The Hormonal Cycle in Human Body

Dec 11 2007

Hormones are chemical messengers produced by endocrine glands. Chemically, they are either steroids (related to lipids or fats) or proteins. These messengers circulate within the body and have specific influences on particular organs, called target organs. These organs include the brain and hypothalamus, where mood and emotions are controlled. Thus, reproductive hormones may have an influence on the emotions.Hormones are manufactured in several locations in the body. The hormones of female reproduction are made in the hypothalamus, the anterior pituitary gland, and the ovary. The hypothalamus, situated at the base of the brain, makes Gonadotropin Releasing Hormone (GnRH), which is transported to the pituitary gland. This hormone stimulates the pituitary to cyclically release several hormones, called gonadotropins which then act on the ovary.

The pituitary is about the size of a seedless grape and is located in a socket in the skull and attached to the base of the brain by a short stalk. The pituitary secretes three gonadotropins -

Follicle-stimulating hormone (FSH) causes the ripening of the part of the ovary containing the egg called a follicle. FSH also stimulates the secretion of estrogen.
Luteinizing hormone (LH) causes the ripe follicle to rupture, releasing the egg to be caught up by the fallopian tube. This is called ovulation. LH then stimulates the ovary to convert the empty follicle into the hormone-secreting structure known as the corpus luteum. It is so named because of its yellowish appearance. The corpus luteum secretes somewhat more estrogen and much progesterone.
Prolactin may have an effect on maturation of the follicle and may also help maintain the corpus luteum. In pregnancy, it stimulates and maintains lactation and breast development. In the non pregnant woman, it reduces the response of the pituitary to GnRH and of the ovary to FSH and LH. An excess of prolactin can be involved in some instances of infertility

The ovaries produce estrogen and progesterone which bring about secondary sexual development and the possibility of reproduction. Throughout the potentially reproductive years, estrogen and progesterone influence changes in the endometrium-the lining of the uterus. Progesterone also plays an important role in the initial maintenance of pregnancy. The ovarian hormones influence the secretion of the pituitary hormones through feedback mechanisms. This means that low or high levels of hormone secreted by one organ influence whether another organ will secrete or not secrete its hormone at a given time.