Structure of mature ovum

In this presentation speaker explains about the structure of ovum in vertebrata.

1. Structure of Mature Ovum G. Ramasubbu Assistant Professor SBK College, Aruppukottai

2. Structure of Mature ovum The Mature egg is covered by a cell membrane or plasmalemma . In most animals except sponges and coelenterates the plasma membrane is surrounded by certain special protective accessory egg membranes. Depending on their origin these may be subdivided into following types.

3. Primary membrane. These membrane develop in the ovary by the ovum itself between the oocyte and follicle cells around the plasmamembrane . In insect, mollusca , amphibians and birds the primary membrane is knows as vitelline membrane. In teleost fishes it is usually called the chorion .

4. The primary envelop of shark, some bony fishes, some amphibians and some reptiles have striated appearance and is called zona-radiata . In mammals, the unstriated and modified zonaradiata is called zonapellucida.It is formed by the joined efforts of ova and follicle cell. All these primary membrane usually adhere closely to the surface of the plasma membrane of oocyte. But at later stage a fluid filled space may appear between the egg plasma membrane and the primary membrane is called pre- viteline space.

5. Secondary membranes These are produced as a basement membranes by layer of follicle cells surrounding ovum. In insect ascidians and cyclostomes membrane is secreted by the follicle cells on the top of vitelline membrane. This second membrane is also called the chorion. An egg while escaping from the mammalian ovary carries with it on the surface of zona pellucida a layer of follicle cells known as corona radiata .

6. Tertiary membranes These membranes are secreted by oviduct of the maternal genital organs when the egg passes from the ovary to the exterior. The oviparous sharks and rays the egg is surrounded by allbumen and a hard shell secreted by shell glands of the oviducts. The shell is drawn out into egg and sometimes serves to make the eggs to adhere to one another and submerged objects such as plants.

7. Most complicated egg membranes are found in the egg of birds where no less than five membranes can be distinguished. In addition to innermost vitelline membrane which is primary in nature , Albumen, Shell membranes, and Shell.

8. Significance of egg membranes The extraneous envelope of egg of different ooviparous vertebrates provide the protection to the contents of egg from different ecological hazards. Zona pellucida in viviparous mammals check polyspermy, prevent egg fusion and maintains necessary means of maintaining normal cleavage of egg. The cortical region of cytoplasm are spherical bodies, the cortical granules surrounded by simple membrane and contain acid mucopolysaccharide .

9. Golgi body is related to the secretory activity whereas the mitochondrial aggregated into mitochondrial cloud. They are bags of oxidative enzymes involved in cell respiration. Ovum has a minute centrosome close to the nucleus. It is active during cell division. Typical endoplasmic reticulum in the form of double lamellae and numerous vesicle with ribosome attached to their surfaces present in a mature ovum.

10. The yolk is nutritional but it also exercises influences on the size and type of eggs, pattern of cleavages morphogenetic movements of blastomeres during gastrulation and the mode of development . Nucleus is essential to the life , growth and reproduction of a cell.