31. Reproductive tissue. Oogenesis. Oocytes.

It consists of gonads which are the ovaries and testes which produce gametes. The gametes in females and males are eggs and sperm, respectively.

The formation of gametes is via meiosis. Reproductive tissue includes highly differentiated germ cells with progenitor and satellite cells. Satellite cells are split into 2 forms; follicular cells, present female reproductive systems and sertolli cells present male reproductive systems. 

Satellite cells are derived from primordial germ cells. 

Primordial germ cells develop in the 3rd embryonal week in the wall of the yolk sac. They are large cells with light cytoplasm and are PAS reaction positive. These cells proliferate and migrate to the primitive sex cords which gives rise to the satellite cells. At this stage the gonad is indifferent as one cannot morphologically distinguish it being male or female.

The formation of an ovum occurs during oogenesis. It take place in the ovaries and has 3 phases; proliferative phase, growing phase and maturation phase. It ends after the fertilisation of an oocyte. 

Oogenesis begins during the 3rd embryonal week during foetal period however it is interrupted and continues when menstruation begins, therefore it is a cyclic process occurring every 28 days. 

Oogenesis starts with a germ cell which gives rise to an oogonia. The oogonia then gives rise to a primary oocyte which then develops into a secondary oocyte and finally an ovum.

Oogenesis consists of mitosis and meiosis; 1st meiosis division ends after ovulation and 2nd meiosis ends after fertilisation.

Oogenesis begins about one or two months before a baby girl is born. By this time most of her oogonia die, and the remaining surviving oogonia enter meiosis I and become primary oocytes. These primary oocytes have their development paused and stay arrested at this stage of development for over a decade until the girl begins her first menstrual cycle

When the primary oocyte finally completes its first meiotic division, it divides the chromosomes evenly, however, it does not divide its cytoplasm equally. Almost all of the cytoplasm goes to one of the two daughter cells, which becomes a secondary oocyte. 

The secondary oocyte still has two copies of each chromosome, it must undergo the second meiotic division in order to produce haploids. This division is also uneven. Ovum achieves its haploid state.

Oocyte development occurs in the ovaries. Each oocyte is surrounded by a number of follicle cells. When the menstrual cycle begins, one, or a few primary oocytes begin to grow larger and the follicle cells increase in number and cause the follicle to grow larger too. The stages of growth of the oocytes during the menstrual cycle occurs as follows: primordial follicle, primary follicle, secondary follicle, tertiary follicle and graafian follicle.

At the time of birth, all surviving primary oocytes are surrounded by thin single layer of primordial follicles. They give rise to the primary follicles. When primary follicles survive, secondary follicles are created. Growing follicle contains the primary oocyte surrounded by zona pellucida, theca interna, theca granulosa and theca externa. 

If the secondary follicles survive, tertiary follicles are produced. If the tertiary follicle survive, graafian follicles are produced.

Oogenesis is regulated by three hormones. FSH and LH which are produced by the pituitary gland and oestrogen which is secreted by the ovaries.