Question

What role does pituitary gonadotropins play during follicular and ovulatory phases of the menstrual cycle? Explain the shifts in steroidal secretions.

Accepted Answer

The gonadotropins help in the proper development, maturation and functioning of the gonads. These are released from the anterior lobe of the pituitary gland. These hormones are luteinising hormone and the follicle-stimulating hormone. The menstrual cycle refers to the cyclical changes in the ovary and uterus that takes place due to changes in the levels of hormones secreted by the ovary and pituitary gland. This cycle lasts for 28 days and repeats itself. The gonadotropins regulate the menstrual cycle and bring out different changes in the ovary are LH– a luteinising hormone, FSH– a follicle-stimulating hormone. The menstrual cycle includes the follicular phase, luteal phase and menstrual phase and the level of these hormones keeps on changing in different phases. It can be seen as follows:

(i) Follicular phase- It lasts for fourteen days. The secretion of LH and FSH increases from the pituitary gland. The FSH triggers the formation of mature Graafian follicle from the primary follicle. Simultaneously, there is a gradual increase in the level of LH which causes the follicle to grow, also causing the secretion of estrogen. Oestrogen inhibits the FSH secretion and stimulates the secretion of the LH. The increased secretion of LH called LH surge results in the release of the ovum from the mature Graaffian follicle into the fallopian tube causing ovulation.

(ii) Luteal phase- It lasts for ten days. The ruptured Graaffian follicle transforms into corpus luteum which secretes the hormone progesterone. It helps in maintaining and preparing the endometrium for the process of embryo-implantation thus it causes the thickening of the endometrium and increases the blood supply to the uterus. When the progesterone level in the blood becomes high, the FSH and LH secretion are inhibited. Hence, maturation of Graafian follicle and the formation of ovum does not take place.

(iii) Menstrual phase- If the released ovum is fertilised by the sperm then this cycle is ceased, if ovum remains unfertilised, it undergoes autolysis and the corpus luteum starts to degenerate. The endometrium starts to disintegrate and the release of FSH is again stimulated, leading to a new menstrual cycle.

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