Oocyte maturation

 

The ovary is the site of the reduction of germ cells in the female. The ovary is covered with germinal epithelium followed by a peripheral cortex and an inner medulla. The  germ cells are located in the cortex. The innermost part of the cortex forms the stroma of the ovary which contains fibrocellular tissue. The primary oocyte destined to be released in one month is surrounded by pregranulosa cells which are flat epithelial cells. This primary oocyte must undergo maturation, or ripening process for its release. The primary oocyte acquires crescentic shaped mitochondrial and fat globules; the nucleus also become eccentric. The oocyte undergoes germinal vesicle breakdown which heralds the resumption of meiosis. The cells surrounding the primary oocyte start to proliferate. They later on form a space which accumulates fluid (follicular fluid). The follicle represents the space between the primary oocyte and the cells surrounding the oocyte. Some cells remain attached to the primary oocyte and they are known as the cumulus oophorus. The innermost single layer that surrounds the oocyte is the corona radiata. Cells that surround the follicles at its margin are called the granulosa cells. The granulosa cell layer is surrounded by two external layers known as the theca interna (more internal) which is vascular and rich in cells. You also have the tunica externa which is fibrous in nature. The oocyte itself develops a thick covering around it called the zona pellucida and its nucleus becomes eccentric. The follicles increases in size as the maturation of the oocytes becomes advanced and it acquires fluid called the liquor folliculi or follicular fluid. The follicle then approaches the innermost part of the ovary, and it is called the Graafian follicle after de Graf who first described it. It then ruptures as a result of acccumulation of fluid and which allows the release of the egg (in a process known as ovulation) to occur. The egg at this stage has already completed its first meiotic division and has already become the secondary oocyte and has extruded its first polar body which can be visualized at the perivitelline space. The cytoplasm of the oocyte released contains considerable amount of deutoplasm necessary for nutriment of the egg as it slides into the peritoneal cavity into the oviduct. Once the first polar body is extruded, the secondary oocyte thus formed as the mature egg that is ready to be fertilized. The meiotic division becomes arrested secondarily until sperm penetration of the egg in the oviduct which then leads to the extrusion of the second polar body. After the extrusion of the second polar body the secondary oocyte become the ovum.

The corpus luteum is formed as an active endocrine gland after the release of the egg from the follicle. The follicular theca externa becomes converted into capsule and trabeculae and the theca interna forms blood vessels and certain small paraluteal cells. The granulosa cells become transformed into large polyhedral cells which become vacuolated and contain the pigment.


      

 

 
Oocyte maturation

 

 

 

 

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