Endocrine system generally controls long term activities of target organs as well as physiological processes such as digestion, metabolism, growth, development and reproduction. The activity of endocrine system involves the release of special chemical substances called hormones which are usually carried by the circulating blood to the site where they may act.
The hormone secreting endocrine glands are also known as ductless glands as they have no external ducts and their secretions are directly discharged into the blood stream. Most of the endocrine systems of all vertebrates and invertebrates are of non-nervous origin.
Endocrine Glands in Invertebrates:
The main sources of hormones in invertebrates are neurosecretions from neurosecretory cells. These neurosecretions appear to regulate growth, regeneration, metamorphosis and reproductive activities.
In annelids, the brain produces a juvenile hormone that influences the reproductive system. Crustacea: Neurosecretory cells are found in various parts of the brain in crustacea. The sinus gland contains chromatophorotropic hormone which regulates the colourchange.
In many crustaceans, the eye-stalk also contains another end organ, the X- organ which is responsible for moulting, pigment changes and retinal pigment movements. It produces hormones which inhibit the ovary and some which inhibit the testis in crab.
The major neurosecretory systems of insect's includeprotocerebrum, corpora cardiaca and corpora allata Corpora cardiaca are responsible for the control of the heart by secreting orthodiphenol which activates the heart. Corpora allata form a juvenile hormone which retards the appearance of adult characteristics.
Endocrine Glands in Vertebrates
Principal endocrine structures of vertebrates include mainly the pituitary, thyroid, parathyroids, pancreas, adrenals, sex glands and gastrointestinal mucosa. All of the endocrine glands excluding the parathyroids are found in all vertebrates.
1. Pituitary gland:
It is also known as hypothalamic system and is the single most important endocrine system in the body. The pituitary gland, also called the hypophysis, is a small unpaired rounded body weighing approximately 5 gm. It has been called the master gland because of the multiplicity of its functions and because it controls many of the other endocrine glands.
It is found at the base of the brain and is connected with brain by means of a short stalk called infundibulum. It is composed of three lobes; anterior, intermediate and posterior lobes.
The anterior and intermediate lobes together are known as adenohypophysis and the posterior one is known as neurohypophysis. Adenohypophysis is made up of three types of cells-basophils, acidophils and chromophils; it secretes six different hormones:
(i) gonadotropins (follicle stimulating hormone)
(ii) lipotropin (iii) somatotropin (iv) luteotropic hormone (v) adrenocorticotropin (ACTH) and (vi) thyroid stimulating hormone (TSH) or thyrotropin.
Gonadotropin hormones are luteinizing hormone in females and interstitial cell stimulating hormone in males. They stimulate various activities of the gonads, the ovaries and testes and also control the sexual cycle.
These hormones stimulate the liberation of fatty acids from stored body fats (adipose tissue).
Also known as growth hormone, secreted by acidophilic cells. It stimulates both body weight and rate of growth of the body. Hyposecretion (low secretion) of this hormone during years of skeletal growth results in dwarfism, while hypersecretion (over secretion) results in gigantism.
This hormone is also called prolactin because of its role in milk production by the mammals.
This hormone is secreted by basophilic cells and stimulates the adrenal cortex. It is also used as a non-disease specific in the treatment of hypersensitive and inflammatory reaction.
(vi) Thyroid Stimulating Hormone:
This hormone is secreted by basophilic cells and regulates the growth of thyroid gland for its proper functioning. It is also believed that it increases the rate of metabolism of all the cells of the body.
Neurohypophysis secretes many biochemical polypeptide hormones which primarily control the water and salt metabolism of vertebrates. The known polypeptides of the neurohypophysis are:
(i) Oxytocin (Pitocin):
Its primary functions relate to uterine contraction and trigger for milk secretion.
(ii) Vasopressin (Pitressin):
Its primary function is increased tubular reabsorption by the kidneys. Thus, it decreases the production of urine. It is also referred as antidiuretic hormone.
2. Thyroid Gland:
All vertebrates have thyroid glands. The mammalian (man) thyroid gland weighing about 25 gm to 40 gm is bilobed and lies close together on either side of the trachea. The two lobes are joined by a thin connective tissue called isthumus. The function of the thyroid gland is to produce mono, di, tri and tetra- iodothyronine (thyroxine). Thyroid hormones are metabolic hormones. In amphibians and teleosts, they control some developmental processes, i.e., metamorphosis.
The under secretion of thyroid hormone causes myxedema (a disease) characterized by low metabolic rate, increased plasma cholesterol and mental lethargy. Thyroxine deficiency leads to the thyroid enlargement known as goitre. Over activity of thyroid produces a disease known as exophthalmic goitre (protrusion of the eye-ball).
3. Parathyroid Gland:
They are located close to the thyroid glands. In mammals (man), parathyroids are four glandular bodies and their combined weight varies from 0.05 to 0.3 gm. They produce the hormones called parathormone and calcitonin.
The primary function of parathormone is to maintain the metabolism of phosphate and calcium. It also plays an important role in homeostasis, e.g., membrane permeability, nerve function, cardiac function and reproduction.
Calcitonin functions opposite to the parathormone. It inhibits bone reabsorption. Under-secretion of parathyroid glands causes tetany (twitching of muscles).
Pancreas has certain specialized glandular cells which are endocrine in function and are known as islets of langerhans. These cells are of two kinds, a-cells and (3-cells. P-cells secrete insulin and'a-cells are the source of glucagon hormone.
Insulin is a protein and performs various functions such as; (i) it increases membrane permeability (ii) it lowers blood glucose level (iii) it induces protein synthesis and (iv) it induces synthesis of enzymes involved in converting glucose to glycogen.
Lack or deficiency of insulin may result in diabetes mellitus which is characterized by hy perglycemia (high blood sugar level) and glycosuria (sugar in urine). In the deficiency of insulin, very little of the glucose absorbed from the gastro intestinal tract is transported into the tissue with the result that the blood sugar concentration is raised from a normal value of 90 mg per 100 ml to as high as 300 to 1200 mg per 100 ml. The other hormone, glucagon is secreted in response to hypoglycemia.
It increases blood sugar level by stimulating the dissolution of glycogan granules. It also lowers the blood levels of calcium and increases the heart rate. The release of both insulin and glucagon is stimulated by blood glucose level. Thus, they are not under nervous or endocrine control. Increased levels of glucose stimulate liberation of insulin while decreased levels bring about the secretion of glucagon.
5. Adrenal glands:
The adrenal glands lie at the superior poles of the two kidneys. These are composed of two distinct parts; an outer cortex and an inner medulla. In mammals (man), the adrenals are richly vascularized. They are smaller in female than in male.
Adrenal cortex secretes several hormones which are steroid compounds and can be divided into three groups:
(i) Glucocorticoids, e.g., cortisone, hydrocortisone and corticosterone.
(ii) Mineralocorticoids, e.g., aldosterone and deoxycorticosterone.
(iii) Sex hormones, e.g., androgen and estrogen.
Adrenal cortex is rich in ascorbic acid (vitamin C) and cholesterol. The chief functions of adrenal cortex hormones are (i) to raise blood glucose level at the expense of liver glycogen (ii) to promote the retention of sodium in salivary glands, sweat glands and colon and (iii) to prevent the appearance of inflammation. Adrenal medulla in mammals, birds and reptiles produces catecholamines, adrenaline and noradrenaline.
Adrenaline is also known as emergency hormone because it converts glycogen to glucose and provides quick energy. It also increases the blood flow and dilates the blood vessels in the heart and skeletal muscles.
Nor-adrenaline is the tonus hormone for circulatory regulation. The main function of this hormone is the normal control of blood circulation.
6. Sex glands:
The sex glands are of two types, the testes (male) and the ovaries (female). Testes in the presence of follicle stimulating hormone (FSH) and interstitial cells stimulating hormone (ICSH) produce two distinct hormones; the testosterone and the and rosterone, collectively called androgens.
Testosterone is secreted by the interstitial cells of leydig and Sertoli cells. Androsterone is also a strong androgen and is secreted in the testes. The androgens are responsible for the normal growth of penis, scrotum, prostate gland, hairs at face and masculine voice.
In the female, ovaries secrete three important hormones; estradiol (oestrogen), progesterone and relaxin.
Estradiol is secreted before ovulation by the developing follicle and is responsible for the development of secondary sexual characters of the female and also for the development of female sexual organs. Lack of estradiol leads to the failure of menstrual cycle and ill-development of genital tracts.
Progesterone is essential for secretory changes in the uterine endometrium, thus preparing the uterus for implantation of the fertilized ovum. It promotes the development of mammary glands to full maturity during pregnancy. Relaxin plays an important role in relaxing pelvic ligaments during pregnancy.
7. Gastrointestinal mucosa:
The hormones secreted by glandular cells of gastrointestine are gastrin, secretin, pancreozymin, cholecystokinin, enterogastrone and enterocrinin.
8. Thymus gland:
Thymus gland in man is found between the upper part of the sternum and the pericardium. It is a temporary gland reaching its greatest development stage at the age of 14 to 15 years after which it gradually atrophies because of the activities of sex glands. It secretes three hormones: thymosin, thymin I and thymin II. Thymosin stimulates the proliferation of lymphocytes.
9. Pineal gland:
In vertebrates, the pineal gland occurs on the dorsal side of the brain and secretes a hormone called melatonin. It is a powerful antioxidant. Melatonin has an important role in reproduction, aging and maintenance of biological clock.
10. Juxtaglomerular Cells of Kidney:
These cells of kidney produce two hormones, renin and erythropoetin. Renin stimulates the release of substances which cause a rise in blood pressure. Erythropoetin stimulates the release of red blood cells (R.B.C.) from the bone marrow. Anoxia, a deficiency of 02 in blood, causes kidneys to produce this hormone.