Pituitary Gland the "Master Gland"

The pituitary gland, which lies is a small depression in the sphenoid bone of the skull called the sella turcica, has often been termed the 'Master Gland' because many of the hormones it releases effect the release of other hormones. However, the pituitary is really not the master. It is controlled by a brain region called the hypothalamus via the release of releasing factors into a special blood vessel network (hypothalamic-hypophyseal portal system) that feeds the pituicytes. These releasing factors then cause or inhibit the release of pituitary hormones which travel via the circulatory sytem to the target organ. For example, as a woman's menstrual cycle progresses toward ovulation, the hypothalamus releases LHRH (luteinizing hormone releasing hormone) that travels via the hypophyseal portal system to the pituitary where it stimulates the production and release of LH (luteinizing hormone). LH then travels to the ovaries where it causes ovulation and the subsequent development of a progesterone secreting corpus luteum.

Anatomically and functionally the pituitary can be divided into three portions:

1) anterior pituritary (adenohypophysis)
Six peptide hormones are secreted by the adenohypophysis: Growth hormone (somatotropin, GH), corticotropin (ACTH), thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), Luteinizing hormone (LH), and prolactin (PRL). All except growth hormone and prolactin regulate the activities of other glands. Somatotropin, PRL and ACTH are polypeptide hormones and LH, FSH, and TSH are glycoproteins having very similar structures.

Growth hormone has no specific target tissue. All cells of the human body are affected by this hormone. It is very important in the growing child but it remains essential to many bodily functions throughout life. GH has effects on the growth of bone and cartilage, protein metabolism, RNA formation, electrolyte balance, fat and glucose metabolism.
ACTH This trophic hormone stimulates the production and release of suprarenal steroids. Normally the amount of circulating ACTH is controlled by the levels of cortisol in the blood, individual biorhythms and stress.
TSH This hormone stimulates the synthesis and secretion of thyroid homones. It is a glycoprotein hormone controlled by feedback from thyroid homones.
FSH The target organs for FSH are the testes, in men, and the ovaries in women. The hormone stimulates the germinal epithelium in the testes to cause and facilitate the making of sperm. In women it stimulates the growth and development of the follicle. It stimulates the production of testosterone in men and estrogen and progesterone in women. Its release from the pituitary is governed by a negative feedback mechanism involving these steroids.
LH The male target organ is the testes and the testosterone producing interstitial cells of Leydig in particular. In women the target of LH is the developing follicle within the ovary where it is necessary for ovulation to occur and a corpus luteum to develop.
Prolactin This hormone is involved in breast development and lactation. In concert with estrogen, it prepares the mammary gland for lactation and then causes the synthesis of milk. Secretion is regulated by a release inhibiting factor and suckling may cause the release of prolactin from the pituitary.
2) intermediate lobe (pars intermedia)
In the adult human this lobe is diminished with poor vascular and neural connections such that secretion is not facilitated. Cells in the pars intermedia may secrete MSH (melanocyte stimulating hormone) which stimulates the activity of melanocytes in the skin.

3) posterior pituitary (neurohypophysis)
This portion of the pituitary is really an extension of the hypothalamus. Neurons with their cell bodies in the hypothalamus and their terminal protions in the neurohypophysis release two hormones. Antidiuretic hormone (ADH) and oxytocin are stored there within the terminal processes of neurons until the signal to release them is received.

ADH In the presence of ADH, the kidneys reabsorb more water from the forming urine within the renal tubules. Without ADH the kidney tubules are almost completely impermeable to water such that a very dilute urine is excreted (diabetes insipidus). ADH has a direct effect on vascular smooth muscle causing vasoconstriction and an increase in blood pressure when present in large doses. The hypothalamus has osmoreceptors that sense the concentration of the blood. They are stimulated by a high blood osmolarity (increased concentration) causing the release of ADH. The hormone then causes the kidney tubules to reabsorb more water to return osmolarity to normal. Volume receptors also play a role when they sense a low blood pressure. Alcohol inhibits ADH secretion.
Oxytocin A major role of this hormone is the stimulation of smooth muscle cells in the pregnant uterus. When labor begins, stretching of the cervix and vagina stimulates a reflex production and release of oxytocin. Oxytocin then travels in the blood to the uterus where it causes more forceful contraction of the smooth muscle. This hormone is also involved in lactation. It causes milk ejection by acting on the smooth muscle surrounding the milk producing cells. Again its production and release is mediated by a neural reflex, the suckling reflex. Emotion, anxiety and pain can inhibit oxytocin release.