Immunology | Immune system| Immunity

Significance of fertilization

1. It stimulates the secondary oocyte to undergo second maturation division to release second polar body and to form haploid ovum.
2. It restores the diploidy in the zygote.
3. Fertilization membrane prevents polyspermy.
4. Metabolic activities are increased as more mitochondria are available.
5. It combines the characters of two parents and introduces variations.So helps in evolution.
6. Centrioles of sperm form spindle to initiate the cleavage of zygote .
7. Sex chromosome of sperm is either X or Y and helps in sex determination.
8. Copulation path sets the axis of division.

Some of the other importance of  fertilization

1.It rises to formation of a new organism/In case of plants it develops into a fruit.
2. It is the successful formation of zygote by the fusion of sex_ germ cells that are female and male gametes.
3. It leads to a successful reproduction attempt.
4. It ultimately leads to a replication of a new organism as an offspring.

Fertilization

Fertilization involves the fusion of haploid male and female gamete to form diploid zygote.Fertilization in the human beings is internal and takes place in the proximal part of fallopian tube of the female. Fertilization involves following processes :

1. Approach of sperm to ovum:

During the copulation, male inserts its erectile penis in the vagina of female and releases about 3.5 ml of seminal fluid. This process is called ejaculation. Seminal fluid contains as many as 400 million sperms.This ensures the reaching of number of sperms.This ensures the reaching of a number sperms to the ovum as many sperms are killed by the acidity of female genital tract and many sperms are engulfed by the phagocytes of the vaginal epithelium so that only about 100 sperms reach the fallopian tube. The sperms swim in the seminal fluid by lashing movements of their tail at the rate of 1-4 mm per minute.The sperms swim towards the fallopian action of the uterus and peristaltic movements of the fallopian tube. Capacitation is the phenomenon of physiological maturation of sperms inside the female genital tract. It occurs in the presence of viscous fluid secreted from the secretory cells of epithelial lining of oviducal mucosa.It takes about 5-6 hours.
Ovum is released from the graafian follicle of ovary on the 15th day of menstrual cycle and the process is called ovulation (the expulsion of an ovum from the ovary). Ovum is trapped by the fimbrae of the ampulla of fallopian tube. Ovum moves in the tube towardsthe uterus by peristalsis and ciliary action. At the time of ovulation,egg is at secondary oocyte stage.
Fertilizability and viability of gametes are limited varying from few minutes to a few hours or days. The fertilizability of human sperms in the female genital tract is of 12 to 24 hours while its survival value is 3 days.Secondly the ovum is non motile and the energy contents of sperm must approach the egg as quickly as possible. Fertilizability period of ovum is only 24 hours though it can live for 72 hours.

2. Penetration of sperms:

Secretions of seminal vesicles,prostate gland and cowper's gland form the major part of seminal fluid. These secretion activate the sperms in fertilizing the egg and neutralise the acidity if vagina.
The sperm generally comes in contact with ovum in the animal pole while the opposite of the ovum is called as vegetal pole.Ovulation in  the human female occurs at secondary oocyte stage in which meiosis-I has been completed and first polar body has been released but second maturation is yet to complete. Penetration of sperm is chemical mechanism. In this acrosome of sperm undergoes acrosomal reaction and release certain sperm lysins which dissolves the egg envelopes locally and make the path for penetration of the sperm. Sperm lysins are acidic proteins.

3. Cortical reaction:

The penetration of sperm into the egg initiates a series of processes like:
Cortical granules appear in the egg cortex. The vitelline membrane starts lifting the cortical surface of the egg. The produces a perivitelline space within the vitelline membrane.Cortical granules are extruded in the peri vitelline space by exocytosis and some of these are attached along inner surface of the vitelline membrane which now thickens and becomes imperviuos to many other sperm entry. It is now called fertilization mambrane. It prevents the polyspermy.
4. Fusion of gametic nuclei:
The sperm entry stimulates the stimulates the secondary oocyte to undergo meiotic II division which produces ovum and second polar body.Inside the ovum,sperm nucleus takes definite path called copulation path. The centrioles of the middle piece of the sperm form a spindle. The nuclear membrane of the gametic nuclei degenerates and two sets of chromosomes initially lie on two poles of the spindle but later these sets of chromosome mix up and the process called ''amphimixis''. The fertilized egg is now called zygote while the zygote nucleus is called synkaryon.

Embryonic development

Embryonic development (Embryogenesis) includes a definite series of phases which are fundamentally similar in all sexually reproducing organisms,and transform one-celled zygote to a multicellular and fully formed developmental stage till hatching or birth. Such a remarkable similarity of embryonic development proves that all metazoans are interrelated and have common ancestory.

Phases of Embryonic Development

Embryonic development involves five dynamic changes and identifiable processes:

1. Gametogenesis : 

Gametogenesis is of two types 
(a) Spermatogenesis- formation of sperms.
(b) Oogenesis- formation of ova.

2.Fertilization:

Menstrual (Ovarian) cycle

Reproductive period of human female(about 30 to 33 yrs)extends from puberty(10-14 yrs) to menopause(45-55 years).During this reproductive period,ovaries and female reproductive tract undergo a series of cyclic changes which are primarily meant to prepare them for fertilization and pregnancy and collectively form the menstrual cycle.Most peculiar feature of this cycle is the periodic vaginal bleeding called menstruation which lasts for about four days.
Period: The lenght of menstrual cycle varies widely in women,but on average it is completed in 28 days .In a female ,successive cycles may vary in lenght by 1 to 2 days.It is absent during pregnancy,may be suppressed during lactation and permanently stops at menopause.
Phase: Menstrual cycle is divided into four phases:

1.Follicular or post-menstrual or pre-ovulatory phase.It follows  the menstrual phase and lasts for about 10 12 days(5th to14th day of menstrual cycle).It involves following changes:
(a) Under the stimulation of FSH-RF of hypothalamus,there is increased secretion of FSH from anterior pituitary.
(b) FSH stimulates the change of a primary follicle of the ovary into graafian follicle.
(c) Follicular cells of Graafian follicle secrete estrogens.

2.Ovulaotry phase: It involves the ovulation from the Graafian follicle of ovary.It occurs midway between two menstrual cycles on the 15th day of the onset of the menstrual cycle.It is caused by increasing turgidity and contraction of smooth muscle fibres around the Graafian follicle. Ovum is received by the fimbrae of the Fallopian tube. Ovum is viable for two days. Ovulation is controlled by the increased level of LH also starts the change of empty Graafian follicle into corpus luteum and secretion of progesterone from corpus.

3.Luteal or Progestational or Pre-menstrual or Secretory phase or Post-ovulatory phase:
It lasts for about 12-14 days and extends from 16th to 28th day of the menstrual cycle.It is characterised by following changes:
(a) Corpus luteum(yellow body) formed from empty Graafian follicle,increases in size,so is called luteul phase.
(b) Corpus luteum secretes progesterone hormone whose level in blood gradually increases.

4.Menstrual phase: It lasts for about 3-5 days and extends from 1st to 4th day of the menstrual cycle.When the ovum remains unfertilized,then the corpus luteum starts degenerating.The level of progesterone in the blood declines.The uterine tissues fail to be maintained. Then the unfertilized ovum along with ruptured uterine epithelium,about 50-100 ml of blood and some mucus is discharged out through the vaginal orifice and is called menstrual flow or menstruation.

Decrease in the level of progesterone and estrogens in the blood stimulates the hypothalamus and the anterior pituitary to release FSH-RF and FSH respectively. FSH starts the follicular phase of next menstrual cycle.
However, if  fertilization occurs then blastula undergoes implantation and a chorionic placenta is formed between the foetus and the mother. Placenta secretes Human Chorionic Gonadotrophin (HCG) which maintains the corpus luteum and the progesterone secretion continues. Progesterone maintains pregnancy and prevents ovulation during gestation period.

ORGANS FROM ECTODERM:

The ectoderm gives rise to the outer layer of the skin over the entire surface of the embryo.
The neural tube into which the medullary groove develops loses its original connection with the surface.Anteriorly it becomes enlarged and forms the brain,the remaining portions developing in to the spinal cord.The thickening of the walls of the spinal tube, which forms the cord,diminishes the central cavity,until it becomes reduced to a fine canal Canalis centralis.
The anterior portion of the tube becomes divided by slight constriction into forebrain ,midbrain, and hindbrain.
Hind brain becomes widened from side to side especially infront its flair and the sides thicken which develops into cerebellum,which remains thin and membranous and becomes thrown into series of folds which support a mass of blood vessels, called choroid plexus.
The portion of the hindbrain which does not form cerebellum is converted into medulla.The midbrain grows out dorsally and laterally into a pair of hollow processes called optic lobes.
The forebrain soon becomes separated into 2 parts, the Thalamencephalon behind and the cerebral hemisphere which grow out from the latter in front hollow out growth- the infundibulum ,which extends downwards.
The sides of thalamencephalon give rise to optic vesicles,which grow out until they come in close contact with the surface ectoderm.
The distal end of the vesicles widens cut and forms Retina of eyes stalk giving rise to the optic nerve.
The lining of the mouth cavity is formed from an invagination of ectoderm-the stomoderm and similar ectodermal invagination Proctoderm forms the lining of a small part of the post end of the alimentary canal.
 The lens,cornea,retina of eye,vesicle of inner ear also take thin origin from this layer.

ORGANS FROM ENDODERM:

Endoderm forms the lining of the alimentary canal and lining of all organs,which arise as outgrowths from it.It forms liver which at the beginning appear as an out pocketing of the ventral side near anterior end.It becomes folded and branched to form number of clusters of tubules, all emptying into into a common canal -the bile duct. A lateral  outgrowth of the bile duct forms the gall bladder forms the gall bladder. The pancreas arises in the same way as the liver, but as a pair of outgrowths instead of one,but they form a single organ and their ducts later become connected with the bile duct.

Bladder arises as an outgrowth of ventral side of the alimentary canal. Lungs appear as a pair of pouches from the sides of aesophagus. The Gill slits in the frog appear in the form of five solid outgrowths on each side of the anterior portion of archenteron.

 Frog Embryology

Fertilization in case of frog takes place by the process of pseudocopulation because there is no external copulatory organ present in case of male and the female are in close proximity during the process it has the name pseudocopulation.

The egg gets surrounded by many sperms and there is specific reaction called as fertilizin-antifertilizin reaction between the two.(Fertilizin is present in the jelly layer of the egg and anti-fertilizin is secreted by the acrosome of the sperm).The sperm traces its path through the jelly layer and gets attached to the vitelline membrane.
The next reaction which takes place is acrosomal reaction followed by cortical action after which the vitelline envelope is converted into thick fertilization membrane.This prevents polyspermy.As soon as the sperm enters the ova(i.e.,the nucleus of sperm enters the egg) the second meiotic division is completed resulting in the release of the second polar body.
The space between the fertilization membrane and PM of the egg is called as peristelline space which is filled with the perivitelline fluid. In this fluid,the egg can rotate freely and also this space allows free exchange of gases.


The egg pronucleus and the female pronucleus fuse together to form the zygotic nucleus,this process is known as Amphimixis or Syngamy.
The entry of the sperm causes formation of a Grey crescent (which is a characteristic of an amphibian embryo);caused due to mixing of cytoplasmic granules or in other words due to the cytoplasmic movement because of the sperm entry.Grey crescent materials the functions as an organizer because if it is removed from the embryo,the embryo fail to develop further. At the same time if a normal embryo is grafted with another Gery crescent then two embryos develop.

Thus,formation of grey crescent fixes up final symmetry of the egg and future embryo.Also grey crescent marks the future dorsal side of the embryo.The sperm penetrates the egg perpendicular to the cortex and after penetration moves perpendicularly along the radius of the egg.This path of sperm in the egg cortex, is called as penetration path.After crossing the cortex,the sperm changes its direction and moves towards the egg nucleus.This changed path is also copulation path because it leads to the copulation (fusion) of the sperm pronuclei.
After fusion of both male and female pronuclei, a zygote nucleus is formed i.e.,a zygote will be formed
Further development of zygote proceeds with its cleavage and formation of blastula,gastrula and so on

STRUCTURE OF SPERMATOZOA:

        Mature spermatozoa of a frog is made up of 3 parts i.e., head,middle piece and tail.Middle piece region is very small as compared to that of the other sperms,The head consists of acrosome and large nucleus. The acrosome secretes enzymes which helps in penetration into the egg. The middle piece of spermatozoon is composed of almost entirely of mitochondria and centriole(Centriole helps in the spindle formation during cleavage).The mitochondrial apparatus supplies the required amount of energy  in the form of ATP.The tail of sperm is flagellar and very long.It helps in locomotion and thus is very essential for the mobility of the sperm.


1.Plasma membrane
2. Outer acrosomal membrane
3.Acrosome
4. Inner acrosomal membrane
5. Nucleus
6. Proximal centriole
7. Rest of the distal centriole
8. Thick outer longitudinal fibers
9. Mitochondrion
10. Axoneme
11. Anulus
12 . Ring fibers

A.Head
B. Neck
C.Mid piece
D. Principal piece
E. Endpiece