The following points highlight the eight types of endocrine glands. The types are: 1. Pineal Gland 2. Thymus 3. Kidneys 4. Gastro-Intestinal Tract 5. Placenta 6. Heart 7. Liver 8. Skin.
Type # 1. Pineal Gland:
Pineal gland has a mass of 0.1—0.2 g.
It develops from the ectoderm of the embryo.
Location and Structure:
The pineal gland is located between the cerebral hemispheres, where it protrudes from the roof of the third ventricle. The pineal gland is a small rounded body which consists of pineal cells and supporting glial cells.
Though the function of the gland is still the subject of current research, it is known to secrete melatonin hormone, also called the “sleep hormone”, because it promotes sleep. Its secretion increases in dim light and decreases in bright light.
Melatonin concentration in the blood appears to flow a diurnal (day-night) cycle as it rises in the evening and through the night and drops to a low around noon.
Because of this light mediated response, the pineal gland may act as a kind of “biological clock” which may produce circadian rhythms (variations following a 24 hour cycle). Melatonin lightens skin colour in certain animals. In mammals melatonin possibly acts as an inhibitory factor for sexual maturation.
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.
In man the pineal gland starts to calcify at about the time of puberty. Such calcium deposits are called the brain sand. There is no evidence that the presence of brain sand is an indication of degeneration. In fact, the presence of brain sand may indicate increased secretory activity.
Type # 2. Thymus:
At birth thymus commonly weighs between 10 and 15 gm. It continues to grow up to the age of puberty when its weight ranges between 30 and 40 gm. Thereafter it generally progressively diminishes in size undergoing gradual atrophy and replacement by fat so that after mid-adult life it may weigh only about 10 gm. Disappearance of thymus causes ageing.
It is derived from the endoderm of the embryo.
Location and Structure:
The thymus gland is located in the mediastinum between the sternum and aorta. 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. Hassall’s corpuscles (macrophages) are spherical or oval bodies present in the thymus. They are phagocytic in function.
Thymus secretes a hormone named thymosin which stimulates the development of certain kinds of white blood cells involved in producing immunity. It also hastens attainment of sexual maturity.
Type # 3. Kidneys:
They develop from the mesoderm of the embryo.
The kidneys secrete three hormones: renin, erythropoietin and calcitriol.
Whenever the rate of ultrafiltration falls, the cells of their juxtaglomerular complex secrete and release into blood a compound named renin. The latter is proteolytic enzyme. It’s some properties are like hormone. It acts upon a plasma-protein, angiotensinogen, separating a compound, called angiotensin-II from it.
Angiotensin-II accelerates heart beat and constricts arterioles, thereby increasing blood pressure. This enhances the rate of ultrafiltration. Simultaneously, the angiotensin-II stimulates adrenal cortex to secrete aldosterone, and enhances water and sodium reabsorption from nephrons. These factors also elevate blood pressure.
The oxygen shortage stimulates the kidney cells to secrete a hormone named erythropoietin (a circulating glycoprotein) into the blood. Erythropoietin stimulates the bone marrow to increase the production of RBCs.
Calcitriol is the active form of vitamin D. It promotes absorption of Ca2+ and phosphorus in the small intestine and accelerates bone formation.
Type # 4. Gastro-Intestinal Tract:
It develops from the endoderm of the embryo. Inner most layer of the wall of the alimentary canal is called mucosa. Certain cells of the mucosa of the stomach and intestine secrete important hormones.
The mucosa of the pyloric region of the stomach synthesizes, stores and secretes the hormone gastrin. This hormone acts on the gastric glands and stimulates the secretion of hydrochloric acid and pepsinogen.
It secretes the following hormones:
It is secreted by the intestinal mucosa of duodenum and jejunum. It acts on the exocrine part of pancreas and stimulates secretion of water and bicarbonate ions. It increases secretion of bile. Probably, it also retards intestinal peristalsis. Secretin also inhibits the secretion and movements of the stomach. Secretin was the first hormone discovered by scientists.
(ii) 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 suggests 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.
(iii) Gastric Inhibitory Peptide-GIP or Enterogastrone:
It is secreted by duodenal mucosa that inhibits gastric secretion and contractions.
This hormone is also secreted by duodenal mucosa. It stimulates the Brunner’s glands to release mucus and a few enzymes into the intestinal juice.
It is also secreted by duodenal mucosa. It stimulates the crypts of Lieberkuhn to secrete the enzymes in the intestinal juice.
(vi) Vasoactive Intestinal Peptide (VIP):
It is secreted by the small intestine. It dilates peripheral blood vessels of the small intestine and inhibits gastric acid secretion.
It is secreted by the mucosa of entire small instestine. It accelerates the movement of intestinal villi to facilitate the absorption of food in the small intestine.
(viii) Somatostatin (SS):
Somatostatin secreted by the delta cells of Langerhans of pancreas inhibits the secretion of glucagon by alpha cells and insulin by beta cells. Somatostatin produced by argentaffin cells of gastric and intestinal glands supresses the release of hormones from the digestive tract.
(ix) Pancreatic Polypeptide (PP):
It is secreted by pancreatic polypeptide cells of islets of Langerhans. It inhibits the release of pancreatic juice from the pancreas. Both somatostatin and pancreatic polypeptide are relatively newly discovered hormones of the pancreas and both are still being studied.
Type # 5. Placenta:
Placenta is the intimate connection between the foetus and the uterine wall of the mother to exchange the materials. Placenta is a temporary endocrine gland. During pregnancy the placenta provides for the exchange of nutrients and wastes between the mother and the developing foetus.
It also has some endocrine functions. It secretes some hormones like oestrogens, progesterone, human chorionic gonadotropin (HCG), human chorionic somatomammotropin- HCS (formerly known as human placental lactogen), chorionic thyrotropin, chorionic corticotropin and relaxin. Oestrogens and progesterone have the same roles as in the non-pregnant state.
However, the placental progesterone also checks contraction of uterine muscles and thus helps to maintain pregnancy. HCG stimulates progesterone release from the corpus luteum and maintains it. Presence of HCG in urine indicates pregnancy.
Human choronic somatomammotropin stimulates the growth of mammary glands. Placental relaxin causes relaxation of the ligaments of pubic symphysis and towards the termination of pregnancy it softens and widens the opening of the cervix (lower part of uterus) for easy child birth (parturition).
Type # 6. Heart:
The cells, called cardiocytes of atria of the heart secrete peptide hormone, called atrial natriuretic factor (ANF) in response to an increased return of the deoxygenated (venous) blood. ANF inhibits the release of renin from juxtaglomerular apparatus (IGA) and thereby, inhibits NaCl reabsorption by the collecting duct and reduces aldosterone release from the adrenal gland.
Type # 7. Liver:
The liver produces a protein angiotensinogen which is changed to angiotensin II by an enzyme renin secreted by the juxtaglomerular apparatus of the nephrons in the kidney. Angiotensin stimulates the adrenal cortex to produce aldosterone. Renin works as hormone.
Type # 8. Skin:
Vitamin D is synthesized in skin epidermis from cholesterol-derived compounds in the presence of sunlight. As stated earlier vitamin D exists in two forms: calciferol or D2 and cholecalciferol or D3. Cholecalciferol is more important. It circulates in the blood. Calcitriol is active form of D3.
It increases absorption of calcium and phosphorus from chyme in the small intestine and accelerates bone formation. It is, therefore, required for growth of body and bone formation. Its deficiency causes rickets in children and osteomalacia in adults.
Eicosanoids (Local Hormones):
Eicosanoids (eicos- twenty forms; оid- resembling) are derived from the 20-carbon fatty acid— arachidonic acid. The eicosanoids are important local hormones. The two major types of eicosanoids are prostaglandins and leukotriene’s.
Because they were first found in semen (which is produced partly by the prostate gland), they were named prostaglandins. These are secreted by many organs (e.g., kidneys, gonads, seminal vesicles, thymus, brain, etc.). Traces of prostaglandins are sufficient to cause contraction of smooth muscles.
Prostaglandins of seminal vesicles mix with semen. When semen is ejected into the female’s vagina prostaglandins contract uterine muscles to facilitate ascending of sperms into the Fallopian tubes.
Synthetic prostaglandins are now used for birth control, for inducing labour pains, abortion, cure of asthma, etc. Although the prostaglandins behave like hormones yet they do not meet the requirements of true hormones for two reasons. First they are not produced by distinct glands. Second, they are metabolized so rapidly after they are released that they cannot travel in the blood for any significant distance.
Leukotriene’s are the mediators of allergic response. They also promote responses during inflammation. The release of leukotriene’s is increased when some allergic agents combine with antibodies like IgE. The leukotriene’s cause bronchiolar constriction, arteriolar constriction, vascular permeability and attraction of neutrophils and eosinophil’s towards the site of inflammation.