The Structure of a typical Ovum
Ovum is the female gamete. lt stores food required for the entire process of development in the form of yolk. lt has three important f unctions:
1. lt supplies a haploid set of chromosomes to the future embryo.
2. lt contributes almost all cytoplasm to the zygote.
3. lt supplies food to the developing embryo.
Shape and Size
Typically, the eggs are spherical or ovoid in shape. But in a few animals like insects, the eggs are elongated and cylindrical in nature. Eggs are generally large rthan the sperms and average somatic cells. The size of a mature egg depends on the amount of yolk present in it. The smallest known egg is that of mouse (0.07mm); the birds possess larger eggs. Ostrich lays the largest egg having a diameter of about 85 mm.
The egg is covered externally by a plasma membrane or plasmalemma. Within the plasma membrane is the granular cytoplasm
Organization of Egg Cytoplasm
The cytoplasm of egg cell is known as ooplasm. lt is granular and contains in addition to the usual cellular organelles certain other inclusions Iike yolk, pigments and cortical granules. The peripheral layer of ooplasm is more viscous and gelatinous. lt is known as the egg cortex which is provided with many microvilli and cortical granules. The microvilli are formed by the outpushings of the plasmalemma and they help in transportation of substances from the outside into the ooplasm during the development of egg. The cortical granules are very small spherical bodies varying in diameter from 0.8 µm to 2µm . They are
rnembrane bound and are formed from golgi complex. They contain homogeneous and granular polysaccharides. Cortical granules are present in the eggs of sea urchins, frogs, fishes, bivalve molluscs, several annelids and certain mammals.
Yolk:Nutritive substances are stored in the cytoplasm of egg in the form of yolk or deutoplasm. This stored food is utilized by the embryo for its early development. The process of formation of yolk is known as vitellogenesis. The yolk is a complex material consisting of proteins, fats, carbohydrates, inorganic salts, vitamins, enzymes, pigments and water. The yolk may be called "protein yolk" when it has more proteins than lipids, or " fatty yolk" when it has more fat contents than the proteins. Most animal eggs contain both kinds of yolk. Since the yolk is heavier ,large quantities of yolk, such as those of the frog and chick, the accumulation of yolk in one region is so marked that they are known as telolecithal eggs. In eggs containing lesser amount of yolk, like those of Amphioxus and man, the york is distributed more uniformry, hence they are known as isolecithatal or homolecithal.
Pigment granules are present in the cytoplasm of eggs of many species. The granules may be brown, black, red, yellow, green or- grey in colour. As the pigment granules are not common to all eggs, they do not play any significant role in development.
Polarity
The constituents of egg are not uniformly distributed throughout the cytoplasm. These are distributed in such a way that two poles distinct can be identified in the egg. These pores are known as animal pole and vegetal pore. The cytoplasm is concentrated in the upper portion or animal hemisphere and the yolk material is concentrated in the lower portion or vegetal hemisphere. A plane passing through these two poles constitute the polar axis. The nucleus is always located in the polar axis, more or less towards the animal pole. The yolk shows a gradation from the animal pole towards the vegetal pole. There is also a metabolic gradation along the polar axis. Metabolic processes are highest at the animal pole and progressively diminish towards the vegetal pole
Classification of Egg
1. On the Basis of the Amount of yolk
Eggs are grouped into three types on the basis of the amount of yolk present in them.
1. AlecithalEgg;When the egg contains no yolk, it is called alecithal egg.
Eg. The eggs ofeutherian mammals
2. Microlecithal Egg:When the egg contain. small or negligible amount of yolk it is said to be microlecithal.Romer and Balinsky named these eggs as oligolecithal eggs Eg'.Amphioxus, Tunicates
3.Mesolecithal egg: In amphibian, Dipnoi and Petromyzon the amount of yolk present is moderate and is not high Hence these eggs are also named as mesolecithal eggs
4. Macrolecithal or Megalecithal Egg
When the egg contains large amount of yolk it is said to be macrolecithal or megalecithal egg. It is also called Polylecithal egg egg.Eg. boney fishes' amphibians,reptiles and birds.
tnonotentcs, etc
1. On the Basis of the distribution of yolk
a. Isolecithal or Homolecithal Egg:ln isolecithal eggs, the very little amount of yolk present is uniformly distributed throughout the ooplasm (eg.. echinoderms, Amphioxus, mammals). This condition is usually observed in eggs with very little amount of yolk.
b. Telolecithal Egg:In eggs containing moderate or large quantity of yolk, the distribution of yolk is not uniform. lt is concentrated more towards the vegeial pole. Such a type of egg, in which the yolk is concentrated towards one pole, is called telolecithai egg.
Telolecithal eggs may further classified into three types:
i.Slightly Telolecithal This type of egg contains only a small quantity of yolk which
is distributed unevenly. The vegetal pole has the highest concentration and the animal pole the lower (e.g. eggs of fishes).
ii.Moderately Telolecithal egg
This type of egg contains a moderate quanilty of yolk which is
Distributed unevenly. Due to high concenteration of yolk in the vegetal hemisphere, the nucleus is shifted more towards the animal hemisphere
(eg. amphibian egg).
iii.Extremely Telolecithal Egg
ln this type of egg, due to the heavy deposition of yolk, the entire vegetal hemisphere and a major portion of the animal hemisphere are occupied by yolk. Due to this extremely uneven distribution of yolk, the ooplasm and nucleus are displaced towards the animal pole (e.9. reptilian and avian eggs).
3. Centrolecithal Egg
Egg of many arthropods and some coelenterates are described as centrolecithal. They are relatively large and elongate and have a very great amount of yolk. The nucleus lies at the geometric centre of the yolk mass, surrounded by a small amount of cytoplasm. A thin cytoplasmic layer covers the surface of the yolk. Fine strands of cytoplasm extend from the peripheral layer to the zone occupied by the nucleus.
Mosaic and Regulative Eggs
a.Mosaic Egg: ln certain eggs, every portion is predetermined with respect to its potentialities foi' further development
lf a small portion of such an egg is removed, a defective embryo iS formed, This is because removal of a portion results in a permanent loss from the egg. The remaining portion of the egg cannot make compensatory development to make good the lost part. Such an egg, in which the future developmental potentialities are predetermined in the form of a mosaic, is called mosaic or determinate egg (e.g annelids, Molluscs and ascidians).
b.Regulative Egg
ln vertebrates and most of the invertebrates, the developmental potentialities are not predetermined in the eggs. Removal of a small portion of the egg, or even one or two early blastomeres will not affect the normal development. This type of egg in which the future developmental potentialities are not predetermined is known as regulative or indeterminate egg.
Ovum is the female gamete. lt stores food required for the entire process of development in the form of yolk. lt has three important f unctions:
1. lt supplies a haploid set of chromosomes to the future embryo.
2. lt contributes almost all cytoplasm to the zygote.
3. lt supplies food to the developing embryo.
Shape and Size
Typically, the eggs are spherical or ovoid in shape. But in a few animals like insects, the eggs are elongated and cylindrical in nature. Eggs are generally large rthan the sperms and average somatic cells. The size of a mature egg depends on the amount of yolk present in it. The smallest known egg is that of mouse (0.07mm); the birds possess larger eggs. Ostrich lays the largest egg having a diameter of about 85 mm.
The egg is covered externally by a plasma membrane or plasmalemma. Within the plasma membrane is the granular cytoplasm
Organization of Egg Cytoplasm
The cytoplasm of egg cell is known as ooplasm. lt is granular and contains in addition to the usual cellular organelles certain other inclusions Iike yolk, pigments and cortical granules. The peripheral layer of ooplasm is more viscous and gelatinous. lt is known as the egg cortex which is provided with many microvilli and cortical granules. The microvilli are formed by the outpushings of the plasmalemma and they help in transportation of substances from the outside into the ooplasm during the development of egg. The cortical granules are very small spherical bodies varying in diameter from 0.8 µm to 2µm . They are
rnembrane bound and are formed from golgi complex. They contain homogeneous and granular polysaccharides. Cortical granules are present in the eggs of sea urchins, frogs, fishes, bivalve molluscs, several annelids and certain mammals.
Yolk:Nutritive substances are stored in the cytoplasm of egg in the form of yolk or deutoplasm. This stored food is utilized by the embryo for its early development. The process of formation of yolk is known as vitellogenesis. The yolk is a complex material consisting of proteins, fats, carbohydrates, inorganic salts, vitamins, enzymes, pigments and water. The yolk may be called "protein yolk" when it has more proteins than lipids, or " fatty yolk" when it has more fat contents than the proteins. Most animal eggs contain both kinds of yolk. Since the yolk is heavier ,large quantities of yolk, such as those of the frog and chick, the accumulation of yolk in one region is so marked that they are known as telolecithal eggs. In eggs containing lesser amount of yolk, like those of Amphioxus and man, the york is distributed more uniformry, hence they are known as isolecithatal or homolecithal.
Pigment granules are present in the cytoplasm of eggs of many species. The granules may be brown, black, red, yellow, green or- grey in colour. As the pigment granules are not common to all eggs, they do not play any significant role in development.
Polarity
The constituents of egg are not uniformly distributed throughout the cytoplasm. These are distributed in such a way that two poles distinct can be identified in the egg. These pores are known as animal pole and vegetal pore. The cytoplasm is concentrated in the upper portion or animal hemisphere and the yolk material is concentrated in the lower portion or vegetal hemisphere. A plane passing through these two poles constitute the polar axis. The nucleus is always located in the polar axis, more or less towards the animal pole. The yolk shows a gradation from the animal pole towards the vegetal pole. There is also a metabolic gradation along the polar axis. Metabolic processes are highest at the animal pole and progressively diminish towards the vegetal pole
Classification of Egg
1. On the Basis of the Amount of yolk
Eggs are grouped into three types on the basis of the amount of yolk present in them.
1. AlecithalEgg;When the egg contains no yolk, it is called alecithal egg.
Eg. The eggs ofeutherian mammals
2. Microlecithal Egg:When the egg contain. small or negligible amount of yolk it is said to be microlecithal.Romer and Balinsky named these eggs as oligolecithal eggs Eg'.Amphioxus, Tunicates
3.Mesolecithal egg: In amphibian, Dipnoi and Petromyzon the amount of yolk present is moderate and is not high Hence these eggs are also named as mesolecithal eggs
4. Macrolecithal or Megalecithal Egg
When the egg contains large amount of yolk it is said to be macrolecithal or megalecithal egg. It is also called Polylecithal egg egg.Eg. boney fishes' amphibians,reptiles and birds.
tnonotentcs, etc
1. On the Basis of the distribution of yolk
a. Isolecithal or Homolecithal Egg:ln isolecithal eggs, the very little amount of yolk present is uniformly distributed throughout the ooplasm (eg.. echinoderms, Amphioxus, mammals). This condition is usually observed in eggs with very little amount of yolk.
b. Telolecithal Egg:In eggs containing moderate or large quantity of yolk, the distribution of yolk is not uniform. lt is concentrated more towards the vegeial pole. Such a type of egg, in which the yolk is concentrated towards one pole, is called telolecithai egg.
Telolecithal eggs may further classified into three types:
i.Slightly Telolecithal This type of egg contains only a small quantity of yolk which
is distributed unevenly. The vegetal pole has the highest concentration and the animal pole the lower (e.g. eggs of fishes).
ii.Moderately Telolecithal egg
This type of egg contains a moderate quanilty of yolk which is
Distributed unevenly. Due to high concenteration of yolk in the vegetal hemisphere, the nucleus is shifted more towards the animal hemisphere
(eg. amphibian egg).
iii.Extremely Telolecithal Egg
ln this type of egg, due to the heavy deposition of yolk, the entire vegetal hemisphere and a major portion of the animal hemisphere are occupied by yolk. Due to this extremely uneven distribution of yolk, the ooplasm and nucleus are displaced towards the animal pole (e.9. reptilian and avian eggs).
3. Centrolecithal Egg
Egg of many arthropods and some coelenterates are described as centrolecithal. They are relatively large and elongate and have a very great amount of yolk. The nucleus lies at the geometric centre of the yolk mass, surrounded by a small amount of cytoplasm. A thin cytoplasmic layer covers the surface of the yolk. Fine strands of cytoplasm extend from the peripheral layer to the zone occupied by the nucleus.
Mosaic and Regulative Eggs
a.Mosaic Egg: ln certain eggs, every portion is predetermined with respect to its potentialities foi' further development
lf a small portion of such an egg is removed, a defective embryo iS formed, This is because removal of a portion results in a permanent loss from the egg. The remaining portion of the egg cannot make compensatory development to make good the lost part. Such an egg, in which the future developmental potentialities are predetermined in the form of a mosaic, is called mosaic or determinate egg (e.g annelids, Molluscs and ascidians).
b.Regulative Egg
ln vertebrates and most of the invertebrates, the developmental potentialities are not predetermined in the eggs. Removal of a small portion of the egg, or even one or two early blastomeres will not affect the normal development. This type of egg in which the future developmental potentialities are not predetermined is known as regulative or indeterminate egg.
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