Mechanism of Ovary Development
The developing egg is known as an oocyte of which several stages can be recognized in the ovary. These are:-
❑ Oocyte 1: This is larger than the oogonium, spherical, and with a central nucleus, having 2 or 3 nucleoli. The cytoplasm is basophilic.
❑ Oocyte II: There is a further increase in the size of the oocytes and the nucleus undergoes divide two or three times consisting of six to nine nuclei. Several nuclei move towards the periphery of the nuclear membrane.
❑ Oocyte iii: This is still larger, distinguished by the appearance of a thin layer of follicular cells around the cytoplasm, a few nuclei pass out of the nuclear membrane.
❑ Oocyte iv: There is a further increase in the size of the oocyte and a large number of small, clear vacuoles called the yolk vesicles appear in the periphery of the ooplasm.
❑ Oocyte v: As the oocyte grows further, the yolk vesicle increases in number and fills the entire ooplasm. A vitelline membrane or zona radiate is also clearly visible, between the ooplasm and follicular layer or zona granulose.
❑ Oocyte vi: This is characterized by the appearance of yolk in the form of minute granules in the extra vesicular ooplasm. The yolk appears first in the peripheral region and accumulates there in large numbers. The yolk granules then proceed centrally the whole ooplasm becomes impregnated with them. The yolk granules fuse to form large globules, and the oocyte is of considerable size.
❑ Oocyte vii: There is heavy deposition of fairly large yolk globules. The yolk vesicles also fuse and become larger. The nucleus migrates gradually towards the periphery. Some yolk vesicles are pushed towards the periphery of the egg and form cortical alveoli.
❑ Ripe egg: It is full of a large amount of yolk vesicles that may lie scattered in it. The nucleus is generally not visible in the ripe egg. An ovary contains several ripe at a time during the spawning period. A mature egg is surrounded by an external layer of theca, followed by the follicular epithelium and the innermost, zona radiate and zona granulose. The granulose cells are believed to be responsible for the deposition of yolk in the developing ovum and responsible for the secretion of ovarian hormones.
