What are the hormonal controls of the menstrual cycle in sexual reproduction?

The hormonal controls of the menstrual cycle involve the interplay of hormones from the hypothalamus, pituitary gland, and ovaries.

The menstrual cycle is a complex process regulated by a series of hormones. It begins with the hypothalamus, a part of the brain that produces gonadotropin-releasing hormone (GnRH). This hormone signals the pituitary gland to release two other hormones: follicle-stimulating hormone (FSH) and luteinising hormone (LH).

FSH stimulates the growth and maturation of follicles in the ovaries. Each follicle contains an egg and as it matures, it starts to produce oestrogen. This hormone has two main roles. Firstly, it helps to build up the lining of the uterus (endometrium) in preparation for a potential pregnancy. Secondly, it inhibits the production of FSH to prevent the maturation of too many follicles at once.

Around the middle of the cycle, a surge in oestrogen levels triggers the pituitary gland to release a large amount of LH. This LH surge causes the most mature follicle to burst and release its egg in a process known as ovulation. The remnants of the burst follicle form a structure called the corpus luteum, which produces progesterone.

Progesterone further prepares the endometrium for pregnancy by making it thicker and more vascular. It also inhibits the release of FSH and LH to prevent further ovulation during the same cycle. If fertilisation does not occur, the corpus luteum breaks down, leading to a drop in progesterone levels. This drop triggers the endometrium to shed, resulting in menstruation.

The decrease in progesterone also removes the inhibition on FSH and LH, allowing the cycle to start again. The interplay of these hormones ensures that only one egg is released per cycle and that the uterus is ready to support a potential pregnancy. This complex hormonal control system is crucial for sexual reproduction.