Follicle-stimulating hormone, follitropin (FSH) is one of gonadotropins – hormones secreted by the anterior part of the pituitary gland, which is responsible for the regulation and control of growth, sexual development, and reproduction.
Follicle-stimulating hormone is secreted from the beta-cells by the anterior portion of the pituitary gland (known as the adenohypophysis) stimulated by the gonadotropin-releasing hormone produced by the hypothalamus.
FSH has a weight of 35.5 kDa and consists of a common glycoprotein with two subunits – α and β. α subunit of FSH is similar to such of other gonadotropins whereas β subunit represents features of follicle-stimulating hormone and can bind with the specific FSH receptors.
- In females: FSH primarily acts on the gonads by binding with FSH receptors in granulosa cells of the ovarian follicles. Thus, the growth and maturation of the follicles and synthesis of estradiol are initiated. In response to FSH, aromatase activity is induced and androstenedione is converted to estradiol.
FSH controls the production of inhibin and induces the expression of LH receptors in granulosa cells of the large follicles. Follitropin stimulates the maturation of the dominant follicle and the recruitment of the next generation of the follicles that will grow during the following menstrual cycles.
- In males: FSH acts on the FSH receptors on the Sertoli cells of the testicles and promotes the production of inhibins, androgen receptors, and androgen-binding proteins, etc. The hormone is responsible for the maturation of spermatozoa.
Biochemical effects of FSH
The physiological effects of the FSH are implemented via its biochemical changes such as:
- Synthesizes its own receptors in the granulosa cells.
- Synthesizes LH receptors in the theca cells.
- Synthesizes LH receptors in the granulosa cells.
- Induces aromatization to convert androgens to estrogens in granulosa cells.
- Enhances autocrine and paracrine function (IGFII, IGF-I) in the follicle.
- Stimulates granulosa cells to produce activin and inhibin.
- Stimulates plasminogen activator necessary for ovulation.
Physiological effects of FSH
Follicle-stimulating hormone as one of the gonadotropins plays a crucial role in the functioning of the human’s reproductive system as it:
- Stimulates the growth and maturation of the ovarian follicles containing oocytes, especially the Graafian follicle preparing it to the ovulation – it means that this hormone is the one which controls the development of oocyte which later will be released for the successful conception;
- In the presence of luteinizing hormone, FSH stimulates secretion of estrogen by the growing follicles;
- Promotes the endometrial changes typical for the proliferative phase of the menstrual cycle – secretion of FSH helps to restore and grow the inner lining of the uterus after the periods;
- Stimulates spermatogenesis (the growth and maturation of the spermatocytes) by the germ cells of the testicles in men;
Serum levels of FSH
The FSH levels start to rise soon after the beginning of the menstruation with its peak at the 12th day of the menstrual cycle (before the ovulation occurs), afterward its levels decrease gradually until the day 18 to the base values.
Therefore, FSH levels vary greatly in women, according to the phase of the cycle and woman’s age.