Sequence of events in ovarian follicular cycle and spermatogenesis.

OVARIAN FOLLICULAR CYCLE :

Throughout their life in ovaries, the egg exist in structure knows as follicle. Follicle begin as primordial follicles, which consist of one primary oocyte surrounded by single layer of cell called granulosa cell. Further development from the primordial follicle stage is characterized by an increase in the size of oocyte, a proliferation of the granulosa cell into multiple layers, and the separation of the oocyte from inner granulosa cell by a thick layer of material the zona pellucida. Despite the presence of zona pellucida, the inner layer of granulosa cell remain closely associated with the oocyte by mean of cytoplasmic protein that transverse the zona pellucida and form gap junction with oocyte. As follicle grows by mitosis of granulosa cells, connective tissue cell surrounding the granulosa cell differentiates and form layer known as theca. Shortly after this, the primary oocyte reaches full size and a fluid filled space, the antrum, begin to form in the midst of the granulosa cell as a result of fluid they secrete. The progression of some primordial follicle to the preantral and early antral stages occur throughout infancy and childhood, and then during the entire menstrual cycle. Although most of the follicle in the ovaries are still primordial, relatively constant number of preantral and early antral follicles are also always present. At the beginning of each menstrual cycle, 10 to 25 of these preantral and early antral follicle begin to develop into larger antral follicles. About one week into the cycle, a further selection process occurs, only one of the larger antral follicle, the dominant follicle, continue to develop. The non-dominant follicle that had begun to enlarge undergo a degenerative process called atresia which is an example of programmed cell death or apoptosis. The dominant follicle enlarges mainly as a result of an increase in fluid, causing the antrum to expand.

SPERMATOGENESIS:

The undifferentiated germ cell, which are termed spermatogonia begin to divide mitotically at puberty. The daughter cell of this first division then divide again and again for a specified number of division cycles. So that a clone of spermatogonia is produced from each stem cell spermatogonium. Some differentiation, occur in addition to cell division. The cells that result from the final mitotic division and differentiation in the series called primary spermatocytes. Each primary spermatocyte increase markedly in size and undergoes the first meiotic division to form two secondary spermatocytes, each of which contains 23 two-chromatid chromosomes. Each secondary spermatocyte undergoes the second meiotic division to form spermatids. Thus, each primary spermatocyte, containing 46 two-chromatid chromosomes, produce four spermatid, each containing 23 one-chromatid chromosomes. The final phase of spermatogenesis is the differentiation of the spermatid into spermatozoa (sperm).