When and where are primary oocytes formed in a human female ? Trace the development of these occytes till ovulation (in menstrual cycle).
How do gonadotropins influence this developmental process ?

Explanation

Oogenesis :
(i) The process of formation of a mature female gamete or ova in the ovary is known as oogenesis.
(ii) Germ cells of female foetus divide to produce oogonia which later grow into ovarian follicles.
(iii) Oogonia start dividing and enter into prophase -I of the miotic division and get temporarily arrested at that stage, called primary oocytes are enclosed by a layer of granulosa cells, the follicle cells, and known as primary follicle.
(iv ) The primary follicles get surrounded by more layers of granulosa cells and a thecal layer known as secondary follicles.
(v) The secondary follicle soon transforms into a tertiary follicle which is characterised by a fluid-filled cavity called antrum.
(vi) Primary oocyte undergoes first meotic division and produce two haploid cells: a large haploid secondary oocyte and a small first polar body. Secondary oocyte moves into the fallopian tube.
(vii) When a sperm enters its cytoplasm, secondary oocyte completes meosis-II.
( viii) The secondary oocyte undergoes the second meotic division and produces a mature ovum and a second polar body.
(ix) Polar bodies have no function and eventually degenerate.
(x) Mature follicle of Graafian follicle ruptures to release the ovum from the ovary by the process called ovulation.
Effect of gonadotropins : The primary follicle in the ovary grows and becomes a fully mature Graffian follicle.
The endometrium of the uterus is regenerated due to the secretion of LH and FSH from anterior pituitary and ovarian hormone, estrogen.
It lasts for about 14 days. Rapid selection of LH (LH surge) induces rupture of Graafian follicle and thereby leading to ovulation (release of ovum).
For figure : See Q. 28 (or), Set-I, Delhi Board-2008