Question

Define menstrual cycle and explain its various phases with diagram.

Answer 1

The menstrual cycle is a reproductive cycle which takes place in the females of the group of primates. The menstrual cycle is divided into four phases:

(i) Menstrual phase: It extends from 1${}^{st}$ to 4${}^{th}$ day of the cycle. It occurs in the absence of fertilization. During this phase, bleeding occurs as the endometrium of the uterus is sloughed off. The menstrual flow consists of secretion of endometrial glands, cell debris, unfertilized ovum. After 4${}^{th}$ day, once again the FSH secretion from the pituitary is resumed and the new follicle starts developing.

(ii) Follicular phase: When the ovary is in this phase, uterus enters in the proliferative phase. This takes place from 5${}^{th}$ to 13${}^{th}$ day of the cycle. During this phase new primordial follicle in the ovary develops due to the action of FSH from the pituitary. It gradually changes into Graafian follicle and production of oestrogen starts. Only one follicle develops in one cycle.Corresponding to the changes in the ovary, uterus also undergoes proliferation. Endometrial glands, stimulated by oestrogen do repair process of uterus.

(iii) Ovulatory phase: During this phase, ovulation takes place. It usually occurs on 14${}^{th}$ day. Mature Graafian follicle ruptures due to LH secreted by pituitary. Graafian follicle bursts and releases the ovum. This ovum along with the follicular fluid is picked up by the fimbriae of infundibulum of fallopian tube. It passes through fallopian tube, where, if it happens to meet a sperm, it is fertilized. If not fertilized, the ovum degenerates.

(iv) Luteal phase: It corresponds with the secretory phase in uterus. It takes place between 15${}^{th}$ to 28${}^{th}$ day of the cycle.

Ovarian changes: In the ovary, corpus luteum is formed from empty Graafian follicle. Progesterone is secreted now. If ovum is fertilized, corpus luteum is retained. LH and LTH from pituitary help in the maintenance of corpus luteum. If ovum is not fertilized, corpus luteum degenerates and forms corpus albicans.

Uterine changes: Under the influence of progesterone, there is increase in the thickness of endometrium. Endometrial glands grow and become secretory. Progesterone is responsible for the maintenance of pregnancy. When fertilized ovum reaches the uterus, it is implanted and placenta is formed. Till placenta becomes functional corpus luteum keeps on producing progesterone. But when progesterone source is cut off, endometrium sloughs off and menstruation begins.