Biochemistry of Hormones or Chemical nature of hormones:
Most hormones are proteins (Polypeptides) derivates of amino acids (amines) or steroids. A few are fatty acid derivates e.g. most invertebrate neurosecretary cells produce polypeptides called neuropeptides. Hormones that the vertebrate pancreas secretes are proteins; those that the thyroid gland secretes are amines. The ovaries, testes and cortex of adrenal glands secrete steroids. Hormones are effective in small amount. Only few molecules of hormone may be enough to produce a dramatic response in a target cell. In target cell hormones help to control biochemical reactions in three ways: (i) A hormone can increase the rate at which other substances enter or leave the cell. (ii) It can stimulate a target cell to synthesize enzymes, proteins or other substances. (iii) It can prompt a target cell to activate or suppress existing cellular enzymes. Hormones are not changed by reaction they regulate.
Lobes of pituitary glands (Hypophysis):
Pituitary gland has two lobes, the anterior lobe (adenohypophysis) and posterior lobe (neurohypophysis). Two lobes differ in several ways (i) Adenohypophysis is larger than neurohypohysis (ii) Secretary cells called pituicytes are in adenohypophysis but not in neurohypophysis (iii) Neurohypophysis has greater supply of nerve endings. Pituicytes produce and secrete hormones directly from adenohypophysis whereas neurohypophysis obtains it hormones from neurosecretory cells in hypothalamus, storing and releasing then when they are needed. These modified hypothalamic nerve cells project their axons down a stalk of nerve cells and blood vessels called infundibulum into pituitary gland directly linking the nervous and endocrine system.
Pancreatic hormones:
Pancreas is an elongated fleshy organ posterior to stomach, endocrine portion of which makes up only about 1% of the gland. This portion synthesizes, stores and secretes hormones from clusters of cells called pancreatic islets of langerhans which are many in number scattered throughout the gland. Each islet has four groups of cells called alpha (α), beta (β), delta (δ), and F cells. Alpha cells produce hormone glucagons and beta cells produce insulin. Delta cells secrete somatostatin. The hypothalamic growth hormone inhibiting factor that also inhibits glucagons and insulin secretion. F cells secrete pancreatic polypeptide into blood stream after a meal and inhibits somatostain secretion, gall bladder contraction and secretion of pancreatic digestive enzymes, when glucose concentration in blood is high such as after a meal, beta cells secrete insulin which promotes uptake of glucose by body cells including liver cells, where excess glucose can be converted to glycogen.
Adrenal glands:
In mammals two adrenal glands rest on top of the kidney. Each gland consists of inner portion medulla and outer cortex.
Adrenal cortex: It secretes three steroid hormones: cortisol, aldosterone and sex hormones (anderogens, estrogens). The cotrisal helps to regulate over all metabolism and the responses to infection or tissue injury. Aldosterone helps to maintain concentration of solutes (such as sodium) in the extra cellular fluid when either food intake or metabolic activity.
Adrenal Medulla: It is under neural control. It contains neurosecretory cells that secrete epinephrine (adrenaline) control heart rate and carbohydrate metabolism. Brain centres and hypothalamus govern the secretions via sympathetic nerves.
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