Follicle-stimulating hormone, follitropin (FSH) is one of the 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) in response to the gonadotropin-releasing hormone produced by the hypothalamus (so-called hypothalamic-pituitary-gonadal axis).
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.
Role of the FSH in the body
The follicle-stimulating hormone controls the development, growth, and maturation of the reproductive system in both men and women. FSH promotes the growth and maturation of follicles containing oocytes in the ovaries and the survival of the only one selected follicle called Graafian follicle whereas the other follicles undergo apoptosis (programmed cell death). Later, when the synthesis of LH increases and reaches its peak, the increased concentrations of both LH and FSH stimulate the secretion of estrogens and finally the rupture of the mature Graafian follicle and the mature oocyte is released (ovulation).
Biochemical effects of FSH
Follicle-stimulating hormone provides a wide range of biochemical reactions and changes in the body:
- Synthesis its own receptors in the granulosa cells.
- Synthesis LH receptors in the theca cells.
- Synthesis LH receptors in the granulosa cells.
- Induction of aromatization to convert androgens to estrogens in granulosa cells.
- Enhancing autocrine and paracrine function (IGFII, IGF-I) in the follicle.
- Stimulation of the granulosa cells to produce activin and inhibin.
- Stimulation of 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. Biochemical effects of FSH are responsible for the physiological changes.
Therefore, in females FSH:
- 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;
In males the hormone:
- Stimulates spermatogenesis (the growth and maturation of the spermatocytes) by the Sertoli cells of the testicle and secretion of inhibin B;
The FSH levels start to rise a few days before the 1 day of the cycle, remains low during the first phase of the cycle and reaches its peak at the 12th day of the menstrual cycle (before the ovulation occurs), afterwards 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.