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.
Development of the germ cells in the ovary
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, keep their connection with the coelom epithelium. Now a cortical zone (cortex ovarii) and a medulla can be distinguished, whereby it should be mentioned that in females the germinal cords never penetrate into the medullary zone. In the genital primordium the following processes then take place:
- A wave of proliferation begins that lasts from the 15th week to the 7th month: primary germ cells arise in the cortical zone via mitosis of oogonia clones, bound together in cellular bridges, that happens in rapid succession. The cell bridges are necessary for a synchronous onset of the subsequent meiosis.
- With the onset of the meiosis (earliest onset in the prophase in the 12th week) the designation of the germ cells changes. They are now called primary oocytes.
The primary oocytes become arrested in the diplotene stage of prophase I (the prophase of the first meiotic division). Shortly before birth, all the fetal oocytes in the female ovary have attained this stage. The meiotic resting phase that then begins is called the dictyotene and it lasts till puberty, during which each month (and in each month thereafter until menopause) a pair of primary oocytes complete the first meiosis. Only a few oocytes (secondary oocytes plus one polar body), though, reach the second meiosis and the subsequent ovulation. The remaining oocytes that mature each month become atretic.
The primary oocytes that remain in the ovaries can stay in the dictyotene stage up to menopause, in the extreme case, without ever maturing during a menstrual cycle.
- While the oogonia transform into primary oocytes, they become restructured so that at the end of prophase I (the time of the dictyotene) each one gets enveloped by a single layer of flat, follicular epithelial cells (descendents of the coelomic epithelium). (oocyte + follicular epithelium = primordial follicle).
From birth there are thus two different structures to be distinguished that, at least conceptually, do not develop further synchronously:
- On the one hand, the female germ cell that at birth is called the primary oocyte, and which can develop further only during (and after) puberty (hormonal cycle is necessary).
- On the other hand, the follicular epithelium that can develop further from the primordial follicle via several follicle stages while oocytes remain in their primary state.
- Primordial germ cell - oogonium - primary oocyte - primary oocyte in the dictyotene
The continuation of the development / maturation of the oocyte begins again only a few days before ovulation (see fertilization module).
- Primordial follicle - primary follicle - secondary follicle - tertiary follicle (graafian follicle)
Since a follicle can die at any moment in its development (= atresia), not all reach the tertiary follicle stage.
Structure of the ovary
An ovary is subdivided into cortical (ovarian cortex) and medullary compartments (ovarian medulla).
Both blood and lymph vessels are found in the loose connective tissue of the ovarian medulla.
In the cortical compartment the oocytes are present within the various follicle stages.
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.