Question

What causes the follicle to mature?

Answer 1

Hint: Follicles are small sac-like structures, filled with fluid and are found inside a woman’s ovaries. Gonadotropin releasing hormone (GnRH) is a releasing hormone which stimulates the pituitary gland to produce follicle stimulating hormone (FSH). This FSH hormone is responsible for the follicle (egg) development as well as maturation of the follicle.

Complete answer:
In the process of oogenesis, around two million oogonia are formed in the ovary. After that, Oogonia divides further mitotically and enters the prophase-1 of the meiotic division. At this stage, the division stops temporarily and the cells are referred to as primary oocytes. These primary oocytes get surrounded by granulosa cells to form primary follicles. Then primary follicles develop into secondary follicles and then to tertiary follicles, which contain a fluid-filled cavity known as the antrum. The tertiary follicle changes into a mature follicle, known as Graafian follicle.
As the levels of follicle stimulating hormone (FSH) and luteinizing hormone (LH) in the blood increase with puberty, the eggs start to mature and a collection of fluid (the follicle) starts to develop around each. The first day of menses is defined as cycle day one, where the estrogen level is very low. Therefore, the pituitary secretes FSH and LH, and this process actually begins before the onset of menses. These hormones, in turn, stimulate the growth of several ovarian follicles. One follicle will soon begin to grow faster than others, called the dominant follicle.
As the follicle grows, blood levels of estrogen rise significantly by day seven of the cycle. The increase in estrogen begins to inhibit the secretion of FSH. The fall in FSH allows the dying away of smaller follicles.

Note:
The first meiotic division of primary oocytes occurs inside the tertiary follicle. It is an unequal division leading to the formation of a haploid secondary oocyte, which is large and has a tiny polar body. Most of the primary follicles degenerate and only 60,000 to 80,000 are left at puberty.