What is oestrogen?

Estrogens are steroidal hormones. They are synthesised from cholesterol through testosterone steroidogenesis mechanisms. Estrogens can be classified into three groups: estradiol, estriol, and estrone. Predominantly female, these hormones are also produced by men in small quantities. In men, estrogens can modulate the synthesis of gonadotropin, a hormone that stimulates the activity of the gonads.

Their synthesis by the ovaries begins at puberty and continues in women until menopause. The amount of oestrogen in the body increases until around the age of 20, where it reaches a peak, before gradually decreasing. Predominantly synthesised by the ovaries, some oestrogens are also produced by other tissues such as the liver or adipose tissue. This "secondary production" is essential, particularly from menopause, when the ovaries cease to synthesise this hormone.

The quantity of oestrogens in women also fluctuates throughout the menstrual cycle, which can be divided into 3 key phases: the menstrual phase, the follicular (pre-ovulatory) phase, and the luteal (post-ovulatory) phase. During the first phase, the level of oestrogen in the body is very low. It then increases significantly during the follicular phase before reaching a peak at ovulation. The synthesis of oestrogen then drops before gradually increasing again during the luteal phase.

Estrogens operate at various levels within the body and have multiple roles, the most significant of which are mentioned below.

• Estrogens contribute to the development of secondary sexual characteristics.

  From puberty, the hypothalamus stimulates the pituitary gland, a small gland located at the base of the brain. The secretions of FSH, a follicle-stimulating hormone, and LH, a luteinising hormone, increase, which accelerates the maturation of ovarian follicles, and with it, the production of oestrogen. By acting on the mammary gland, these hormones are responsible for the pubertal anatomical transformations observed during this period, such as the growth of the uterus, the feminisation of the voice and the enlargement of the breasts.

• Estrogens also contribute to the preparation of pregnancy.

  Estrogens play a role in preparing for pregnancy by developing the uterine lining to allow for the implantation of the egg. Just before ovulation, the level of estrogen increases, which also triggers the sudden secretion of LH. This surge of luteinising hormone triggers the rupture of the follicle containing the egg, and thus the expulsion of the latter by one of the two ovaries into the Fallopian tube. Once released, the egg travels down the tube towards the uterus where it persists for 24 hours waiting to be fertilised.

• Estrogens play a role in bone growth and development.

  According to several studies, oestrogens appear to have beneficial effects on bone formation in both men and women by inducing the secretion of proteins from the extracellular bone matrix, as well as the synthesis of growth factors, such as TGF-β3. These, in turn, are believed to be responsible for an increase in cell multiplication and differentiation. This is why, after menopause, bones can become more fragile.

Estrogens are capable of stimulating the fibroblasts, cells in the dermis that contribute to the synthesis of hyaluronic acid, collagen, and elastin. Found in the extracellular matrix, all three contribute to the well-being of the skin. Hyaluronic acid is a macromolecule that works by retaining water in the skin, while the collagen and elastin provide tone and flexibility to the skin.

Estrogens thus have a beneficial effect on skin hydration. This property was highlighted in studies conducted with non-menopausal individuals at different times of the menstrual cycle and menopausal individuals. After menopause and during menstruation, when estrogen levels are at their lowest, the skin tends to be more sensitive and sometimes drier.

Low levels of oestrogen can be associated with a slow renewal of skin cells, as well as an impairment of the skin barrier function. This also promotes the appearance of wrinkles and fine lines, as well as skin sagging.

• SKOUBY S. & al. Menstrual cycle and skin reactivity. Journal of the American Academy of Dermatology (1991).

• STEVENSON J. & al. Role of oestrogen in the development of osteoporosis. Calcified Tissue International (1991).

• THORNTON M. Oestrogen roles in skin and skin appendages. Expert Opinions on Therapeutic Targets (2005).

• MILLINGTON G. & al. The menstrual cycle and the skin. Clinical and Experimental Dermatology (2015).