Question

Part A:

Outline and paraphrase the negative feedback cycles involved in the development and maturation of sperm in the human male. Include the role of follicle stimulating hormone(FSH), luteinizing hormone (LH), androgen binding protein (ABP)and inhibin.

Part B:

Outline and paraphrase the negative feedback cycles involved in the development and ovulation of an egg in the female. Include the role of follicle stimulating hormone (FSH), luteinizing hormone (LH), estrogen and progesterone.

Answer 1

Part A)
FSH --Follicular stimulating hormone
LH --Luteinising hormone
*Hypothalamic control-GnRH:
Arcuate nuclei of the hypothalamus secrete GnRH into the hypothalamic-hypophysial portal blood. GnRH stimulates the anterior pituitary to secrete FSH and LH.

*Anterior pituitary-FSH and LH
FSH acts on the sertoli cells in seminiferous tubules to maintain spermatogenesis. The sertoli cells also secrete inhibin, which help in negative feedback of FSH secretion.
LH acts on the Leydig cells to promote testosterone synthesis. Testosterone acts via an intra-testicular paracrine mechanism to increase the spermatogenic effects of FSH in the sertoli cells.

*Negative feedback control-testosterone and inhibin:
Testosterone inhibits the secretion of LH by inhibiting the release of GnRH from the hypothalamus and by directly inhibiting the release of LH from the anterior pituitary. Inhibin (produced by the sertoli cells) inhibits the secretion of FSH from the anterior pituitary

Part B)

*Pulsatile release of GnRH from the hypothalamus (every 60 to 90 minutes) triggers a corresponding pulsatile release of LH and FSH from the gonadotrophs of the anterior pituitary to secrete FSH and LH, which stimulate ovarian cells to secrete estrogens and progestins.

*LH and FSH bind to specific receptors on the surface of their target cells. The theca cells of the follicle have affinity for LH receptors, whereas the granulosa cells have both LH and FSH receptors affinity .

*Ovarian steroids (estrogens and progestins) exert both negative and positive feedback on the hypothalamic-pituitary axis.  

*Negative feedback by ovarian steroids: Throughout most of the menstrual cycle, the estrogens and progestins feedback negatively on both the hypothalamus and the gonadotrophs of the anterior pituitary. The net effect is to reduce the release of both LH and FSH. The estrogens exert negative feedback at both low and high concentrations, whereas the progestins have their action only at high concentrations

*Although ovarian steroids feed back negatively on the hypothalamic-pituitary axis during most of the menstrual cycle, they have the opposite effect at the end of the follicular phase. Levels of estrogen, mainly estradiol, rise gradually during the first half of the follicular phase of the ovarian cycle and then steeply during the second half .

*After the estradiol levels reach a certain threshold for a minimum of 2 days the hypothalamic-pituitary axis reverses its sensitivity to estrogens; that is, estrogens now feed back positively on the axis. This switch to positive feedback promotes the LH surge. The gonadotropin surge (LH) causes ovulation and luteinization.

Luteinization in the granulosa cells of the follicle causes these cells to secrete progesterone rather than estradiol. As the luteal phase of the menstrual cycle begins, circulating levels of LH and FSH rapidly decrease. This fall-off in gonadotropin levels reflects negative feedback by three ovarian hormones—Estradiol ,Progesterone and Inhibin .