Fertilizaton is the fusion of a haploid male gamete and a female gamete to form a diploid cell, the zygote, is called fertilization.
First Newport studied the entry of sperm into the egg in frog. O. Herting observed the fusion (amphimixis)of egg nucleus and sperm nucleus in Sea urchin. Fusion of gametes in the fertilization is called as syngamy. Fusion of cytoplasm is plasmogamy and fusion of nucleus is karyogamy.
TYPES OF FERTILIZATION
A. On the basis of place of fertilization.
(a) Internal fertilization - In this fertilization occurs in the body of female. By the process of copulation (Mating) male deposits sperms in the body of female. This process is mainly seen in terrestrial animals and endoparasites. e.g. Few Nematods, Insects, Birds, Reptiles and mammals.
(b) External fertilization - Fertilizaton occurs outisde the body of female. Mostly in water or in cocoon in few cases. eg. Most of the invertebrates, frog, fish etc.
B. On the basis of individuals involved
(a) Self fertilization - It is only possible when animal is hermaphrodite i.e. It have both sex organs. Self male gametes fertilizes female gametes. It is rare in animal kingdom. eg. Tapeworm.
(b) Cross fertilization - In this gametes of different individuals fuses with each other. It occurs in unisexuals and most of the hermaphrodites animals also. Hermaphrodites ensures it by protogyny (Maturation of egg first) or protoandry (Maturation of sperms first).
MECHANISM OF FERTILIZATION
Fertilization completes in three steps.
APPROACH OF SPERM TO EGG : Sperms fuses with egg just by chance,
certainadaptations are found in animals to increase the chances of fusion of sperms and egg. These are also called as approximation of gametes.
These are -
Sperms are produced and released in large amount : In humans 100 million sperm/ml
of semen are present. In one ejaculation 3 to 5 ml of semen are introduced in female body at a time.
(ii) Size of egg is very large as compaired to sperms : This increase the possiblities of fusion. The size of egg about 0,1 mm while the sperm of human is of 60μ
(B) ATTACHMENT OF SPERM WITH OVUM:
Some special types of glycoproteins are found on the surface of eggs and sperms which are helpful in their attachment.
Fertilzin is found on the surface of egg. Fertilizin was discovered by R.R. Lillie. Antifertilizin are present on the surface of sperms. It is a type of acidic protein. (discovered by Ballinsky). Fertilizin and antifertilizin are specific for each species of animals (species specificity). By the reaction of both sperm and egg, both combine each other, this reaction is known as agglutination. The mechanism resembels like key-lock mechanism.
GAMONS
On the surface of egg and ova some hormones are also present These hormones are known as Gamons. Androgamones are found on surface of the sperms and Gyanogamones are found on the surface of eggs. Androgamones are of two types-
(i) Androgamone I — Conserves the energy of sperms, inactivates them.
(ii) Androgamones II Dissolves the viscous layers of egg and forms rough surface on the egg. This helps in attachment.
Gyanogamones are of two types — Present on egg surface
(1) Gyanogamones I: When sperm reach to the surface of egg then the Gyanogamones I neutrilize the androgamone I. Therefore sperm becomes more active.
(ii) Gyanogamone II: This hormone makes the head of sperm viscous. And viscous sperm head attaches strongly with rough surface of egg. Gamones are hypothetical hormones these are not yet isolated nor their chemical nature is known.
(C) ACROSOME REACTION
The enzymes of acrosome dissolves the egg membranes, head of sperm now attaches with plasma membrane of egg.
After it the inner membrane of Acrosome spread out and form a simple tube. This tube is known as Acrosomal filament. This filament joins and stimulates the plasma membrane of egg.
In mammals the acrosomal filament is not formed because of hyperactivity of sperm. so that the sperm can stimulate the plasma membrane of egg without the aid any filament.
(D) ACTIVATION OF EGG : The stimulus of sperm induces many changes in ovum. These changes are collectively called as egg activation or Gynogenesis.
Joining of sperm to egg plasm membrane activates adenyl cyclase enzyme. It converts ATP to CAMP, CAMP is a second messanger thant induces various developmental chain reactions in egg cytoplasm.
The plasma membrane of egg extends outside at the contact place of sperm and forms a reception cone or fertilization cone. Later the sperm is drawn into the cytoplasm of egg from reception cone. The process of entry of sperm into egg is a type of phagocytosis.
The stimulation of sperm induces 2nd maturation division in egg and egg form a polar body and the ova becomes mature.
Along with entry of sperm cortical granules ruptures & releases mucopolysaccharide. These form a membrane which surrounds the egg it is known as Fertilization membrane. This membrane forms on inner side vitelline membrane. Along with the perivitelline space becomes broad and increases amount of fluid in it. The fertilization membrane and liquid in perivitelline space prevent the entry of another sperm into the egg.
The egg plasma membrane becomes more permeable for Na+, H+ and Ca++ ions. The Ca++ ions neutralize the cytostatic factor. This factor inhibite the cleavage to start in unfertilized egg. The egg is triggered for cleavage after neutralization.
By the stimulation of cytostatic factor sperm induces H+ - Na+ pump in plasma membrane of egg.
Few informosomes activates m—RNA quickly synthesizes different types of enzymes thus metabolic rate of egg increases.
The stimulus of sperm induces the NAD kinase enzyme and this enzyme converts NAD into NADP and due to availability of high NADP the respiratory rate of egg increases. Rapid oxidation of stored food occurs and energy is released for egg metabolism.
(E) AMPHIMIXIS : In most animals head and middle part of sperm enters into egg and tail remain outside but in few animals only head part of sperm enters into egg e.g. Hydra, Neries. In mammals the whole sperm enters into the egg.
MONOSPERMY POLYSPERMY
Generally only one sperm enters into a egg, this phenomenon is known as Monospermy. Rarely more than one sperm enters in one egg, this phenomenon is known as polyspermy. Podyspermy is of two types —
(a) Pathological Polyspermy— All the nucleus of sperms fuse with nucleus of egg. Egg degenerate and embryo do not develop.
(b) Physiological polyspermy— Nucleus of only one sperm fuse with nucleus of the egg and all the other sperms dies in cytoplasm of egg. The dead sperms in cytoplasm of egg are known as "merocyte". It is commonly found in polylecithal eggs of Reptiles, Aves, Prototheria etc.
Just after entry sperm rotates at 180o. Thus middle part of sperm comes in anterior position and head becomes posterior in position.
Sperm and egg nuclei moves in egg cytoplasm, these are called as path.
(i) Sperm penetration path : The movement of sperm nucleus in a straight line from entry site.
(ii) Sperm copulation path - The movement of sperm nucleus towards egg nucleus.
(iii) Egg copulation path -The movement of egg nucleus towards sperm nucleus.
(iv) Cleavage path - The movement of both pronucleus towards animal pole.
Proximal centriole is given to egg cell. Sperms diastal centriole aquires opposite pole position in egg and start to develop spindle fibres, as the mammalian egg lacks centrioles of its own. The centrioles of egg degenerates during second maturation division. The nucleus of sperm is enlarged by absorbing water from cytoplasm of egg. Now this nucleus of sperm is known as male pronucleus.
Egg nucleus also slightly enlarges and now called as female pronucleus, both pronuclei fuses & moves by cleavage path.
The nuclear membranes of both pronucleus degenerate and chromosomes form pairs with each others. With this fusion the amphimixis is completed.
First Newport studied the entry of sperm into the egg in frog. O. Herting observed the fusion (amphimixis)of egg nucleus and sperm nucleus in Sea urchin. Fusion of gametes in the fertilization is called as syngamy. Fusion of cytoplasm is plasmogamy and fusion of nucleus is karyogamy.
TYPES OF FERTILIZATION
A. On the basis of place of fertilization.
(a) Internal fertilization - In this fertilization occurs in the body of female. By the process of copulation (Mating) male deposits sperms in the body of female. This process is mainly seen in terrestrial animals and endoparasites. e.g. Few Nematods, Insects, Birds, Reptiles and mammals.
(b) External fertilization - Fertilizaton occurs outisde the body of female. Mostly in water or in cocoon in few cases. eg. Most of the invertebrates, frog, fish etc.
B. On the basis of individuals involved
(a) Self fertilization - It is only possible when animal is hermaphrodite i.e. It have both sex organs. Self male gametes fertilizes female gametes. It is rare in animal kingdom. eg. Tapeworm.
(b) Cross fertilization - In this gametes of different individuals fuses with each other. It occurs in unisexuals and most of the hermaphrodites animals also. Hermaphrodites ensures it by protogyny (Maturation of egg first) or protoandry (Maturation of sperms first).
MECHANISM OF FERTILIZATION
Fertilization completes in three steps.
APPROACH OF SPERM TO EGG : Sperms fuses with egg just by chance,
certainadaptations are found in animals to increase the chances of fusion of sperms and egg. These are also called as approximation of gametes.
These are -
Sperms are produced and released in large amount : In humans 100 million sperm/ml
of semen are present. In one ejaculation 3 to 5 ml of semen are introduced in female body at a time.
(ii) Size of egg is very large as compaired to sperms : This increase the possiblities of fusion. The size of egg about 0,1 mm while the sperm of human is of 60μ
(B) ATTACHMENT OF SPERM WITH OVUM:
Some special types of glycoproteins are found on the surface of eggs and sperms which are helpful in their attachment.
Fertilzin is found on the surface of egg. Fertilizin was discovered by R.R. Lillie. Antifertilizin are present on the surface of sperms. It is a type of acidic protein. (discovered by Ballinsky). Fertilizin and antifertilizin are specific for each species of animals (species specificity). By the reaction of both sperm and egg, both combine each other, this reaction is known as agglutination. The mechanism resembels like key-lock mechanism.
GAMONS
On the surface of egg and ova some hormones are also present These hormones are known as Gamons. Androgamones are found on surface of the sperms and Gyanogamones are found on the surface of eggs. Androgamones are of two types-
(i) Androgamone I — Conserves the energy of sperms, inactivates them.
(ii) Androgamones II Dissolves the viscous layers of egg and forms rough surface on the egg. This helps in attachment.
Gyanogamones are of two types — Present on egg surface
(1) Gyanogamones I: When sperm reach to the surface of egg then the Gyanogamones I neutrilize the androgamone I. Therefore sperm becomes more active.
(ii) Gyanogamone II: This hormone makes the head of sperm viscous. And viscous sperm head attaches strongly with rough surface of egg. Gamones are hypothetical hormones these are not yet isolated nor their chemical nature is known.
(C) ACROSOME REACTION
The enzymes of acrosome dissolves the egg membranes, head of sperm now attaches with plasma membrane of egg.
After it the inner membrane of Acrosome spread out and form a simple tube. This tube is known as Acrosomal filament. This filament joins and stimulates the plasma membrane of egg.
In mammals the acrosomal filament is not formed because of hyperactivity of sperm. so that the sperm can stimulate the plasma membrane of egg without the aid any filament.
(D) ACTIVATION OF EGG : The stimulus of sperm induces many changes in ovum. These changes are collectively called as egg activation or Gynogenesis.
Joining of sperm to egg plasm membrane activates adenyl cyclase enzyme. It converts ATP to CAMP, CAMP is a second messanger thant induces various developmental chain reactions in egg cytoplasm.
The plasma membrane of egg extends outside at the contact place of sperm and forms a reception cone or fertilization cone. Later the sperm is drawn into the cytoplasm of egg from reception cone. The process of entry of sperm into egg is a type of phagocytosis.
The stimulation of sperm induces 2nd maturation division in egg and egg form a polar body and the ova becomes mature.
Along with entry of sperm cortical granules ruptures & releases mucopolysaccharide. These form a membrane which surrounds the egg it is known as Fertilization membrane. This membrane forms on inner side vitelline membrane. Along with the perivitelline space becomes broad and increases amount of fluid in it. The fertilization membrane and liquid in perivitelline space prevent the entry of another sperm into the egg.
The egg plasma membrane becomes more permeable for Na+, H+ and Ca++ ions. The Ca++ ions neutralize the cytostatic factor. This factor inhibite the cleavage to start in unfertilized egg. The egg is triggered for cleavage after neutralization.
By the stimulation of cytostatic factor sperm induces H+ - Na+ pump in plasma membrane of egg.
Few informosomes activates m—RNA quickly synthesizes different types of enzymes thus metabolic rate of egg increases.
The stimulus of sperm induces the NAD kinase enzyme and this enzyme converts NAD into NADP and due to availability of high NADP the respiratory rate of egg increases. Rapid oxidation of stored food occurs and energy is released for egg metabolism.
(E) AMPHIMIXIS : In most animals head and middle part of sperm enters into egg and tail remain outside but in few animals only head part of sperm enters into egg e.g. Hydra, Neries. In mammals the whole sperm enters into the egg.
MONOSPERMY POLYSPERMY
Generally only one sperm enters into a egg, this phenomenon is known as Monospermy. Rarely more than one sperm enters in one egg, this phenomenon is known as polyspermy. Podyspermy is of two types —
(a) Pathological Polyspermy— All the nucleus of sperms fuse with nucleus of egg. Egg degenerate and embryo do not develop.
(b) Physiological polyspermy— Nucleus of only one sperm fuse with nucleus of the egg and all the other sperms dies in cytoplasm of egg. The dead sperms in cytoplasm of egg are known as "merocyte". It is commonly found in polylecithal eggs of Reptiles, Aves, Prototheria etc.
Just after entry sperm rotates at 180o. Thus middle part of sperm comes in anterior position and head becomes posterior in position.
Sperm and egg nuclei moves in egg cytoplasm, these are called as path.
(i) Sperm penetration path : The movement of sperm nucleus in a straight line from entry site.
(ii) Sperm copulation path - The movement of sperm nucleus towards egg nucleus.
(iii) Egg copulation path -The movement of egg nucleus towards sperm nucleus.
(iv) Cleavage path - The movement of both pronucleus towards animal pole.
Proximal centriole is given to egg cell. Sperms diastal centriole aquires opposite pole position in egg and start to develop spindle fibres, as the mammalian egg lacks centrioles of its own. The centrioles of egg degenerates during second maturation division. The nucleus of sperm is enlarged by absorbing water from cytoplasm of egg. Now this nucleus of sperm is known as male pronucleus.
Egg nucleus also slightly enlarges and now called as female pronucleus, both pronuclei fuses & moves by cleavage path.
The nuclear membranes of both pronucleus degenerate and chromosomes form pairs with each others. With this fusion the amphimixis is completed.
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