The most interesting in connection with oogenesis is the development of the different follicle stages. The complex processes that are connected with it are treated in the fertilization module.

Following the immigration of the primordial germ cells into the gonadal ridge, they proliferate, are enveloped by coelomic epithelial cells, and form germinal cords that , though, keeptheir connection with the coelom epithelium. Now a cortical zone(cortex ovarii) and a medulla can be distinguished, whereby itshould be mentioned that in females the germinal cords neverpenetrate into the medullary zone. In the genital primordium thefollowing processes then take place:

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Stages of the first meiotic prophase of the oocyte.

From birth there are thus two different structures to be distinguished that, at least conceptually, do not develop further synchronously:

Birth The continuation of the development / maturation of the oocyte begins again only a few days before ovulation (see fertilization module).

Since a follicle can die at any moment in its development (= atresia), not all reach the tertiary follicle stage.

The sex hormones influence the primordial follicles to grow and a restructuring to take place. From the primordial follicles the primary follicles, secondary follicles, and tertiary follicles develop in turn. Only a small percentage of the primordial follicles reach the tertiary follicle stage - the great majority meet their end beforehand in the various maturation stages. Large follicles leave scars behind in the cortical compartment and the small ones disappear without a trace. The tertiary follicles get to be the largest and, shortly before ovulation, can attain a diameter up to 2.5 mm through a special spurt of growth. They are then termed graafian follicles.

Fig. 18 The follicles in various stages are shown in the ovarian cortical compartment. This very schematic drawing shows the relationships shortly before ovulation. In reality the primordial follicles are the most prevalent numerically.

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Oogenesis - Embryology