All endocrine structures develops from epithelia or nervous tissues, or from both. It is noteworthy that some structures described here have other important functions, e.g., kidney, gonads, pancreas.
1. The Hypothalami are paired thickenings in the thalamencephalon and thus nervous in origin. They control most of the endocrine structure, acting through the pituitary and autonomic systems. Groups of cells produce neurosecretions; droplets produced in cells traverse the axons into the pars nervosa, to be stored and later released into the blood. Other neurosecretions (releasing factors) pass in the blood of the hypothalamo – hypophyseal portal system into the pars anterior where they stimulate production of other hormones. Two hormones plus a number of releasing factors are produced. Oxytocin stimulates contractions of the uterus during birth, and promotes secretion of milk during suckling. Vasopressin causes a rise in blood – pressure by contraction of arterioles, and is also anti – diuretic, favoring resorption of water and Na+ ions from the renal tubules. Specific releasing factors affect the secretion of all hormones produced in the pars anterior and pars intermedia.
2. The Pituitary Body is an endocrine structure that lies beneath the thalamencephalon. It is formed by fusion of a hollow down – growth from the brain, the infundibulum, with a dorsal invagination from the stomodaeum, the hypophysis, and is thus nervous and epithelial in origin. The infundibulum becomes the pars nervosa; the endocrine parts of the hypophysis are the pars anterior and pars intermedia. The former secrets six hormones. Somatotropin (STH or growth hormone) stimulates rate of protein synthesis, excess producing gigantism, and deficiency, dwarfness. Thyrotropic hormone stimulates development of the thyroid and promotes production of thyroxin. Adrenocorticotropic hormone induces production of corticoids from the andrenals. Follicle – stimulating hormone promotes development of Graafian follicles in females, and of sperm in males. Luteinizing hormone (LTH) stimulates ovulation and formation of corpora lutea. Prolactin affects the development of the mammary glands and the maintenance of milk secretion. The pars intermedia produces two hormones, both melanocyte – stimulating; they affect formation and distribution of melanin granules in the chromatophores.
3. The Thyroid Gland is another endocrine gland that is a mass of soft, red, vesicular tissue on each side of the larynx. It develops from epithelium in the floor of the pharynx and is homologous with the endostyle of lower chordates. The hormone thyroxin regulates the basic rate of metabolism (BRM); deficiency lowers the BRM and leads to hypothyroidism. In the young, this causes retardation of development, a condition known as cretinism, of mentally retarded dwarfness; in adults, deficiency leads to sluggish metabolism, atrophy of the gland and swelling of the neck – the condition of myxoedema. Excess of thyroxin causes hyperthyroidism; there is a high BRM, restless activity, rapid heart – beat and general wasting of the body. The gland becomes enlarged and the eye – balls protrude; a condition known as exophthalmic goiter. Deficiency of iodine may cause simple goiter. The effects of thyroxin goiter. Deficiency of iodine may cause simple goiter. The effects of thyroxin in promoting metamorphosis in amphibian and some fishes are notable.
4. The Suprarenal Glands are small, yellow, ovoid structures, one anterior to each kidney. The cortex is derived from neural crests and the medulla from coelomic epithelium. The medulla secretes adrenalin and nor – adrenalin after sympathetic stimulation. These produce dramatic changes in the body in response to fear, stress and shock. The three zone of the cortex secrete different hormones; the outer zone produces mineralocorticoids called aldosterone and deoxycorticosterone which play a vital part in osmoregulation; the middle zone secretes the glucocorticoids, cortisome and cortisol, which affect carbohydrate metabolism; the inner zone produces androgens and oestrogens which promote development of the secondary sexual characteristic.
5. The Parathyroids are two pairs of small glandular masses, lateral to the thyroid, and derived from epithelium of the third and fourth visceral clefts. The secretion, parathormone affects Ca++ and PO4- – – ions between bones and blood; it also affect excretion and resorption of these ions.
6. The Thymus Gland, derived from epithelium dorsal to the visceral clefts, is a large, soft, pink mass above the heart: it degenerates considerably with age. In the young, it is the main source of lymphocytes and is important in immunological reactions. The hormone, thymosin, stimulates lymphocyte production in the lymphatic glands.
7. The Pineal Body is an endocrine structure that is a hollow dorsal outgrowth of the thalamencephalon. In lamprey larvae and some reptiles, it has a complete eye like structure, and in the former, it has an undoubted effect on skin colour. In amphibians, the hormone melatonin causes blanching of the skin; the same effect may occur in mammals. Melatonin certainly stimulates maturation of the gonads, probably by stimulating secretion of gonadotropic releasing factors from the hypothalamus.
8. The Pancreas is derived from endodermal epithelium. The islets of Langerhans contain two types of cells; large β cells which secrete insulin, and smaller ά cells are stimulated by STH. Deficiency of insulin results in diabetes mellitus.
9. The Ovaries, like the testes, are derived from coelomic epithelium. The follicular phase of the oestrous cycle is activated by FSH; this induces development of the follicle, stimulating them to produce oestrogens, the most potent being oestradiol. These activate the reproductive tract in preparation for pregnancy and stimulate development of mammary glands and secondary sexual characteristics. The luteal phase is activated by LH which induces repair of the ruptured follicles and growth of corpora lutea; the latter, activated by prolactin, secrete progesterone which inhibits further ovulation, prepares the uterus for implantation and induces further development of the mammae. Relaxin from the corpora lutea, promotes dilation of the cervix and relaxation of the public symphysis during birth.
10. The Placenta as an endocrine structure is epithelial in origin, part from the uterine lining, and part from the trophoblast. Three hormones are secreted; a gonadotropin, an oestrogen and progesterone; they influence growth of the uterus and mammae after ovaries secretions have wanted, and assist in maintenance of corpora lutea until birth.
11. The Testes, after stimulation by FSH, produce at least two androgens, androsterone and testosterone, which play a part in spermatogenesis, control activity of the seminal vesicle and prostate glands and help in the formation of the male secondary sexual characteristics.
12. The Kidneys is a major endocrine structure involved in osmoregualtion. The kidneys are derived from coelomic epithelium. A thickened pad of cells on each tubule, the juxtaglomerular complex, secrets rennin in response to falling concentration of Na+ in the blood. Rennin converts a plasma globulin, hypertensinogen, into hypertension, which then causes vasoconstriction and stimulates release of mineralocorticoids; these raise the level of Na+ resorption from the tubules.
13. The Alimentary Canal: The gastic mucosa secretes gastrin, which induces continued flow of gastric juice; enterocrinin, which starts enzyme secretion of salt solution from the pancreas. The duodenal mucosa produces secretin, which stimulates further flow of pancreatic salt solution; pancreomysin induces secretion of pancreatic enzymes; cholecystokinin causes contraction of the gall bladder, and enterogastrone inhibits acid secretion by the stomach.
14. Nerve Endings secrets hormones which excite adjacent cells, sympathetic fibres secrete sympathin, identical in effects with adrenalin; parasympathetic and somatic motor fibres produce acetylcholine, which also facilitates transmission of impulses across synapses.
15. Tissues Hormones: Histamine and acetylcholine, both powerful depressors, are probably present in inactive form in all cells. They are activated when cells are damaged and play an important part in the process of healing.