertilization.
where fertilization takes place
In human being, fertilization takes place mostly in the ampullary-isthmic junction of the oviduct (Fallopian tube).
Arrival of sperms:
Male discharges semen into the female’s vagina close to the cervix during coitus (copulation). This is called insemination. A single ejaculation of semen may contain 300 million sperms.
Movement of sperms:
From the vagina the sperms travel up the uterus but only a few thousand find their way into the openings of the fallopian tubes. Primarily, contractions of the uterus and fallopian tubes assist in sperm movement but later on they move by their own motility. Sperms swim in the fluid medium at the rate of 1.5 to 3 mm per minute to reach the site. The leucocytes of the vaginal epithelium engulf millions of sperms.
Arrival of secondary oocyte:
In human being, the secondary oocyte is released from the mature Graafian follicle of an ovary (ovulation). The oocyte is received by the nearby Fallopian funnel and sent into the Fallopian tube by movements of fimbriae and their cilia. The secondary oocyte can be fertilized only within 24 hours after its release from the ovary.
The secondary oocyte is surrounded by numerous sperms but only one sperm succeeds in fertilizing the oocyte. Since the second meiotic division is in progress, so the sperm enters the secondary oocyte. Second meiotic division is completed by the entry of the sperm into the secondary oocyte. After this secondary oocyte is called ovum (egg).
Capacitation of sperms:
The sperms in the female’s genital tract are made capable of fertilizing the egg by secretions of the female genital tract. These secretions of the female genital tract remove coating substances deposited on the surface of the sperms particularly those on the acrosome. Thus, the receptor sites on the acrosome are exposed and sperm becomes active to penetrate the egg. This phenomenon of sperm activation in mammals is known as capacitation. It takes about 5 to 6 hours for capacitation.
The secretions of seminal vesicles, prostate gland and bulbourethral glands (Cowper’s glands) in the semen contain nutrients which activate the sperms. The secretions of these glands also neutralise the acidity in the vagina. Alkaline medium makes the sperms more active.
Physical and Chemical Events of Fertilization:
These events include the following processes:
(i) Acrosomal reaction:
After ovulation, the secondary oocyte reaches the Fallopian tube (oviduct). The capacitated sperms undergo acrosomal reaction and release various chemicals contained in the acrosome. These chemicals are collectively called sperm lysins.
Important sperm lysins are:
(i) hyaluronidase that acts on the ground substances of follicle cells,
(ii) corona penetrating enzyme that dissolves corona radiata and (iii) zona lysine or acrosin that helps to digest the zona pellucida.
Optimum pH, Ca++, Mg++ ions concentration and temperature are essential for acrosomal reaction. Ca++ plays major role in acrosomal reaction. In the absence of Ca++, fertilization does not occur.
Due to acrosomal reaction, plasma membrane of the sperm fuses with the plasma membrane of the secondary oocyte so that the sperm contents enter the oocyte. Binding of the sperm to the secondary oocyte induces depolarization of the oocyte plasma membrane. Depolarization prevents polyspermy (entry of more than one sperm into the oocyte). It ensures monospermy (entry of one sperm into the oocyte).
(ii) Cortical reaction:
Just after the fusion of sperm and plasma membranes of oocyte, the secondary oocyte shows a cortical reaction. The cortical granules are present beneath the plasma membrane of the secondary oocyte. These granules fuse with the plasma membrane of the oocyte and release their contents including enzymes between the plasma membrane and the zona pellucida. These enzymes harden the zona pellucida which also prevents entry of additional sperms (polyspermy).
(iii) Sperm entry:
At the point of contact with the sperms, the secondary oocyte forms a projection termed the cone of reception or fertilization cone which receives the sperm. The distal centriole of the sperm divides and forms two centrioles to generate the mitotic spindle formation for cell division. The mammalian secondary oocyte (egg) does not have centrioles of its own.
(iv) Karyogamy (Amphimixis):
Sperm entry stimulates the secondary oocyte to complete the suspended second meiotic division. This produces a haploid mature ovum and a second polar body. The head of the sperm which contains the nucleus separates from the middle piece and the tail and becomes the male pronucleus. The second polar body and the sperm tail degenerate.
The nucleus of the ovum is now called, the female pronucleus. The male and female pronuclei move towards each other. Their nuclear membranes disintegrate mixing up of the chromosomes of a sperm and an ovum is known as karyogamy or amphimixis. The fertilized ovum (egg) is now called zygote (Gr. zygon- yolk, zygosis- a joining). The zygote is diploid unicellular cell that has 46 chromosomes in humans. The mother is now said to be pregnant.
(v) Activation of egg:
Sperm entry stimulates metabolism in the zygote. As a result, the rates of cellular respiration and protein synthesis increase greatly.
Process of fertilization:
The process of fertilization includes the following steps which are as follows:
(a) Activation of the egg:
It is completed in the following stages:
(i) Movement of the sperm towards the egg:
Encounter between the sperm and ovum is purely accidental, because the movements of spermatozoa are entirely at random. But in some species the sperms are guided towards the ovum by chemical substances. The fertilizins and antiferlilizins become active after the chance collision of the sperms with the ova.
Egg secretes a chemical substance known as fertilizin (composed of glycoprotein). Sperm has on its surface layer a protein substance called antifertilizin (composed of acidic amino acids). The fertilizin of an egg interacts with the antifertilizin of a sperm of the same species. This interaction makes the sperms stick to the egg surface. Adhesion of spermatozoa to the surface of the egg is brought about by linking of fertilizin molecules which establish an initial bond.
(ii) Activation of the sperm:
The peripheral portion of the acrosome of sperm breaks and releases its contents, the sperm lysins. The central portion of the acrosome elongates and forms a thin, long tube known as the acrosomal filament.
When the sperm possesses such an acrosomal filament protruding out from the sperm head it is said to be activated for the penetration in the unfertilized egg. The released enzyme hyaluronidase (sperm lysin) by the acrosome dissolves the corona radiata, zona pellucida and vitelline membrane, enabling the sperm to penetrate these coverings.
The activation of egg insemination:
At the point of contact with the sperm, the egg forms a projection, termed the cone of reception or fertilization cone which receives the sperm. The penetration of the sperm in the egg is known as the insemination. Just after the entry of the sperm into the egg, a fertilization membrane is formed in the egg to prevent the entry of other sperms.
During the insemination the entire sperm may enter in the egg as in mammals or the sperm leaves its tail outside the egg or sheds it shortly after entering the egg’s cytoplasm. The ovum completes the second meiotic division and extrudes the second polar body. The head of the sperm swells to form the male pronucleus and the nucleus of the ovum becomes the female pronucleus. The fusion of the haploid male pronucleus with the haploid female pronucleus forms the nucleus of the fertilized egg or zygote.
The term cleavage or segmentation is applied to the series of rapid mitotic division of the zygote which converts the single celled zygote into a multicellular structure blastomeres called morula which ultimately transforms into blastula, having unilayered thick blastoderm around a central space, blastocoel. Types of Cleavage
A Based on the amount and pattern of distribution of yolk in the zygote, cleavage is of two types: holoblastic and meroblastic.
1. Holoblastic cleavage:
It divides the zygote and blastomeres completely into daughter cells. It is of two types: equal and unequal.
(i) Equal Holoblastic Cleavage:
It forms equal blastomeres. It occurs in star fish.
(ii) Unequal Holoblastic Cleavage:
It forms unequal blastomeres. Blastomeres are micromeres (smaller) and macromeres (larger). It is found in frog.
2. Meroblastic cleavage:
In this type of cleavage, the divisions are confined to the animal pole or peripheral region of egg. The yolk remains undivided. It is of two types: discoidal and superficial.
(i) Discoidal Cleavage:
The divisions are confined to the cytoplasmic disc located at the animal pole. It occurs in reptiles, birds and egg laying mammals.
(ii) Superficial Cleavage:
The cleavage remains restricted to the peripheral portion of the egg. It occurs in arthropods especially insects.
B. Based on the potentiality of the blastomeres, cleavage is of two types: determinate and indeterminate.
1. Determinate (Mosaic) Cleavage:
In this type of cleavage a complete embryo is formed only if all the blastomeres remain together e.g., annelid eggs.
2. Indeterminate (Non-mosaic) Cleavage:
In this type of cleavage each early blastomere on separation from other blasomeres may give rise to complete embryo e.g., chordate eggs.
what is Gastrulation in fertilization
Formation of gastrula from the monoblastic blastula is called gastrulation. Gastrulation is that phase of embryonic development during which the cells of blastula (in frogs) / blastodermic vesicle (in mammals) move in small masses or as a sheet of cells to attain the final location. Such movements of cells are called morphogenetic movement. It is differentiated into two types.
what is Epiboly in fertilization
The term epiboly is derived from Greek language, meaning ‘throwing on’ or ‘extending upon’. Epiboly means overgrowth of the ectoderm – forming regions around the endoderm forming region. It occurs in frog where the micromeres divide rapidly in the animal half and spread over the megameres over the vegetal half.
what is Emboly in fertilization?
The term emboly is also derived from Greek language meaning “throw in” or “thrust in”. Migration of prospective endodermal and mesodermal cells from the surface into the interior of the embryo is called emboly. It includes invagination, involution, ingression and delamination.
what is Invagination in fertilization?:
It is the process of infolding or inpushing of the vegetal pole of the embryo (blastula) into its cavity (blastocoel), forming a double-walled structure. It is just like the pushing in one side of a rubber ball with a thumb. Invagination occurs in the blastula of frog.
what is Involution in fertilization?:
or ‘rolling under’. Involution is the process of rolling or turning in of the surface cells into the interior of the embryo. It occurs in frog’s blastula.
what is Ingression in fertilization?::
The term ingression means “inward migration”. In ingression the blastomeres form new cells from their surface. New cells migrate into the blastocoel of the blastula to form a solid gastrula. It means, it forms solid gastrula or sterogastrula without archenteron (primitive cavity). Archenteron is formed later by splitting the internal cell mass. Ingression is of two kinds.
what is Unipolar Ingression in fertilization?::
Inward migration of cells is restricted to the vegetal pole only. It is seen in Obelia.
(b) Apolar Ingression:
Inward migration of cells occurs from ail sides of the blastoderm (wall of blastula). It occurs in Hydra.
The term delamination means ‘splitting off’. Delamination is a process in which the separation of a layer of cells occurs from the original layer of the blastula. It occurs during gastrulation of chick and rabbit.
In all the triploblastic animals, three germs layers namely ectoderm, mesoderm and endoderm, are formed by the morphogenetic movements.
In human, the germ lavers are formed so quickly that it is difficult to determine the exact sequence of events.
Formation of Embryonic Disc in fertilization?:
We have seen that early blastocyst consists of inner cell mass and trophoblast. The inner cell mass contains cells called stem cells which have the potency to give rise to all tissues and organs. The cells of the inner cell mass differentiate into two layers around 8 days after fertilization, a hypoblast and epiblast. The hypoblast (primitive endoderm) is a layer of columnar cells and epiblast (primitive ectoderm) is a layer of cuboidal cells. The cells of the hypoblast and epiblast together form a two layered embryonic disc.
Formation of Amniotic Cavity in fertilization?:
A space appears between epiblast and trophoblast, called amniotic cavity filled with amniotic fluid. The roof of this cavity is formed by amniogenic cells derived from the trophoblast, while its floor is formed by the epiblast.
Formation of Extra-embryonic Coelom:
The cells of the trophoblast give rise to the mass of cells called the extra-embryonic mesoderm. This mesoderm is called extraembryonic because it lies outside the embryonic disc. It does not give rise to any tissue of the embryo itself. The extraembryonic mesoderm is differentiated into outer somatopleuric extra-embryonic mesoderm and inner splanchnopleuric extraembryonic mesoderm. Both these layers enclose the extraembryonic coelom.
Formation of Chorion and Amnion:
At this stage, two very important embryonic membranes, the chorion and amnion, are formed. The chorion is formed by the somatopleuric extra-embryonic mesoderm inside and the trophoblast outside. The amnion is formed by the amniogenic cells inside and splanchnopleuric extraembryonic mesoderm outside.
As mentioned earlier the amniogenic cells are derived from the trophoblast. Later on chorion becomes the main embryonic part of the placenta. The chorion also produces human chorionic gonadotropin (hCG) an important hormone of pregnancy.
Amnion surrounds the embryo creating the amniotic cavity that is filled with amniotic fluid. The amniotic fluid serves as a shock absorber for the foetus, regulates foetal body temperature and prevents desiccation.
Formation of Yolk Sac:
Flattened cells arising from the hypoblast spread and line inside the blastocoel. These are endodermal cells lining the primary yolk sac. With the appearance of the extraembryonic mesoderm and later of the extraembryonic coelom, the yolk sac (embryonic membrane) becomes much smaller than before and is now called the secondary yolk sac.
This change in size is due to change in the nature of the lining cells. These cells are no longer flattened but become cubical. The secondary yolk sac consists of outer splanchnopleuric extra embryonic mesoderm and inner endodermal cells.
The yolk sac is a source of blood cells. It also functions as a shock absorber and helps prevent desiccation of the embryo.
Formation of Primitive Streak in fertilization?:
Gastrulation involves the rearrangement and migration of cells from the epiblast. A primitive streak which is a faint groove on the dorsal surface of the epiblast is formed. It elongates from the posterior to the entire part of the embryo. The primitive streak clearly establishes the head and the tail ends of the embryo as well as its right and left sides.
Formation of Embryonic Layers:
After the formation of the primitive streak, cells of the epiblast move inward below the primitive streak and detach from the epiblast. This inverting movement is called invagination. Once the cells have invaginated, some of them displace the hypoblast forming the endoderm. Other cells remain between the epiblast and newly formed endoderm forms the mesoderm. Cells remaining in the epiblast form ectoderm.
Thus three germ layers, namely endoderm, mesoderm and ectoderm are formed which give rise to all the tissues and organs of the body.
The primitive germ layers formed during gastrulation spilt into groups of cells called as primary organ rudiments and the process of formation of organs from the three germ layers is known as organogenies. The primary organ rudiments further subdivide into secondary organ rudiments which are the initial stages in the formation of organs and their parts. A this stage the embryo acquires resemblances with the adult or a larva.
153. FUNGAL DISEASES