There are two female hormones, estrogen and progesterone, which are responsible for the various processes associated with the sexual characteristics in females. Both estrogen and progestrone are mainly produced in the ovaries along with a small quantity of testosterone (androgen – male sex hormone).
The production and release of these hormones are regulated by other hormones in the hypothalamus and pituitary gland. Gonadotropin-releasing hormone (GnRH) is secreted by the hypothalamus and acts on the anterior pituitary gland which responds by releasing follicle-stimulating hormone (FSH) and luteinizing hormone (LH). This process is similar to the regulation of the sex hormones in males.
The secretion of the female hormones and regulating hormones are not constant but fluctuate to varying degrees during the course of the menstrual cycle.
Regulation of Female Hormones
The gonadotropic hormones, FSH and LH, are absent in childhood and its presence in the early teens causes the ovaries to become active. This results in the development of the secondary sexual characteristics of females (puberty) and eventually the onset of the first menstrual cycle (menarche).
FSH and LH act on receptors on the ovarian cell membranes causing the ovary to grow and enlarge. In sufficient quantities, these hormones will also cause the follicles in the ovary to mature and begin secreting estrogen and progesterone. FSH is responsible for the first part of follicle maturation and growth which is then spurred on by LH and estrogen. Although many follicles in the ovaries respond to the hormones, only one follicle will fully mature each month.
The Female Hormones Estrogen and Progesterone
The two types of female sex hormones secreted from the ovaries include the estrogens and progestogens. These are groups of hormones with estradiol being the most important of the estrogens, and progesterone being the most abundant progestogen. Broadly estrogen is responsible for the growth and development of tissues while progesterone increases secretory functioning of specific tissues. Both these hormones are therefore essential for pregnancy.
Estrogens and progestogens are synthesized from cholesterol and acetyl co-enzyme A. Initially the ovaries produce mainly progesterone and testosterone. Estrogen is then produced from some of this progesterone and almost all of the estrogen. The hormones can then enter the blood stream and bind to blood proteins, including albumin, estrogen-binding globulin and progesterone-binding globulin. Estrogen and progesterone can then be transported to the various tissues in the body. It is degraded by the liver and the byproducts are passed out in the bile and urine.
Most of the estrogens are secreted by the ovaries with small amounts secreted by the adrenal glands (cortex). There are three types of estrogens – estradiol, estrone and estriol. Estradiol is the considered as the main estrogen due to its potency relative to estrone and estriol. Even with estrone and estriol combined, it cannot match the effect of estradiol.
Functions of Estrogens
Estrogens have the following effects on various organs and tissues :
- Female reproductive organs – increase in size
- External genitalia – deposition of fat in the mons pubis, enlargement of labia
- Vagina – change of the epithelial lining after puberty
- Uterus – thickening of the endometrium (inner lining of uterus) and development of endometrial glands
- Fallopian tubes – increase in ciliated epithelial cells in the fallopian tube to help push the ovum out
- Breasts – growth of the duct network of the breasts, increase in fat and stroma
- Bone – stimulate bone growth by reducing bone break down and increasing bone deposition
- Metabolism – increases the metabolic rate
- Fat deposition – increases deposition of fat in the subcutaneous tissues, particularly on the breast, thighs and buttocks
- Skin – increases vascularity, contributes to softer smoother skin
- Kidneys – electrolyte and water retention which is only prominent with high quantities of estrogen like during pregnancy
The most abundant progestogen is progesterone, with small quantities of another type of progestogen known as 17-alpha-hydroxyprogesterone, also being secreted by the ovaries.
Functions of Progesterone
- Uterus – increases secretory actions of the endometrial lining in preparation for pregnancy
- Fallopian tubes – increases secretions to nourish the rapidly developing fertilized ovum
- Breasts – promotes development of breast alveoli and lobules