Chapter II - Endocrine System
The pituitary gland, or hypophysis, is an endocrine gland about the size of a pea that sits in the small, bony cavity (sella turcica) at the base of the brain. Its posterior lobe is connected to a part of the brain called the hypothalamus via the infundibulum (or stalk), giving rise to the tuberoinfundibular pathway. The posterior lobe is thus derived from neural ectoderm while the anterior lobe is derived from oral ectoderm. The anterior pituitary lobe receives releasing hormones from the hypothalamus via a portal vein system. The pituitary gland secretes hormones regulating a wide variety of bodily activities, including trophic hormones that stimulate other endocrine glands. For a while, this led scientists to call it the master gland, but now we know that it is in fact regulated by releasing hormones from the hypothalamus. It is physically attached to the brain by the pituitary, or hypophyseal stalk connected with the median eminence.
The pituitary gland is divided into two sections: the anterior lobe (adenohypophysis) and the posterior lobe (neurohypophysis). The posterior pituitary is, in effect, a projection of the hypothalamus. It does not produce its own hormones, but only stores and releases the hormones oxytocin and antidiuretic hormone (ADH - also known as vasopressin).
The anterior lobe of pituitary produces six hormones, which are instrumental in controlling the activities of various other endocrine organs. These are:
1. Growth hormone: It regulates general body growth, growth of bones, cartilages, and also muscle growth.
2. Adrenocorticotrophic hormone (ACTH): ACTH controls activity of adrenal cortex to release glucocorticosteroid, especially cortisol.
3. Thyroid-stimulating hormone (TSH) or thyrotropin: TSH controls the activity of thyroid gland and helps in the secretion of hormones.
4. Follicle-stimulating hormone: In women, the follicle-stimulating hormone stimulates the development of ova in the Graafian follicles of ovary and also stimulates the secretion of estrogen by follicle cells. In male, it stimulates formation of spermatozoa in testes.
5. Luteinizing hormone (LH) or interstitial cell-stimulating hormone (ICSH): In females, it stimulates ovulation, formation and maintenance of corpus luteum, secretion of progesterone in the ovary, and prepares the uterus for implantation. In men, it stimulates development and activity of interstitial cells to produce testosterone and semen. (Note: Both LH and FSH are together called pituitary gonadotrophic hormones as they control the growth and activity of the gonads and the functions associated with them.
6. Prolactin: Prolactin initiates and maintains milk secretion by the mammary glands after childbirth.
Posterior lobe of pituitary secretes
I. Antidiuretic hormone (ADH) or vasopressin: ADH regulates the amount of water passed by the kidneys. It also increases t he reabsorption of water by the kidneys and raises the blood pressure by constricting the peripheral blood vessels.
2. Oxytocin: Oxytocin stimulates contraction of a pregnant uterus during labor and childbirth. It also stimulates the milk expulsion from the mammary glands.
There is also an interaction between the hormones from the hypothalamus, i.e. TRH induces the release of prolactin. The control of release of hormones from the pituitary is in a negative feedback loop. Their release is inhibited by increasing levels of hormones from the target gland on which they act.
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Chapter II - Endocrine System