Oogenesis is the production of eggs and is made ​​in the ovaries . The eggs are part of the ovarian follicles . This process is regulated by hormones from the pituitary gland : follicle stimulating hormone and luteinizing hormone.

In mammals it is very common to speak of folliculogenesis, to indicate the different phases through the follicle development. Folliculogenesis is a dynamic and complex process, through which the follicle goes through several stages of development.

Early embryonic development (from eight to twelve weeks gestation) may be seen in the cortex of the ovary oocytes surrounded by the first layer of flattened follicular cells. The whole oocyte and its companion cells called primordial follicle (Fig.1A). Primordial follicles are small in size and enclose an oocyte (arrested in prophase of meiosis I), approximately 35 microns in diameter. During the reproductive life of a woman, more than 90% of the follicles are primordial follicular reserve constituting the fixed resting follicles, which is consumed gradually in each menstrual cycle and when depleted, ovarian activity ceases and menopause ensues .

When girls reach puberty, the ovaries begin a phase of activity characterized by cyclic crecimieno and maturation of small groups of follicles. Normally at each cycle, only one follicle reaches full maturity and the oocyte is released from the ovary . So the 400,000 oocytes present at birth, only 400 reach complete maturity during the woman’s fertile life.

The progression of folliculogenesis, i.e. the passage through the various stages of follicle development is based on the ability of follicular cells to divide and differentiate.

Continuously primordial follicles leave the reserve to become primary follicles (Fig. 1B, C). This transition involves changes. It reaches a diameter two to three times greater than the primordial follicles and as the oocyte increases in size, the single layer of flattened follicular cells is initially converted into a cubic cell layer and then cylindrical. These cells show increased mitotic activity gradually proliferate and assuming the appearance of a stratified epithelium. As their cytoplasm is granular appearance, they are often also called granulosa cells. In this phase the oocyte with a diameter of 50 to 80 microns, is separated from the follicle cells by a transparent and homogeneous membrane, which is called the zona pellucida. As follicles grow in size, they gradually moving into the cortex.

Continuing with follicular development and when the granular layer reaches a thickness of several rows of cells therebetween appear several small cavities filled with fluid which is secreted by the follicle cells themselves. At this stage the hair follicle is called secondary or antral (Fig. 1D) and has a diameter of 10 mm or more. Subsequently these cavities converge to form a crescent-shaped space called follicular antrum. The presence of cavities eccentrically moving the oocyte is also more developed (125 microns).

The fully mature follicle is called Graafian follicle (Fig. 1E) characterized by the presence of large antral follicles. The follicular antrum determines that the oocyte be moved to one pole of the follicle and is situated in a small cell called cumulus oophorus accumulation.

In this phase, the follicle occupies virtually the entire thickness of the bark and starts to make embossed on the surface of the ovary. The impending rupture of the follicle is preceded by a rapid growth: the follicle wall near surface thins, becomes translucent, avascular and breaks, releasing the oocyte that immediately completes its first meiotic division. Once produced the release of the oocyte remains empty follicle and the surrounding tissue are transformed into a structure known as corpus luteum gland (Fig. 1F). The corpus luteum is an endocrine structure formed by follicular cells and theca remaining follicle that released the egg Its main function is to secrete progesterone, which has many functions, including the preparation of the uterus for pregnancy period and regulating contractionsoviduct to transport the egg into the uterus .

Fig 1: oogenesis. A primordial follicle, B, primary follicle, C, advanced primary follicle, D, secondary follicle, E, mature follicle, F, corpus luteum. One can observe the different schemes follicles and next a photograph thereof taken under an optical microscope. Follicles are not to scale.




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