Tag: control and coordination in living organisms

Progesterone is a hormone secreted by ovaries. It maintains the lining of the uterus during the middle part of the menstrual cycle and during pregnancy. If a pregnancy occurs, progesterone is produced in the placenta, and levels remain elevated throughout the pregnancy. One of the progesterone's most important functions is to cause the endometrium to secrete special proteins during the second half of the menstrual cycle, preparing it to receive and nourish an implanted fertilized egg. If implantation does not occur, estrogen and progesterone levels drop, the endometrium breaks down and menstruation occurs.

In females, ovulation of mature follicles on the ovary is induced by LH secretion known as the preovulatory LH surge. Residual cells within ovulated follicles proliferate to form corpus luteum, which secretes the steroid hormones progesterone and estradiol. Progesterone is necessary for the maintenance of pregnancy, and, in most mammals, LH is required for continued development and function of corpora lutea. The name luteinizing hormone derives from this effect of inducing luteinization of ovarian follicles.

Leydig cells, also known as interstitial cells of Leydig, are found adjacent to the seminiferous tubules in the testicle. They produce testosterone in the presence of luteinizing hormone (LH). 

In men, testosterone plays a key role in the development of male reproductive tissues, such as the testis and prostate as well as promoting secondary sexual characteristics, such as increased muscle, bone mass, and the growth of body hair.

The male sex hormone testosterone is produced by Leydig cells. These cells are located in the connective (interstitial) tissue, that holds the tubules together within each lobule. The tissue becomes markedly active at puberty under the influence of the interstitial-cell-stimulating hormone of the anterior lobe of the pituitary gland.

Prolactin is produced both in the anterior pituitary gland. It stimulates the mammary glands to produce milk (lactation); increased serum concentrations of prolactin during pregnancy cause enlargement of the mammary glands of the breasts and prepare for the production of milk. 

In men, testosterone plays a key role in the development of male reproductive tissues, such as the testis and prostate as well as promoting secondary sexual characteristics, such as increased muscle, bone mass, and the growth of body hair.

In women, relaxin is secreted into the circulation by the corpus luteum in the ovary. During pregnancy it is also released from the placenta, the membranes, which surround the foetus, and the lining of the uterus. During pregnancy, relaxin levels are at their highest in the first trimester. At this time it is believed to promote implantation of the developing foetus into the wall of the uterus and the growth of the placenta. Towards the end of pregnancy, relaxin promotes rupture of the membranes surrounding the foetus and the growth, opening and softening of the cervix and vagina to aid the process of childbirth. Relaxin can relax the ligaments at the front of the cervix to ease delivery of the baby. 

Gonadotropins are secreted from the anterior pituitary gland and act on the gonads (i.e., the ovaries or testes). Gonadotrophs secrete two primary gonadotropins: luteinizing hormone (LH) and follicle-stimulating hormone (FSH).