The mammalian endocrine system consists of the following endocrine glands.
(1) Hypothalamus, pituitary and pineal glands, which are associated with the brain.
(2) Thyroids, parathyroid and thymus, associated with the pharynx and bronchial pouches.
(3) Pancreas, adrenals and gonads, which lie in the coelom.
Pituitary gland is one of the most important endocrine glands. It produces several hormones, some of which profusely influence the activities of other endocrine tissues are known as pituitary gland hormones. Its own activity is influenced by hypothalamus and pineal body.
The pituitary is a small ovoid gland of pea size and weighs 0.5 - 1.0 g in man and slightly more in a woman during pregnancy. It remains suspended from the floor of the third ventricle of the brain by a narrow funnel shaped stalk called infundibulum that lies in a depression on the upper surface of the sphenoid bone. Pituitary gland is divisible into two major parts: (i) the anterior pituitary or the pars anterior and (ii) the posterior pituitary or the pars posterior.
The anterior pituitary gland hormones
The six hormones synthesized by the anterior pituitary are as follows: -
i) Growth hormone (GH) or somatotropic hormone (STH)
ii) Adrenocorticotropic hormone (ACTH) or corticotrophin
iii) Prolactin or lactogenic hormone or mammotropin (LTH)
iv) Thyrotrophic hormone or thyroid stimulating hormone (TSH) or thyrotropin
v) Follicle stimulating hormone (FSH)
vi) Luteinizing hormone (LH) or interstitial cell stimulating hormone (ICSH)
Note that, two of these hormones, FSH and LH, are collectively known as gonadotropic hormones.
The pituitary gland hormones GH or STH or somatotropin produced by somatotrophs (acidophil cells). Functionally, somatotropin is the most important hormone for normal growth of the body. It does so I stimulating retention of proteins and calcium in the body, synthesis and disposition of proteins in the tissues, growth and elongation of long bones, and proportionate growth of the muscles and visceral organs. It affects
carbohydrate and fat metabolism. It also increases blood sugar level and lipolysis of the adipose tissue. Normal growth pattern is disturbed by the secretion of abnormal levels of GH. Its deficiency leads to warfism, while excessive secretion produces gigantism and acromegaly. Deficiency of GH in childhood produces profound impairment of growth. Children with deficiency of GH grow less than half of the normal rate, leading to dwarfism Secretion of GH in excess amounts in childhood or puberty causes gigantism.
Adrenocorticotropic hormone (ACTH)
The pituitary gland hormones ACTH is produced by corticotrophs (basophil cells), located chiefly in the central part of the anterior pituitary. Corticotropin releasing factor (CRH) of hypothalamus has a very strong influence on ACTH secretion. Cortisol level of plasma affects the hypothalamus, thereby exerting a feedback control on the hypothalamic CRH. Both, mental and physical stresses cause stimulation of ACTH secretion.
ACTH is necessary for normal growth, development and maintenance of the adrenal cortex. It stimulates the secretion of cortisol and adrenal androgens of the adrenal cortex. It also acts on melanocytes. Thus hypersecretion of ACTH is the cause of pigmentation in Addison's disease. It stimulates lipolysis in the adipose tissue. Moreover, ACTH, along with cortisol and CRH, forms an axis which is the main weapon against various forms of stresses.
Lactogenic hormone or mammotropin (LTH)
Prolactin, one of the anterior pituitary gland hormones is secreted by lactotrophs (mammotrophs), which are acidophil cells. There is a considerable increase in the number of lactotrophs during pregnancy. Prolactin inhibitory factor (PIF) released from the hypothalamus inhibits the secretion of prolactin.
However, mammary teats sucking by baby generate afferent impulses which reflex to stimulate the secretion of prolactin through hypothalamus. Prolactin is responsible for the maintenance of corpus luteum and continued production of progesterone in female mammals. It also supplements the actions of gonadal hormones in stimulating the growth and activity of the female mammary gland during pregnancy and lactation.
Thyroid stimulating hormone(TSH)
Another of the anterior pituitary gland hormones is thyrotropin, synthesized by thyrotrophs, which are basophilic cells. Secretion of thyrotropin is controlled by hypothalamus and the level of circulating thyroxine. Thyrotropin releasing factor (TRF) of hypothalamus causes stimulation of thyiotrophs and thus increases TSH level. TSH secretion is also under the feedback control of thyroxine. The major action of TSH is to stimulate the thyroid gland to produce thyroid hormones.
Follicle stimulating hormone (FSH)
FSH is secreted by gonadotrophs, which are basophilic in nature. Gonadotropin, one of the anterior pituitary gland hormones releasing hormone of hypothalamus. The concentration of sex hormones in the blood also determines the FSH secretion. In females, FSH causes growth and maturation of Graafian follicles; the growing follicle in turn secretes estrogen. However, in males, FSH helps in spermatogenesis and normal functioning of seminal vesicles.
Luteinizing hormone (LH)
LH is also secreted by gonadotrophs and is glycoprotein in nature. In females, LH is chiefly responsible for ovulation, formation and maintenance of corpus luteum and secretion of progesterone. However, in males, it is responsible for the stimulation of interstitial cells which in turn produce the male sex hormone, testosterone.