Menstrual Cycle (Ovarian Cycle)

Menstrual cycle:

• The reproductive cycle in female primates (e.g. monkeys, apes, and human beings) is called the menstrual cycle. 
• The first menstruation begins at puberty and is called menarche.
• The menstrual or ovarian cycle occurs approximately once in every 28/29 days during the reproductive life of the female from menarche (puberty) to menopause except during pregnancy.
• The cycle of events starting from one menstrual period till the next one is called the menstrual cycle during which cyclic changes occurs in the endometrium every month.
• One ovum is released (ovulation) during the middle of each menstrual cycle.
• Cyclic menstruation is an indicator of the normal reproductive phase and extends between menarche and menopause.

The menstrual cycle comprises of the following phases:

1. Menstrual phase:
   The cycle starts with the menstrual phase when menstrual flow occurs and lasts for 3-5 days. Menstrual flow is due to the breakdown of the endometrial lining of the uterus, and its blood vessels which forms a liquid that comes out through the vagina due to a decline in the level of progesterone and oestrogen. Menstruation occurs only if the released ovum is not fertilized. The absence of menstruation may be an indicator of pregnancy. However, it could also be due to stress, hormonal disorder, and anaemia.

2. Follicular or proliferative phase:
   The follicular phase extends from the 5^th day of the cycle until the time of ovulation. During this phase, the primary follicle in the ovary grows to become a fully mature Graafian follicle, and simultaneously, the endometrium of the uterus regenerates through proliferation. These changes in the ovary and the uterus are induced by the changes in the level of the pituitary and ovarian hormones. The secretion of gonadotropins like FSH and LH increase gradually during the follicular phase and stimulates follicular development as well as secretion of oestrogen by the follicle cells.

3. Ovulatory phase:
   Both LH and FSH attain peak level in the middle of the cycle (about the 14^th day). Maximum secretion of LH during the mid-cycle called LH surge induces the rupture of the Graafian follicle and the release of the ovum (secondary oocyte) from the ovary wall into the peritoneal cavity. This process is called as ovulation.

4. Luteal or secretory phase:
   During luteal phase, the remaining part of the Graafian follicle is transformed into a transitory endocrine gland called corpus luteum. The corpus luteum secretes a large amount of progesterone which is essential for the maintenance of the endometrium. If Fertilization takes place, it paves way for the implantation of the fertilized ovum. The uterine wall secretes nutritious fluid in the uterus for the foetus. So, this phase is also called the secretory phase. During pregnancy, all events of the menstrual cycle stop and there is no menstruation. In the absence of Fertilization, the corpus luteum degenerates completely and leaves a scar tissue called corpus albicans. It also initiates the disintegration of the endometrium leading to menstruation, marking the next cycle.
   

Menopause:

Menopause is the phase in a women’s life when ovulation and menstruation stops. The average age of menopause is 45-50 years. It indicates the permanent cessation of the primary functions of the ovaries.