Gonadotropins (FSH & LH)
Two hormones from the adenohypophysis, the follicle‑stimulating hormone (FSH) and luteinizing hormone (LH/ICSH) in females/males affect the gonadal activity and known as Gonadotropins.
The basal or tonic output of FSH and LH is pulsatile during the reproductive functions in males and females.
FSH output is under hypothalamic control by GnRH and a negative feedback mechanism involving the gonadal hormones, rising levels of estrogen and inhibin production.
Follicle stimulating hormone (FSH) is a glycoprotein made up of α- (92 amino acids) and β- (118 amino acids) subunits with carbohydrate moiety.
Luteinizing hormone (LH) or Interstitial-cell stimulating hormone (ICSH) – LH in female and ICSH of the male is the same hormone.
LH is a glycoprotein made up of α- and β- subunits having 92 and 115 amino acids with carbohydrate moiety.
The pituitary content of LH is highest in cattle, sheep and cat but lowest in horse and man.
Environmental conditions like changing seasons and day light length is mediated from exteroceptors like eye to the hypothalamus to influence GnRH output.
Basal pulsatile FSH secretion stimulates follicular growth and estrogen secretion in ovaries.
FSH acts synergistically with LH to effect ovulation. In the male FSH acts on the seminiferous tubules and stimulates spermatogenesis.
The actions of LH and FSH are synergistic. Increased production of estrogen by the Graafian follicle stimulates LH surge to effect ovulation by positive feedback effect.
LH secretion is increased greatly 24 to 48 hours before ovulation to effect ovulation but the increase in FSH is to a lesser extent.
LH causes maturation of ovum, ovulation and formation of corpus luteum (CL). The CL produces progesterone which inhibits LH secretion and prevents further follicular growth and ovulation, thus preventing estrus during the period of CL.
In induced ovulators e.g., cat and rabbit, LH secretion is stimulated by neuroendocrine reflex of the hypothalamus.
In male LH stimulates interstitial cells of testis and causes testosterone production .
The regulation of LH output from the pituitary gland is dependent upon the hypothalamic GnRH by a negative-feedback control mechanism.
During pregnancy, placenta also secretes placental luteotropic hormone.
Estrogen and progesterone in large amounts inhibit FSH and LH release from the adenohypophysis and the feedback effect operates through the hypothalamus and anterior pituitary.
In addition inhibin secreted from the large graafian follicles in females and Sertoli cells in males inhibits FSH to a major extent and LH to a lesser extent.