Human endocrine system includes the following glands.
Hypothalamus (Supreme commander)
Hypothalamus produces several neurogenic hormones and its target organ is pituitary. Hypothalamus is the basal part of diencephalon, forebrain and it regulates a wide spectrum of body functions. It contains several groups of neurosecretory cells called nuclei which produce hormones. These hormones regulate the synthesis and secretion of pituitary hormones.
However, the hormones produced by hypothalamus are of two types, the releasing hormones (which stimulate secretions of pituitary hormones) and the inhibiting hormones (which inhibit secretions of pituitary hormones).
Adrenocorticotropic releasing hormone (ARH) or corticotropin releasing hormone: It stimulates the anterior lobe of the pituitary gland to secrete its adrenocorticotropic hormone (ACTH).
Thyrotropin releasing hormone (TRH): It stimulates the anterior lobe of the pituitary gland to secrete its thyroid stimulating hormone (TSH) or thyrotropin.
Growth hormone-releasing hormone (GHRH): It stimulates the anterior lobe of the pituitary gland to release its growth hormone (GH) or somatotrophin. Growth hormone-inhibitory hormone (GHIH): This hormone is also called somatostatin (SS). It inhibits the secretion of growth hormone from the anterior lobe of the pituitary gland.
Gonadotropin releasing hormone (GnRH): It stimulates the anterior lobe of the pituitary gland to secrete two gonadotropic hormones; follicle stimulating hormone (FSH) and luteinising hormone (LH).
Prolactin releasing hormone (PRH): It stimulates the anterior lobe of the pituitary gland to secrete its prolactin.
Prolactin inhibitory hormone (PIH): It inhibits the secretion of prolactin from the anterior lobe of pituitary gland.
MSH releasing hormone (MSHRH): It stimulates the intermediate lobe of the pituitary gland to secrete its melanocyte stimulating hormone (MSH).
MSH inhibitory hormone (MSHIH): It inhibits the secretion of melanocyte stimulating hormone from the intermediate lobe of the pituitary gland.
Pituitary gland (Master endocrine gland)
It is smallest endocrine gland having the weight of about half a gram. The gland is attached to the brain by a stalk called infundibulum which is continuous with the hypothalamus. The pituitary gland is formed of two main lobes of different origin.
1. Adenohypophysis has three parts – pars distalis, pars tuberalis and pars intermedia.
2. Neurohypophysis also possesses three regions – pars nervosa, hypophysial stalk and median eminence. The last two are together called infundibulum. Therefore, histologically pituitary gland has six parts.
Morphologically there are three regions – anterior lobe, middle lobe (intermediate) and posterior lobe. Anterior and middle lobes functionally constitute adenohypophysis while posterior lobe forms neurohypophysis. Adenohypophysis is formed of reticular connective tissue and epitheloid or parenchyma cells. Pars distalis is the largest part of adenohypophysis, as well as entire pituitary.
Neurohypophysis is formed of pituicytes, nervous tissues and blood vessels.
Pituicytes are large, spindle shaped, ocassionally branched cells which store hormone granules but are otherwise nonsecretory.
Hormones of the anterior lobe
The anterior lobe of the pituitary gland secretes the following hormones, most of them are trophic hormones.
(i) Growth hormone (STH or GH) or somatotrophin
(soma-body, trophe – nourishment).This hormone stimulates growth. Growth hormone promotes protein anabolism, the absorption of calcium from the bowel and the conversion of glycogen to glucose.
(ii) Thyroid stimulating hormone (TSH) or thyrotropin. This hormone controls the growth and activity of the thyroid gland. It influences the uptake of iodine, the synthesis of the hormones, thyroxine and triiodothyronine by the thyroid gland and the release of stored hormones into the blood stream.
(iii) Adrenocorticotropic hormone (ACTH). This hormone stimulates the cortex of the adrenal gland to produce its hormones.
(iv) Prolactin hormone (PRL) or mammotropin hormone (MTH) or luteotrophic hormone (LTH).
Prolactin is also called the “hormone of maternity” because its main physiological effect is to activate growth of breasts during pregnancy and secretion of mammary glands after child birth.The name luteotrophic hormone (LTH) refers to it because it also stimulates the corpus luteum of the ovary to secrete progesterone hormone.
(v) Gonadotropic hormones.
These are as follows:
(a) Follicle-stimulating hormone (FSH). It stimulates growth of ovarian follicles and their secretion of oestrogens in the female, and spermatogenesis (formation of sperms) in the male.
(b) Luteinizing hormone (LH). In female it stimulates the corpus luteum of the ovary to secrete progesterone. In male it activates the Leydig’s (interstitial) cells of the testis to secrete androgens hence it may be called interstitial cell stimulating hormone (ICSH) in male.
Hormone of the intermediate lobe
The intermediate lobe of the pituitary gland secretes melanotrophin or melanocyte stimulating hormone (MSH). This hormone causes dispersal of pigment granules in the pigment cells, thereby darkening the colour in certain animals like fishes and amphibians. It is believed that it is associated with the growth and development of melanocytes in man which give colour to the skin. Hormones of the posterior lobe The secretion of the posterior lobe is known as pituitrin and it contains two hormones.
(i) Oxytocin (OT): Oxytocin promotes contraction of the uterine muscle and contraction of the myoepithelial cells of the lactating breast, squeezing milk into the large ducts
behind the nipple. In late pregnancy the uterus becomes very sensitive to oxytocin. The amount secreted is increased just before, and during labour, and by suckling of the baby. Because of its role, oxytocin is called “birth hormone” and “milk ejecting hormone”.
(ii) Antidiuretic hormone (ADH) or vasopressin or pitressin. This hormone has two main functions.
(a) Antidiuretic effect: It increases the reabsorption of water in the distal convoluted tubule and collecting ducts of the nephrons of the kidneys. As a result, the reabsorption of water from the glomerular filtrate is increased.
(b) Pressor effect: Involuntary muscles in the walls of the intestine, gall bladder, urinary bladder and blood vessels are stimulated to contract.
Pineal secretes a hormone called melatonin. Melatonin plays a very important role in the regulation of a 24 hour (diurnal) rhythm of our body. For example, it helps in maintaining the normal rhythms of sleep-wake cycle, body temperature. In addition, melatonin also influences metabolism, pigmentation, the menstrual cycle as well as our defense capability.
Thyroid gland (largest endocrine gland) is present in the neck between the trachea and larynx. It is bilobed with a connecting isthmus (a narrow non glandular median part). The microscopic structure of the thyroid gland shows thyroid follicles composed of cubical epithelium and filled with a homogenous material called colloid. Small amount of loose connective tissue forms stroma of the gland. Besides containing blood capillaries, the stroma contains small clusters of specialized parafollicular cells or ‘C’ cells.
Thyroid hormones are produced by the secretory cells lining the follicle and stored in the colloid until needed. So each follicle accumulates a storage form of the circulating thyroid hormones – thyroglobulin. It is a large protein molecule that contains multiple copies of one amino acid tyrosine. Thyroid gland produces two hormones – thyroxine (T4) and tri-iodothyronine (T3) together called thyroidal hormone. Both are iodinated forms of an amino acid called tyrosine. T3 & T4 contain 3 & 4 iodine atoms respectively. T3 is more potent & active than T4.
Thyroid hormones (i) increase the metabolic rate of the body, enhance heat production and maintain BMR (basal metabolic rate), (ii) promote growth of body tissues and mental faculties, (iii) stimulate tissue differentiation (e.g. promote metamorphosis of tadpoles into adult frogs).
These hormones also support the process of red blood cell formation. Thyroid hormones control the metabolism of carbohydrates, proteins and fats. Maintenance of water and electrolyte balance is also influenced by thyroid hormones.
Thyroid gland also secretes a protein hormone called thyrocalcitonin (TCT) or calcitonin which regulates the blood calcium levels.
The parathyroid glands consist of four separate glands located on the posterior surface of the lobes of the thyroid gland. They consist of two types of cells : chief cells (small) and oxyphil cells (large). The chief cells secrete parathormone or Collip’s hormone. Parathyroids are under the feed back control of blood calcium level. Parathyroid hormone (PTH) increases the Ca2+ levels in the blood by withdrawing calcium from the bones into the plasma. PTH also stimulates reabsorption of Ca2+ by the renal tubules and increases Ca2+absorption from the digested food. It is, thus, clear that PTH is a hypercalcemic hormone, i.e., it increases the blood Ca2’ levels. Along with TCT, it plays a significant role in calcium balance in the body.
The thymus gland is located in the upper part of the thorax near the heart. It is a soft, pinkish, bilobed mass of lymphoid tissue. It is a prominent gland at the time of birth but it gradually atrophies in the adult. As a result the immune responses of old persons become weak. Hassall’s corpuscles are spherical or oval bodies present in the thymus. They are phagocytic in function. Thymus secretes a hormone named thymosin which stimulates the development and differentiation of T-cells, increasing resistance to infections.
Adrenal gland or suprarenal gland Adrenal glands are paired structures (conical, yellowish bodies) located on the top of the kidneys. Each adrenal gland has two parts: external adrenal cortex and internal adrenal medulla.
The medulla of the adrenal gland secretes two hormones: norepinephrine (noradrenaline) and epinephrine (adrenaline) which are commonly called catecholamines. Norepinephrine and epinephrine are derived from the amino acid tyrosine.
Adrenaline and noradrenaline are rapidly secreted in response to stress of any kind and during emergency situations and are called emergency hormones or hormones of fight or flight. These hormones increase alertness, pupilary dilation, pilo erection (raising of hairs), sweating, etc. Both the hormones increase the heart beat, the strength of heart contraction and the rate of respiration. Catecholamines also stimulate the breakdown of glycogen resulting in an increased concentration of glucose in blood. In addition, they also stimulate the breakdown of lipids and proteins.
The adrenal cortex is divided into three layers:
(i) Zona glomerulosa is the outer zone that constitutes about 15% of the gland and has closely packed cells arranged in spherical clusters and arched columns which secrete hormones called mineralocorticoids.
(ii) Zona fasciculata is the middle zone and consists of cells arranged in long, straight columns which secrete glucocorticoids.
(iii) Zona reticularis is the inner zone and consists of cells arranged in branching cords which secrete gonadocorticoids or sex corticoids.
Functions of various hormones:
(a) Mineralocorticoids maintain water and electrolyte balance and blood volume in the body by regulating mineral metabolism. The major mineralocorticoid is aldosterone, commonly called salt-retaining hormone.
Aldosterone acts mainly at the renal tubules and stimulates the reabsorption of Na+ from urine, saliva, bile and sweat to reduce its loss from the body and water and excretion of K+ and phosphate ions. Thus, aldosterone helps in the maintenance of electrolytes, body fluid volume, osmotic pressure and blood pressure.
(b) Glucocorticoids regulate the metabolism of carbohydrates, proteins and fats. They increase the blood-glucose level by converting proteins and fats into carbohydrates which are, in turn, converted to glucose (gluconeogenesis). The most important glucocorticoid is cortisol.
Glucocorticoids stimulate lipolysis and proteolysis; and inhibit cellular uptake and utilisation of amino acids. Cortisol is also involved in maintaining the cardiovascular system as well as the kidney functions. Glucocorticoids, particularly cortisol, produce antiinflammatory reactions and suppress the immune response. Cortisol stimulates the RBC production.
(c) Sexcorticoids include small amounts of both male and female sex hormones. However, more male sex hormone (testosterone) is produced than female sex hormone (oestrogen). Testosterone stimulates the development of male secondary sexual characters. Oestrogen stimulates the appearance of female secondary sexual characters.
Pancreas is a composite gland which acts as both exocrine and endocrine gland. The endocrine pancreas consists of groups of cells called ‘Islets of Langerhans’. There are about 1 to 2 million Islets of Langerhans in a normal human pancreas representing only 1 to 2 percent of the pancreatic tissue.
Four kinds of cells have been identified in the islets. These are :
– Alpha cells (about 15%) producing glucagon.
– Beta cells (about 65%) producing insulin.
– Delta cells or D-cells (about 5%) producing somatostatin (SS).
Pancreatic polypeptide cells or PP cells or F cells (15%) producing pancreatic polypeptide (PP).
Glucagon is a peptide hormone, and plays an important role in maintaining the normal blood glucose levels. Glucagon acts mainly on the liver cells (hepatocytes) and stimulates glycogenolysis resulting in an increased blood sugar (hyperglycemia). In addition, this hormone stimulates the process of gluconeogenesis which also contributes to hyperglycemia. Glucagon reduces the cellular glucose uptake and utilisation. Thus, glucagon is a hyperglycemic hormone Insulin is a peptide hormone antagonistic to glucagon, which plays a major role in the regulation of glucose homeostasis. Insulin acts mainly on hepatocytes and adipocytes (cells of adipose tissue), and enhances cellular glucose uptake and utilisation. As a result, there is a rapid movement of glucose from blood to hepatocytes and adipocytes resulting in decreased blood glucose levels (hypoglycemia). Insulin also stimulates conversion of glucose to glycogen (glycogenesis) in the target cells.
The glucose homeostasis in blood is thus maintained jointly by the two – insulin and glucagon.
Prolonged hyperglycemia leads to a complex disorder called diabetes mellitus which is associated with loss of glucose through urine.
Loss of glucose is followed by water loss which causes excessive thirst (polydipsia). Cells are unable to utilize glucose and other carbohydrates for energy production, subsequently proteins and fats are used. Degradation of fats increases, producing ketone bodies (ketosis). Gonads
Gonads are the sex glands, the ovaries and testes. They produce ova and sperms respectively, but also secrete hormones.
Testis performs dual functions as a primary sex organ as well as an endocrine gland. Testis is composed of seminiferous tubules and stromal or interstitial tissue. The Leydig’s cells or interstitial cells, which are present in the intertubular spaces produce a group of hormones called androgens, mainly testosterone. Testosterone stimulates growth and development of male sex organs and secondary sexual characters like beard, moustache and low pitch voice. It also stimulates formation of sperms (spermatogenesis) and promotes growth of bones and muscles. This explains the body growth at puberty and a higher stature of the male’s body.
Androgens act on the central neural system and influence the male sexual behaviour (libido). These hormones produce anabolic (synthetic) effects on protein and carbohydrate metabolism.
Ovary produces two groups of hormones called estrogen and progesterone. Ovary is composed of ovarian follicles and stromal tissues. The estrogen is synthesised and secreted mainly by the growing ovarian follicles. After ovulation, the ruptured follicle is converted to a structure called corpus luteum, which secretes mainly progesterone.
Estradiol is the principal estrogen. It stimulates the development of external female sex characters and maturation of ova. It also regulates female sexual behaviour.
Progesterone brings about most of the pregnancy changes such as development of uterine lining and mammary gland, formation of placenta and also maintains pregnancy.
Placenta is the intimate connection between the foetus and the uterine wall of the mother to exchange the materials. It is a temporary endocrine gland. During pregnancy the placenta provides for the exchange of nutrients and wastes between the mother and the developing foetus.