Endocrine System2

ADRENAL (OR SUPRARENAL) GLAND • Small, yellow-brown, paired gland situated at anterior end of each kidney like a cap, is a semi-circular structure weighing 3.5 – 4.5 gm (in adult). • Covered by a fibrous connective tissue capsule. The structure is internally differentiated as outer mesodermal cortex and inner ectodermal medulla of nervous origin. Cortex : Thick layer constitutes 70-75% part of the gland distinct as following three zones of cells: (i) Zona glomerulosa : outermost layer below the capsule, made of five layers of small cuboidal basophilic cells densely set a long axis parallel to the surface. (ii) Zona fasciculata : thickest and middle part having large polyhedral cells arranged in radiating columns perpendicular to the surface. (iii) Zona reticularis : the innermost part, having cells arranged in a layer to form network. Medulla: consists of groups of round granulated cells which are modified postganglionic cells of sympathetic nervous system having exclusively secretory function. These are chromaffin cells, distinct in two layers: (i) Zona compactum : densely packed rounded cell with thick plasma membrane, without any intercellular spaces, the average size of cells in 190-210 μm (diameter). (ii) Zona spongiosum : the innermost layer of irregular cells in 5-12 layers with large intercellular spaces with matrix and blood vessels, the average size of cells is 260 μm – 360 μm.

Cortical hormones : About 50 types of steroids have been isolated from adrenal cortex hence called as corticosteroids. But, only few are biologically active. 1. Mineralocorticoids (Aldosterone) A 21-carbon steroid molecule is secreted by zona glomerulosa. Increases the reabsorption of Na+ and Cl¯ by uriniferous tubules, also controls the level of other salts, thus play role in osmoregulation. Increases the removal all K+ in urine. Hypersecretion causes Conn’s disease symptomatized by hypertension (hypernatremia), muscular weakness (hypokalemia), nervous disorders resulting in death. Hyposecretion causes hyponatremia (low B.P.) and hyperkalemia. 2. Glucocorticoids (Cortisol or Hydrocortisone and Corticosterone) ¾ A steroid of 20 carbons it is secreted from zona fasciculate and zona reticularis. ¾ It controls metabolism of carbohydrates, proteins and fats, and stimulates gastric secretions. ¾ Stimulates the conversion of fat and amino acids into carbohydrate (gluconeogenesis). ¾ Reduces amino acid level in tissues, increases ketone formation in blood. ¾ Anti-inflammatory in nature, suppresses the WBC movement, but increases RBC count.

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Immunosuppressive in nature, hence used for treatment of allergy and in transplantation surgery. ¾ Demotes synthesis of collagen, hence cortisol is used for treatment of arthritis of rheumatism. Sex corticoids ¾ Secreted by zona fasciculata and zona reticularis it is a 12 carbon steroid ¾ Influcences sex related changes in muscles and bones, maintains the normal growth of external sex organs, hair growth etc., in pre-puberty stage , also influences sexual behaviour. ¾ Hypersecretion of this hormone causes gynaecomastia in male with female like characters like overgrowth of mammary gland. ¾ Hirsutism (Adrenal virilism) in female : growth of facial hairs, temporal baldness, enlargement of clitoris, atrophy of breast, hoars voice etc. Control of Secretion : ACTH from pituitary maintains its growth and secretion which in turn is controlled by corticoitropin releasing hormone (CHR) from hypothalamus. Secretion of aldosterone is mainly controlled by rennin-angiotensin system. Hypersecretion causes Cushing’s syndrome, with following symptoms : ¾ High glucose level (hyperglycemia), excess deposition of glycogen in liver. ¾ Enhanced protein break-down decreasing its level (muscular weakness) ¾ Osteoporosis due to decrease of protein (ossein) in bone. Hyposecretion causes Addison’s disease with following symptoms : ¾ Low blood pressure, high urea level in blood. ¾ Lower NaCl level and high K+. ¾ Degeneration of seminiferous tubules (in male) and ovarian follicles (in female).

HORMONES OF ADRENAL MEDULLA ¾ ¾

Medullary hormones are formed from the amino acid tyrosine. Chromaffin cells take up tyrosine which is first hydroxylated and then decarboxylated to from dopamine which is further hydroxylated to form noradrenalin, this is then methylated to form adrenalin. ¾ All these, dopamine, noradrenalin and adrenalin together are called as catecholamines that means “catch all”. Adrenaline (or Epinephrine) : Its secretion (from zona compactum) is stimulated by emergency conditions like fear, anger, accident, injury, stress etc. hence is also called emergency hormone. Its effects is like sympathetic nervous system. Norepinephrine (or Noradrenaline) : • Secreted from zona spongiosum and it is a polypeptide. • It is vasoconstrictor and increases blood pressure (both systolic and diastolic) and controls normal circulation of blood. Control of Adrenal Medulla : • Its secretion is controlled by sympathetic (autonomic) nervous system and feed back system through level of adrenaline and nor-adrenaline in the blood. • Hyposecretion causes low B.P. and depression, cocaine and amphetamines are useful for treatment. • Hypersecretion causes high B.P. (hypertension); tranquilizers are useful for treatment. ISLETS OF LANGERHANS • Pancreas act as both exocrine and endocrine gland hence it is referred as heterocrine gland. • The endocrine components of pancreas are Islets of Langerhans [discovered by Langerhans (1950)]. • Covered by thin layer of connective tissue it has many primary lobes, some further divided as lobules. • Each lobule is made of many pancreatic acini. • In between the acini the discrete heaps of cells called islets of Langerhans, constituting only 2% part of the gland with 4 types of cells : (i) α-cell or A-Cell constitute about 20% of total islet cells, peripheral in position, acidophilic in nature, secretes glucagons hormone. Diazoxide and Phenytoin destroy these cells. (ii) β-cells or B-Cell constitute about 60% of islet cells central in position, basophilic in nature; secrete insulin; Alloxan treatment destroys these cells. (iii) δ-cells or D-cell constitute about 10% of the islets; secretes somatostatin which balances the level of glucagon and insulin. Similar hormone is also secreted from hypothalamus known as GHIH. (iv) F-cell constitutes about 5% of the islet cells, secretes pancreatic polypeptide (PP) which restricts the secretion of somatostain from D-cell.

HORMONES OF ISLETS OF LANGERHANS Insulin • Isolated by Banting and Best (1992) and its primary structure was determined by Sanger (1950). • It is a polypeptide with mol. Wt. of about 6,000; comprises of 51 amino acids, arranged in two chains, α-chain of 30 amino acid and β-chain with 21 amino acids, disulphide bonds connect both chains, hypoglycemic (it converts glucose into glycogen) • Increases carbohydrate metabolism, fat deposition in adipose tissue, storage and synthesis of protein. • Increases the permeability of membrane system of cells for glucose, amino acids, K+ and PO4. • Along with growth hormone (GH) it influences the growth of the body. • Its hyposecretion causes diabetes mellitus. • Hypersecretion causes hypoglycemia (extreme case insulin shock). Glucagon • Discovered by Kimbell and Murlin (1953) a polypeptide with molecular weight of 2495; comprises 29 amino acids in a single chain. • It has no effected on muscle glycogen. • Hyperglycemic, (enhance glycogenolysis in liver). • Promotes gluconeogenesis, uptake to amino acids and its deamination in liver. • Lowers blood calcium level by increasing its renal elimination. • It has no effect on muscle glycogen. • It’s hypersecretion causes diabetes mellitus. Somatostatin • Secreted from δ-cells called as GHIH and it balances the level of glucagon and insulin as per requirement of the body.

GONADS •

These are mixed tissues and perform two functions i.e., gametogenic function and endocrine function.

A. Testes •

The interstitial cells (or cells of Leydig) in the stroma secrete androgen (steroid) from which the male sex hormone testosterone is synthesized.

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Induces the production of male gametes (sperms) and their morphogenesis.

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Promotes growth and virility of external genitalia (accessory sex organs), maintains secondary sex characters in puberty age.

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Its hyposecretion causes eunuchoidism, with following symptoms : (i) Less growth of secondary sex organs (prostate, seminal vessels, penis). (ii) Improper spermatogenesis (infertility) and loss of secondary sexual characters. (iii) Extreme hyposecretion causes complete sterility. (iv) Sertoli cells secrete inhibin hormone that inhibits secreation of FSH by anterior pituitary.

B. Ovaries •

Follicular cells and theca of maturing follicles secrete estrogens. Stimulates production, growth and maturation of ovum, functions of secondary sex organs (uterus, fallopian tubes and ducts of mammary glands).

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Maintains external or secondary sexual characters. FSH from anterior pituitary controls the level of estrogen.

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Corpus luteum formed from ruptured follicle after ovulation secretes another hormone progesterone (a steroid) the pregnancy maintaining hormone.

Helps in thickening of uterus wall (endometrium) increasing blood supply. Implantation of zygote on uterus wall, and formation of placenta. It also secrete inhibin hormone which has negative feedback effect with FSH.

THYMUS GLAND (TRAINING SCHOOL OF T-LYMPHOCYTE) • It is a single, bilobed, flattened, pyramidal gland, situated in the mediastinal space just in front of the heart. • Grows in early age and becomes fully developed at 16 years but starts degenerating after 18 years and atrophies after the age of 25 years. • It is partly endocrine and partly lymphoid, covered by a connective tissue capsule and consists of two zones : (i) Outer cortex of lymphocyte-like cells, and (ii) Inner medulla comprising reticular cells.

Thymosin hormones secreted is from this gland with following functions : Stimulates T-lymphocytes to counter the attack of pathogens and antigens. Helps in the development of sex glands. According to immunity theory of ageing, degeneration of thymus is one of the causes of ageing. HYPOTHALAMUS •

It is the floor of diencephalon (fore brain) with many neurosecretory cells which secrete variety of hormones to control the secretion of pituitary gland.

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It also contains several such nerve cells whose axons terminate into posterior pituitary and secrete hormones.

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The ability of endocrine secretion in hypothalamus is to monitor metabolism and hormone level in the body as per the information gathered by brain. Hypothalamic hormones released in blood come to pituitary through hypothalamo-hypophyseal portal circulation.

PINEAL BODY (BIOLOGICAL CLOCK) •

Situated on the roof of 3rd ventricle (diencephalon), covered by capsule from which many septa (or trabeculae) extend inside dividing it into many lobules; contain two types of cells : (i)

Parenchymal pinealocytes with minute processes, neuroglia with dark stained nucleus.

(ii)

A few acervuli or brain sand or corpora arenacea are present with the capsule.

(iii)

It is prominent during childhood but with the onset of puberty it atrophies.

(iv)

Earlier it was considered as vestigial in humans as its functions were not known.

(v)

It secretes two hormones, melatonin and serotonin. Level of melatonin is more during night time and serotonin is more during day time.

(vi)

Serotonin is a powerful vasoconstrictor and also a neurotransmitter.

(vii) Melatonin regulates mood, behaviour, ovarian cycle, sleep wake cycle also. (viii) Melatonin is antigonadal in humans i.e., it delays the onset of puberty. Congenitally blind girls attain puberty at an early. Both melatonin and serotonin are formed from the amino acid tryptophan.