Structure and types of eggs

STRUCTURE OF AN EGG OF ANIMALS
DIFFERENT TYPES OF EGGS IN ANIMALS ARE DESCRIBED

1. STRUCTURE AND TYPES OF EGGS G RAVITEJA, LECTURER IN ZOOLOGY YVNR GDC KAIKALUR This Photo by Unknown Author is licensed under CC BY-NC This Photo by Unknown Author is licensed under CC BY-SA-NC

2. STRUCTURE OF EGG (OVUM) Definition: The female haploid gamete formed as a result of meiotic division of the mother cells of ovary (oogonia) is called as egg or ovum. The formation of ovum, thus, is called as Oogenesis.

3. STRUCTURE OF EGG (OVUM) The mature ovum or unfertilized egg is typically more or less spherical and large-sized . It has a plasma membrane called as plasmalemma or oolemma inside which are situated the cytoplasm (ooplasm), yolk ( deutoplasm ) and other organelles and inclusions.

4. STRUCTURE OF EGG (OVUM) i . Plasma Membrane: It is living, dynamic, fluid-mosaic membrane made up of lipids and proteins. ii. Vitelline Envelope: It is the membrane present around the plasma membrane. It is made up of a fibrous mat. It is essential for the species-specific binding of sperm . In mammals, it is a separate and thick extra-cellular matrix called Zona pellucida .

5. STRUCTURE OF EGG (OVUM) iii. Ooplasm: It is differentiated into an outer Ectoplasm, also called egg cortex and inner endoplasm. a. Ectoplasm: It is present beneath the plasma membrane and contains small membrane bound granules called cortical granules which contain mucopolysaccharides . b. Endoplasm: It contains many mitochondria , golgi complexes, RER , late-translating mRNAs , tRNAs , tubulin , actin , enzymes etc. It is rich in large amount of yolk , glycogen granules and fat droplets ( lipochondria ) which are required as food for the developing embryo.

6. STRUCTURE OF EGG (OVUM) iv. Nucleus: Greatly enlarged having large amount of nucleoplasm and a prominent nucleolus - germinal vesicle . It is the part of endoplasm which is clear and without yolk pigment granules granting a polarity to the egg. In mature oocyte, these are concentrated in the half of the egg known as animal hemisphere while the other less pigmented, yolk-laden half is known as vegetal hemisphere . The transitional - marginal zone . The animal pole of the egg also can be recognized by the fact that during maturation, polar bodies are always given off from this end only.

7. Types of Eggs:

8. 1. Based on amount of yolk, there are three types of eggs… a. Microlecithal Eggs: These are small-sized eggs -very small amounts of yolk. Kent (1969) described these as alecithal eggs . Eg : Hydra, Sea Urchin, Amphioxus, Tunicates, Marsupials and Eutherian mammals b. Mesolecithal Eggs: moderate amount of yolk. Eg : Annelids, Molluscs, Petromyzon, Dipnoi and Amphibia c. Macrolecithal or Megalecithal or Polylecithal Eggs: enormous amount of yolk. Eg : Insects, Myxine, Chondrichthyes, Reptiles, Birds, Prototherian mammals Types of Eggs:

9. 2. Based on distribution of yolk, there are three types of eggs in animal kingdom viz., Isolecithal , Telolecithal and Centrolecithal eggs. i . Isolecithal or Homolecithal Eggs: A mount of yolk is very less and is found distributed almost uniformly throughout the egg cytoplasm. These are found in microlecithal eggs. Eg : Echinoderms, Molluscs, Ascidians, Eutherian Mammals Types of Eggs:

10. ii. Telolecithal Eggs: These types of eggs have polarized distribution of yolk in the ooplasm and are found in the mesolecithal and macrolecithal eggs. Thus, the yolk, due to its gravity, is concentrated more in one hemisphere called as vegetal pole and the other with less yolk is called as animal pole. These are again of two types– a. Moderately Telolecithal Eggs: - moderate quantity of yolk which is distributed unevenly. Due to high concentration of yolk in the vegetal hemisphere, the nucleus is shifted more towards the animal hemisphere. Eg : Amphibians b. Heavily Telolecithal Eggs: - D ue 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. Eg : Reptiles, Birds Types of Eggs:

11. iii. Centrolecithal Eggs: Here, the yolk is concentrated in the centre of the egg and the active cytoplasm forms a thin peripheral layer around the yolk. The nucleus lies at the geometric centre of the yolk mass, surrounded by a small amount of cytoplasm. Eg : Insects, Hydrozoans (Coelenterates) Types of Eggs:

12. 3. Based on development potency, eggs are of two types viz., Mosaic and Regulative eggs. 1. Mosaic Egg: In certain eggs, every portion of the egg is predetermined with respect to its potentialities for 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 . Eg : annelids, Molluscs and ascidians). 2. Regulative Egg: In 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 . Eg : Echinoderms and Vertebrates Types of Eggs:

13. 4. On the Basis of Shell, two of types or eggs recognized - Cleidoic eggs and Non-cleidoic eggs. 1. Cleidoic eggs - These eggs contain a thick and hard outermost shell. This hard shell is permeable for gases. Yolk, Salts and Water is present in large amount in cleidoic eggs. Cleidoic egg is a terrestrial adaptation. Eg : Insects, Birds, Reptiles and Prototherian mammal 2. Non-Cleidoic eggs - Egg membranes are soft in these eggs Eg : All viviparous animals and in oviparous animals which lays eggs in water. Types of Eggs: