When and where are primary oocytes formed in a human female?Trace the development of these oocytes till ovulation. How do gonadotropins influence this developmental process?
a. The primary oocytes are formed in the foetal ovary during the development of the foetus. The ovum is formed by the process of oogenesis.
b. During embryonic growth,millions of gamete mother cells are formed in the foetal ovary. These cells undergo meiosis, but get temporarily arrested at the prophase and are called primary oocytes. Before reaching puberty, a large number of primary oocytes degenerate and the remaining ones get surrounded by layers of granulosa cells and a new theca. These are called secondary follicles.The secondary follicles are then converted into tertiary follicles that have characteristic fluid-filled cavity called antrum. At this stage, the primary oocyte present within the tertiary follicle completes meiosis, which results in the formation of haploid secondary oocyte and a tiny polar body. This tertiary follicle further changes into the Graafian follicle. The secondary oocyte in the Graafian follicle is surrounded by the zona pellucida. Then, the Graafian follicle ruptures to release the ovum by ovulation.
c. Role of gonadotropins : The gonadotropins are released by the anterior portion of the pituitary gland. The maturation of the primary follicles into the Graafian follicles occurs during the follicular phase. The secretion of gonadotropins increases during this phase and cause follicular growth and the growing follicles produce oestrogen.The LH and FSH are at their peak in the middle of the cycle, and cause the rupture of the Graafian follicles to release ovum. This phase is called the ovulatory phase.