Oogenesis

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Oogenesis

The production of female gametes begins before birth. Primordial germ cells migrate from the yolk sac of the embryo to the gonads (reproductive organs), where they differentiate into oogonia. Oogonia undego multiple mitotic divisions, and are arranged in clusters surrounded by a layer of epithelial/follicular cells by the end of the 3rd month of development. The oogonia continues to divide, however some of them start meiosis, arresting at the diplotene stage of meiosis 1(stage when crossing over occurs). These are now called primary oocytes. The number of germ cells in the ovaries reaches its maximum at about 5 months of gestation, at around 7 million. After 5 months, a portion of oogonia and primary oocytes begin to degenerate becoming atretic. By the 7th month, most cells hav degenerated except for those near the surface, these have all entered meiosis 1 and most are surrounded by a layer of epithelial cells. A primary oocyte together with the epithelial cells is called a primordial follicle. At birth, there are an estimated 700,000 to 2,000,000 primordial follicles.


The primary oocytes do not finish the first meiotic divisions until puberty because of oocyte maturation inhibitor secreted by follicular cells. By puberty, more of the cells would have undergone atresia (process of degeneration) and only around 400,000 will remain.

At puberty, 15 to 20 follicles will begin to grow and mature each month, although only one will develop fully and be released. There are 3 stages to this maturation which links closely to the stages in the ovarian cycle (menstruation). These 3 stages are called the primary or pre-antral, secondary or antral, and the preovulatory phase.

The preantral period occurs over the first 5 days of menstruation. During this time, the follicular cells (epithelial) change from flat cells to cuboidal granulosa cells. They replicate rapidly to create a stratified (layered) epithelium. The granulosa cells together with the oocyte is now called the primary follicle. Granulosa cells surrounded by a basement membrane which is surrounded by stromal cells (connective tissues) These stromal cells form the theca folliculi. The granulosa cells secrete a layer of glycoprotiens around the surface of the oocyte forming the zona pellucida. As the follicle continues to grow, the theca folliculi splits into the theca interna, made up of mainly secretory cells and the theca externa, a fibrous capsule.

The granulosa cells secrete fluids which join to form the antrum on about day 5 of the menstrual cycle. The follicle is now termed a secondary follicle and enters the antral phase.  The granulosa cells surrounding the oocyte form the cumulus oophorus. The secondary follicle grows until around day 13 and can be up to 25 mm in diameter when fully mature. The theca interna gain the characteristics of steroid secretory cells and is rich in blood supply and the theca externa merges with the connective tissues of the ovary.

About 27 hours before ovulation on day 14, a spike in LH levels induces the preovulatory phase. Meiosis one is completed and the 2 daughter cells split unequally creating a polar body. Meiosis 2 then begins but is arrested at metaphase. The first polar body also divides. The oocyte is ovulated and will only complete meiosis if fertilized. It would otherwise degenerate within around 24 hours of ovulation. 

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