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Origin, position and structure : This is a small, (0.1 to 0.29) whitish and somewhat flattened ectodermal gland situated at the tip of a small, fibrous stalk that arises from dorsal wall of diencephalon, i.e. the roof (epithalamus) of third ventricle of the brain. Due to its location, it is also called epiphysis cerebri. It is covered over by a thin capsule formed of the piamater of the brain. Septa from this membrane extend into the gland, dividing in into lobules having two types of branched cells, viz the large and modified nerve cells, called pinealocytes, and interstitial or neuroglial cells forming the supporting tissue. In the pineal gland starts degenerating after the age of about 7 years because of deposition of granules of calcium salts (brain sand) in it. Function of pineal body : Hormone, though the function of the gland is still the subject of current research, it is known to secrete one hormone, melatonin. Melatonin concentration in the blood appears to flow a diurnal (day-night) cycle as it arises in the evening and through the night and drops to a low around noon. Melatonin lightens skin colour in certain animals and regulates working of gonads (testes and ovaries). Light falling on the retina of the eye decreases melatonin production, darkness stimulates melatonin synthesis. Girls blind from birth attain puberty earlier than normal, apparently because there is no inhibitory effect of melatonin on ovarian function. Serotonin, a neurotransmitter found in other locations in the brain, is also found in the pineal gland. Research evidence is accumulating to support the idea that the pineal gland may be involved in regulating cyclic phenomena in the body. Melatonin also is a potent antioxidant. Melatonin causes atrophy of gonads in several animals.

(1) Position and structure : These are four in number which are wholly or partially embedded in the dorsal surface of the thyroid gland two glands in each lobe of thyroid gland. Each is oval shaped, small sized \[(5\times 5\text{ }mm)\] and yellow coloured. Histologically, a parathyroid gland is formed of masses of polygonal cell arranged in cords. Endocrine cell are two types principal or chief and oxyphil cells. Parathyroid is endodermal in origin.     (2) Hormones of parathyroid : Active hormone secreted by parathyroids is parathormone (PTH), also called Collip's Hormone (Phillips collip, 1925). It was discovered and purified by Collip in 1925. Its crystals were first prepared by Craig and Ras mussen in 1960. Its molecular structure was worked out by potts and his associates in 1971. The latter is a protein of 84 amino acid monomer. It is a polypeptide hormone. Parathyroids are present in all vertebrates except fishes. Its secretion is stimulated by low level of calcium in blood than normal level through feedback control. Functions of parathormone : Parathormone is essential for survival, because it significantly contributes to "homeostatis" by regulating the amount of calcium and phosphate ions in ECF. Our body requires an optimum calcium level (10.0 to 11.5 mg per 100mL.) in ECF (total 1000 to 1120 grams in a 70 kg man), because calcium is a key element in many physiological functions like proper permeability of cell membranes, muscular activities, nerve impulse conduction, heartbeat, blood coagulation, bone formation, fertilization of ova, etc. Calcium is most abundant of all minerals found in the body and about 99% of calcium and phosphorous are contained in the bones. Maintenance of proper calcium level under 'homeostasis' is, in fact, a combined function of parathormone, thyrocalcitonin and vitamin D3 (cholecalciferol). Parathormone promotes absorption of calcium from food in the intestine and its reabsoption from nephrons in the kidneys. Simultaneously, it accelerates elimination of phosphates in urine (phosphaturic action). Thus, calcium level tends to rise in the ECF due to the effect of parathormone. This calcium is, then, utilized by bone-forming cells – osteoblast – in bone formation under the influence of vitamin D3. Bones are asymmetrical when first formed. Their unnecessary parts are, therefore, dissolved by bone-eating cells called osteoclasts. This process also proceeds under the influence of parathormone. It results in release of calcium and phosphate in blood. Vitamin D3, is a steroid hormone which is first synthesized in an inactive form in skin cells from 7-dehydrocholesterol under the influence of ultraviolet (UV) rays of sunlight. Skin cells release it in blood. Liver cells take it from blood, change in into 25-hydroxycholecalciferol and release back into blood. Finally, the cells of proximal convoluted tubules of nephrons in the kidneys change 25-hydroxycholecalciferol into 1-25-dihydroxycholecalciferol under the influence of parathormone. This last compound is released in blood as active vitamin D3 named as cholecalciferol (calcitriol). In addition to its role in bone-remodelling, D3 more...

Location and origin : Pancreas (Gr. pankreas = sweet bread; Fr., pan = all + kreas = flesh) is a flattened and pinkish mixed gland (both exocrine and endocrine) situated in the concavity formed by duodenum just behind the stomach. It measures about 15 cm in length and 4 to 5 cm in breadth. It forms by fusion of two bilateral endodermal processes of embyronic intestine (duodenum of future adult). Structure : About 99% part of the gland is exocrine and formed of hollow pancreatic acini or lobules embedded in a connective tissue stroma. In the stroma, there are numerous (approximately 1 to 2 million in human pancreas) small (0.1 to 0.2 mm in diameter) clusters of endocrine cells, called islets of Langerhans after the name of their discoverer, Paul Langerhans (1869).     Cells Types in the Pancreatic Islets Each pancreatic islet includes four types of hormone-secreting cells : (1) Alpha or A cells constitute about 15% of pancreatic islet cells and secrete glucagon. (2) Beta or B cells constitute about 80% of pancreatic islet cells and secrete insulin. (3) Delta or D cells : It’s constitute about 5% of pancreatic islet cells and secrete somatostatin (identical to growth hormone-inhibiting hormone secreted by the  hypothalamus). (4) F cells : It’s constitute the remainder of pancreatic islet cells and secrete pancreatic polypeptide. Hormones of pancreas and their role The \[\beta \] and \[\alpha \] cells of islets of Langerhans respectively secrete insulin and glucagon hormones which are important regulation of carbohydrate, protein and fat metabolism in the body. (1) Insulin : In 1889, Minkowski and Mehring discovered that pancreas is related with the disease of diabetes mellitus in humans. Normal concentration of glucose in blood is about 100 mg (0.1 gm) per 100 ml. It increases somewhat after a carbohydrate rich food. Then, the secretion of insulin increases. It increases the permeability of all cells for glucose several times, except that of brain cells and red blood corpuscles (RBCs). The brain cells and RBCs are already highly permeable to glucose. After taking more glucose from blood, the cells utilize it for energy-production. Consequently, the basal metabolic rate (BMR) and RNA and protein synthesis increases in cells. In 1923, two Canadian scientists, Banting and Best succeeded in preparing a pure extract of insulin from the pancreatic islets of a new born calf with the help of Macleod, Banting and Macleod won the 1923 Nobel prize for this work. Later, Abel (1926) succeeded in preparing pure crystals of insulin. F. Sanger (1955) worked out the molecular structure of bovine insulin and won the 1958 Nobel Prize. He discovered that insulin is a small protein whose molecule consists of two polypeptide chains, \[\alpha \]and \[\beta ,\] joined by disulphide linkages and respectively formed of 21 and 30 amino acid residues. Insulin is the first protein to be crystallized in pure form, first protein whose molecular structure was worked out, more...

Hormones described so far are called circulating hormones, because these circulate in whole body with blood. When stimulated by physical or chemical stimuli, all body cells, except red blood corpuscles (RBCs), secrete certain such compounds which transmit coded informations of metabolic adjustments between neighbouring cells and hence remain ECF instead of diffusing into the blood. These compounds are called local tissue hormones or autocoids. These are short-lived, because various enzymes present in ECF continue degrading these at a fast rate. Local hormones are of two main categories-paracrine and autocrine. Paracrine hormones affect metabolism of cells located in the neighbourhood of those which secrete them. Autocrine hormones affect metabolism of the every cells which secrete them. Most local hormones are paracrine. These belong to the following categories :  (1) Eicosanoids : These are a category of lipids derived from a fatty acid, arachidonic acid, synthesized in the plasma membrane of cells, and released in ECF. These are of four categories, viz. Prostaglandins, prostacyclins, thromoboxanes and leukotrienes. (i) Prostaglandins (PGs) : In 1935, Ulf von Euler discovered that human semen contains a very active compound presumably secreted by prostate gland and, hence, named as such. He found that after the semen is discharged in woman’s vagina, this compound contracts uterine muscles to facilitate the sperms to ascend into fallopian tubes and reach ova to fertilize these. (ii) Prostacyclins : These are found in walls of blood vessels and induce vasodilation. These also facilitate flow of blood in vessels and prevent thrombosis by inhibiting aggregation of platelets. (iii) Thromboxanes : These are secreted by blood platelets. These help in blood clotting by instigating aggregation of platelets at the place of injury. These also instigate vasoconstriction at places of injury to prevent excessive loss of blood. (iv) Leukotrienes : These are secreted by eiosinophils of blood and mast cells of connective tissues. These serve as mediators in inflammatory and allergic reactions, induce bronchoconstriction (constriction of bronchioles), constrict arterioles and induce migration of neutrophils and eosinophils towards the places of inflammation. These can cause asthma, arthritis, colitis, etc. (2) Neuroregulators : These are a category of proteins which function as paracrine hormones in nervous tissues. These can be classified in three categories as follows : (i) Neurotransmitters : These are synthesized in nerve cells and are secreted by exocytosis by axon terminals of these cells. These serve to transmit nerve impulses from one neuron to other neighbouring neurons, or muscles, or glands across synapses. About 60 to these have so far been discovered, but the most common of these are acetylcholine, norepinephrine, dopamine, serotonin and histamine. (ii) Neuromodulators : In nervous tissues, the neurons secrete such paracrine hormones which modulate (increase or decrease) the excitability of other neighbouring neurons. These hormones are called neuromodulators. The main positive neuromodulators which increase the excitability of other neurons are the amino acids glutamate and aspartate, and polypeptide named ‘P’ substance. Contrarily, the main negative modulators which decrease the excitability of neighbouring neurons are the amino acid glycine and more...

Position and Structure : Hypothalamus is the floor of diencephalon. It is formed of masses of grey matter, called hypothalmic nuclei, containing neurosecretory cells. It is connected with anterior pituitary lobe by blood capillaries of hypophyseal portal system and with the posterior pituitary lobe by axons of its neurons, both passing through the pituitary stalk. Hormones of hypothalamus : Neurosecretory cells of hypothalamus secrete neurohormones called releasing factors (RF) or inhibiting factors (IF). These neurohormones are carried by hypophyseal portal system to adenohypophysis (primary target organ) and stimulate or inhibit the release of trophic hormones from adenohypophysis. These neurohormones are proteinous in nature and formed of \[320\] amino acids.   Neurohormones of Hypothalamus
S.No. Neurohormones Physiological effects
1. TRH (Thyroid Releasing Hormone - Releasing Factor) Increased TSH secretion from adenohypophysis.
2. ACTH-RF (Adrenocorticotrophic Hormone-Releasing Factor) Increased ACTH secretion from adenohypophysis.
3. STH-RF  
  more...
Hormones are informational molecules secreted by the endocrine cells in one part of the body and carried by blood to another part where they stimulate or inhibit specific physiological process. Discovery : First hormone discovered was secretin. It was discovered by two English physiologists : William M Bayliss and Ernest H. Starling in 1903. Term hormone was coined by starling (1905) from Greek word Homone means to excite. It is a mishomer because a number of hormones are known to have inhibitory effect (e.g., Somatostatin). General function of hormones (1) Some hormones control Basic Metabolic Rate (BMR) e.g., thyroxine of thyroid gland. (2) Some hormones control the secretion of other endocrine glands, e.g., Tropic hormones of Anterior pituitary control Thyroid, Adrenal cortex, gonads, etc.  (3) Some hormone control blood pressure e.g., Aldosterone, Atrial Natriuretic Hormone (ANH) of heart, Vasopressin or ADH’, oxytocin and Renin of kidney. (4) Increase production of RBC e.g., erythropoietin of kidney. Properties of hormones (1) These are secreted by endocrine gland (biogenic in origin). (2) Their secretions is released directly into blood (except local hormones e.g., gastrin). (3) These are carried to distantly located specific organs, called target organ. (4) These have specific physiological action (excitatory or inhibitory). These co-ordinate different physical, mental and metabolic activities and maintain homeostasis. (5) The hormones have low molecular weight e.g., ADH has a molecular weight of \[6002000\] daltons. (6) These act in very low concentration e.g., around \[{{10}^{10}}\] molar. (7) Hormones are non antigenic. (8) These are mostly short-lived. So have a no cumulative effect. (9) Some hormones are quick acting e.g., adrenalin, while some acting slowly e.g., oestrogen of ovary. (10) Some hormones secreted in inactive form called Prohormone e.g., Pro-insulin. (11) Hormones are specific. They are carriers of specific information to their specific target organ. Only those target cell respond to a particular hormone for which they have receptors. (12) Hormones after their action destroyed in liver and kidney. Classification of hormones (1) On the basis of chemical nature : On the basis of chemical composition hormones are classified into three categories. (i) Amine hormones : These are derived form tyrosine amino acid and have amino \[(-N{{H}_{2}})\] group e.g., Thyroxine, Epinephrine, Nor-epinephrine. (ii) Steroids : These are fat soluble and have sterol group. These are derived from cholesterol e.g., hormones of adrenal cortex (cortisol, cortisone, corticosterone, aldosterone) testes (testosterone) and ovaries (estrone, estradiol, progesterone etc.) (iii) Proteinous and peptide hormones : These are formed of \[3200\] amino acids interlinked by peptide bonds and are water soluble e.g., (a) Proteinous hormones like STH, TSH, FSH, LH etc. Out of these FSH and LH are glycoproteins. (b) Long peptide hormones like insulin and glucagon, ACTH, Paratharmone. (c) Short peptide hormones like oxytocin, ADH, MSH. These hormones formed of a few amino acids. (2) On the basis of mode of action (i) Quick acting hormones : These hormones initiate immediate response from their target cells. There receptor is always located on the outer surface of plasma membrane of target cell more...

The gonads are the sex glands, the testes and the ovary. Testes is the male gonad and ovary is the female gonads. Besides producing gametes, the gonads secrete sex hormones from the onset of puberty (sexual maturity) to control the reproductive organs and sexual behaviour. The sex hormone were discovered by Adolf Butenononal in 1929 and 1931. He won the 1939 Nobel prize jointly with Leopold Ruzicka. Testes Location and structure : In testes between the siminiferous tubules, special types of cells are present called interstitial cells or cells of leydig. These cells secrete male hormones (androgens) derived from cholesterol. The main androgen is testosterone other less important androgens include androstenedione and dehydroepiandrosterone (DHEA). It is a masculinizing hormone. From puberty to the age of about twenty year i.e. adolescence or the period of sexual maturation or attainment of adult hood.       Function (1) It stimulates the male reproductive system to grow to full size and become functional. (2) It stimulates the formation of sperms (spermatogenesis) in the seminiferous tubules. (3) It stimulates the development of male accessory sex characters such as hair on the face (beard and moustaches), growth and distribution of hair on the body, thickening of skin, deepening of voice, enlargement of larynx, broadening of shoulders, Narrow lips enlarged and stronger bones and muscles. It also maintains these characters. (4) It also determines the male sexual behaviour sex urge, aggressive behaviour.   (5) Under its effect protein anabolism increases. (This function is obvious in the heavier muscle and bone mass of most men as compared to women). (6) Grythropoisis in bone marrow increases. (7) In brief, testosterone determines libido or sex drive. It is also required, together with the follicle stimulating hormone (FSH) of pituitary, for initiation and completion of spermatogenesis. All androgens are also secreted in traces from adrenal glands in both boys and girls. (8) Increased sebaceous gland secretion. Development of testis : Under the effect of chorionic gonadotropic hormone, secreted by placenta during pregnancy, the testes of eight to nine months old fetus start secreting testosterone. The latter regulates differentiation and development of urinogenital system, accessory genital organs and external genitalia in the embryo. During childhood i.e. from birth to puberty (age of 11 to 13 years), testes remain quiescent, so that androgens are not secreted. At puberty, the gonadotropic hormones (FSH and ICSH) of pituitary reactivate the testes which, therefore, start producing sperms and resume secreting androgens. Upto the age of about 40 years, androgens are secreted in sufficient amounts. There after, their secretion starts gradually declining, but the capability of reproduction still continues for many years. Castration : Surgical removal of testes is called castration or orchidectomy. Castration, or deficient secretion of testosterone (hypogonadism) before puberty (due to congenital defects or injury to testes) retards growth of genitalia, muscles and bones, as well more...

(1) Gastro-intestinal mucosa : Inner most layer of the wall of the alimentary canal is called mucosa. Certain cell of the mucosa of the stomach and intestine secrete important hormones. Gastro-intestinal mucosa is endodermal in origin. (i) Stomach : The mucosa of the pyloric stomach near the duodenum secretes a hormone called gastrin. Presence of food in the stomach provides a stimulus for gastrin secretion. Gastrin stimulates the gastric glands to produce the gastric juice. It also stimulates the stomach movements. (ii) Intestine : The intestinal mucosa secretes six hormones : secretin, cholecystokinin, enterogastrone, enterocrinin, duocrinin and villikinin. Entry of acidic food from the stomach into the duodenum serves as a stimulus for the release of these hormones. (a) Secretin : It is produced by the small intestinal mucosa. It causes the release of sodium bicarbonate solution from the pancreas for pancreatic juice and from the liver for bile. It also inhibits the secretion and movements of stomach. (b) Cholecystokinin-pancreozymin (CCK-PZ) : This hormone is secreted by the mucosa of entire small intestine. The actions of cholecystokinin and pancreozymin were discovered independently. But it has been discovered that both hormones have similar effects and hence it is considered one hormone. As the name suggest \[CCK-PZ\] has two main functions. The word cholecystokinin is derived from three roots : Chol meaning bile, Cyst meaning bladder, and kinin meaning to remove. The word pancreozymin is derived from pancreas and Zymin, which means enzyme producer. This hormone stimulates the gall bladder to release the bile and also stimulates the pancreas to release its enzymes. (c) Enterogastrone : It is secreted by the duodenal mucosa. It shows gastric contractions and stops the secretion of gastric juice. (d) Enterocrinin : It is secreted by duodenal mucosa. It stimulates crypts of Lieberkuhn to secrete the enzymes in the intestinal juice. (e) Duocrinin : It is secreted by the duodenal mucosa. It stimulates the release of viscous mucus from Brunner’s glands into the intestinal juice. (f) Villikinin : It is secreted by the mucosa of the entire small intestine. It accelerates the movements of villi to quicken absorption of food. (2) Placenta : When the early embryo reaches into the uterus from fallopian tube, it becomes implanted with uterine wall by a placenta for support and nutrition. The cells of placenta secrete two steroid hormones (estradiol and progesterone) and two protein hormones (human chorionic gonadotropin-hcG and human chorionic somato mammotrophin-HCS). Early placenta secretes so much of chorionic gonadotropin that the latter starts being exerted in mother’s urine just after about two weeks of pregnancy. Its presence in urine is used for pregnancy test. It serves to maintain the corpus luteum, and to stimulate it for secretion. Due to its effect, the corpus luteum continues secreting estrogens, progesterone and relaxin. It also serves to maintain pregnancy by preventing contraction of uterine wall. After about three months of pregnancy, secretion of progesterone by the placenta increases. Hence, importance of corpus luteum decreases, and it starts degenerating. If therefore, ovaries more...

Adrenal gland was first reported by Eustachius. Origin and position : The adrenals are paired glands placed on the top or superior of the kidneys as cap. Hence, they are also called suprarenal glands. Adrenals have a dual origin, they are originated from ectoderm and mesoderm both. Like thyroid, adrenals also highly vascular in nature.     Structure : Each adrenal is a small ( 5 cm long, 3 cm broad and 1 cm thick), triangular and yellowish cap like structure. Its weight in humans is about 3.5 to 5.09 gm. At birth Adrenal gland best developed. Each gland has two parts – Outer cortex and inner medulla. (1) Outer cortex : The cortex is derived from mesoderm and forms about 80% part of the gland. Cortex consists of fatty, cholesterol rich cells. These cells distinguish the cortex into three zones or regions. (i) Zona glomerulosa : It is the outer part of the cortex (15% of the gland), which consists of small polyhedral cells. It secrets mineralocorticoids e.g., Aldosterone. (ii) Zona fasciculata : It is the middle part of the cortex (50% of the gland). Which consists of large polyhedral cells. This part secrets gluco-corticoids. e.g., Cortison, Corticosterone. (iii) Zona Reticularis : It is the inner part of the cortex (7% of the gland). In which the parallel cell cords of the zona fasciculata branched to form a loose anastomosing network. It secrets sex hormones. (2) Inner medulla : The medulla is derived from ectoderm and forms about 20% part of the gland. Adrenal medulla is reddish brown in colour and colourless of rounded groups of short cords of relatively large and granular cells. These cells are modified postganglionic cells of sympathetic nervous system. These are called chromaffin cells or phaeochromocytes. Adrenal medulla secrets adrenalin and nor-adrenalin which are collectively called as catecholamines.  Hormones of adrenal cortex About 20 steroids (steroidogenic) compounds have secreted from adrenal cortex. These are called adrenocorticoids (corticosteroids). Only few of them are biologically active as hormone. These hormones are steroid in nature. The later, however account about 80% of the secretion of adrenal cortex and are classified into three categories.     (1) Mineralo-corticoids : The principal mineralocorticoid is aldosterone. It is also called salt-retaining hormone. It promotes reabsorption of sodium ions from kidney and excretion of potassium ions in urine. It also reabsorb Cl– ions from kidney. Thus aldosterone has a important contribution in homeostasis by controling osmatic pressure of ECF (Extra cellular fluid). Remember that doctors administer saline drip to the patients who lose excessive water and salts due to diarrhoea, cholera, etc. Aldosterone also helps in maintaining acid-base equilibrium and blood pH (7.35) by promoting reabsorption of \[HC{{O}^{}}_{3}\] and regulating excretion of \[{{H}^{+}}\]by kidneys. It also promotes absorption of water and salt in intestine, mainly in colon. The Renin-angiotensin-aldosterone or RAA pathway controls more...

(1) Exteroreceptors : Receive stimulation directly from external environment. These may be of following type (i) Photoreceptor : Sensitive to light (Eye)                            (ii) Thigmoreceptor : Sensitive to touch. (iii) Tectoreceptor : Sensitive to touch. (iv) Tangoreceptor : Sensitive to touch pressure. (v) Phonoreceptor : Sensitive to sound (Ear). (vi) Olfactoreceptor : Sensitive for smell (Nose). (vii) Gustoreceptor : Sensitive to taste (Tongue). (viii) Thermoreceptor : Sensitive to temperature. (ix) Calo receptor : Sensitive to heat. (x) Frigido receptor : Sensitive to cold. (xi) Galvano receptor : Sensitive to electric current. (xii) Rheoreceptor : Sensitive to water or air current. (xiii) Geo receptor : Sensitive to gravity. (xiv) Telero receptor : Sensitive to distance. (Receptors of vision, hearing and smell receive stimuli from a distance hence called teleroreceptor). (2) Proprioceptors : Proprioceptors are located in skeletal muscles, joints, tendons etc. It is from these receptors that we know the position of our arm or leg without having to look at it. (3) Intero receptor : These are present in internal organ. Ex. receptor for hunger, thirst, pain and balancing.


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