The Endocrine Glands II

The endocrine' pancreas consists of 'Islets of Langerhans'. There are about 1 to 2 million Islets of Langerhans in a normal human pancreas representing only 1 to 2 per cent of the pancreatic tissue. The two main types of cells in the Islet of Langerhans are called a-cells and b-cells. The a-­cells secrete a hormone called glucagon, while the b-cells secrete insulin.

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 ill an increased blood sugar (hyperglycemia).

Glucagon reduces the cellular glucose uptake and utilisation, Thus, glucagon is a hyperglycemic hormone.

Insulin is a peptide hormone, 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.

Insulin also stimulates conversion of glucose to glycogen (glycogenesis) in the target cells.

Prolonged hyperglycemia leads to a complex disorder called diabetes mellitus which is associated with loss of glucose through urine and formation of harmful compounds known as ketone bodies.

Testis

A pair of testis is present in the scrotal sac (outside abdomen) of male individuals.

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 cells or interstitial cells, which are present in the intertubular spaces produce a group of hormones called androgens mainly testosterone. 

Androgens regulate the development, maturatior and functions of the male accessory sex organs like epididymis, vas deferens, seminal vesicles, prostate gland, urethra etc. These hormones stimulate muscular growth, growth of facial and axillary hair, aggressiveness, low pitch of voice etc. Androgeru playa major stimulatory role in the process of spermatogenesis (formation of spermatozoa), 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

Females have a pair of ovaries located in the abdomen.

Ovary is the primary female sex organ which produces one ovum during each menstrual cycle. In addition, ovary also produces two groups of steroid hormones called estrogen and progesterone. 

Ovary is composed of ovarian folliclos 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.

Estrogens produce wide ranging actions such as stimulation of growth and activities of female secondary 'sex organs, development of growing ovarian follicles, appearance of female secondary sex characters (e.g., high pitch of voice, etc.), mammary gland development.

Progesterone supports pregnancy. Progesterone also acts on the mammary glands and stimulates the formation of alveoli (sac-like structures which store milk) and milk secretion.
 

Mechanism of Hormone Action

Hormones produce their effects on target tissues by binding to specific proteins called hormone receptors located, in the target tissues only.

Each receptor is specific to one hormone only and hence receptors are specific.

Hormone-Receptor complex formation leads to, certain biochemical changes in the target tissue, 'Target tissue metabolism and hence physiological functions are regulated by hormones.

The hormone receptor complex may acts in one or the two ways –

Formation of cAMP: Mechanism of formation of cAMP was discovered by E.W. Sutherland in 1950. The hormone receptor complex causes the release of an enzyme adenyl cyclase, from the receptor site. This enzyme hydrolyses the A. TP into c-AMP. The c-AMP activates the existing enzvmo system of the cell. This accelerates the metabolu­reactions in cell. The hormone is called firs: messenger and the c-AMP is termed the second messenger. e.g., Adrenaline causes the secrertion of glucose from the liver cell from this mechanism. 

(ii) Change in membrane permeability: The receptor proteins of some hormones are large transmembrane intrinsic protein acting as ion channels for facilitated diffusion of Na⁺, K⁺, Ca²⁺ etc. On binding with specific hormone these receptor proteins undergo conformational changes, so that the membrane permeability for ions is altered, resulting into important changes in metabolism.

For example, insulin promotes the entry of glucose from blood into the muscles cells by increasing the permeability of- sarcolemma to glucose.

The steroid hormones act within the cell. Their small, lipid soluble molecules pass through the cell membrane and bind to specific receptor molecules present in the cytoplasm. The receptor molecules carry them into the nucleus. Here, the receptor hormone complex binds to a specific receptor site on the chromosome and activates certain genes that were previously repressed. The activated gene transcribe m-RNA which directs the synthesis of enzyme (protein molecule) in the cytoplasm. The enzyme molecule promote the metabolic reactions in the cell. 

Note:

The atrial wall of our heart secretes a very important peptide hormone called atrial natriuretic factor (ANF), which decreases blood pressure. When blood pressure is increased, ANF is secreted which causes dilation of the blood vessels. This reduces the blood pressure.

The juxtaglomerular cells of kidney produce a peptide hormone called erythropoietin which stimulates erythropoiesis (formation of RBC).

Endocrine cells present in different parts of the gastro-intestinal tract secrete four major peptide hormones, namely gastrin, secretin, cholecystokinin (CCK) and gastric inhibitory peptide (GIP).

Gastrin acts on the gastric glands and stimulates the secretion of hydrochloric acid and pepsinogen. Secretin acts on the exocrine pancreas and stimulates secretion of water and bicarbonate ions.

CCK acts on both pancreas and. gall bladder and stimulates the secretion of pancreatic enzymes and bile juice, respectively. GIP inhibits gastric secretion and motility.

Hormones which interact with intracellular receptors (e.g., steroid hormones, iodothyronines, etc.) mostly regulate gene expression or chromosome function by the interaction of hormone-receptor complex with the genome.

Heterocrine gland: These are those endocrine glands which are involved in hormone secretion as well as some other function eg. pancrease, gonads, placenta, GI mucosa and kidneys.

In females prolactin induce maternalism i.e. strong emotional attachment.

In male prolactin promotes paternalism i.e. protective attitudes towards family members and intensive food gathering for the family.

Contrary to thyroid dwarf (cretins), the pituitary dwarf have a normal mental development and proportionate body.

Sporadic cases of simple goitre (sporadic goitre) in a population are normally due to genetic defect.

Muller organ and subneural gland is homologous to pituitary gland.

Tropic hormone: A hormone which stimulates another endocrine gland to secrete its hormone is called trophic hormone.

Simmond's disease: This condition is due to atrophy of the anterior lobe of pituitary gland.

Pheochromocytoma: It is due to hypersecretion of adrenaline causes, high blood pressure, high level of sugar in blood and urine, high metabolic rate, nervousness and sweating.

Sexual pseudoprecocity results from adrenal cortex, testes, ovary or from other sources, including extragonadial tumours.

Eunuchoidism: Failure of testosterone secretion in male-causes eunuchoidism.

eunuch has a undeveloped secondary sex organs like prostrate, seminal vesicle and penis

does not produce sperm.

Growth hormone: Stimulate the liver to form "Somatomedins" ("Insulin like growth factors"). This somatomedins potent effect to bone growth.

In heart cGMP has antagonistic effect to cAMP, cAMP mediate muscle contraction in response to adrenaline, while cGMP slow down muscle contraction in response to acetylcholine.

cGMP is used as second messenger in atrial natriuretic peptide and nitric oxide.
 

Difference between Nervous and Endocrine Coordination

Thyroid gland is the largest endocrine gland in the body.

Endostyle of lower vertebrates like Herdmania, Amphioxus is homologous of thyroid gland.

Thyroid is the only endocrine gland in the body which stores its hormone in its inactive state.

The oxidation of iodine is promoted by the enzyme peroxidase.

21 October is Iodine deficiency day

Thyroxine stimulates the metamorphosis of tadpole larva in amphibians.
 

Difference between Hormone and Enzymes

Number of Hormones Secreted by Different Endocrine Glands

Anterior Pituitary Hormones

Posterior Pituitary Hormones

Hormones, Their Chemical Nature and Functions

Disease Caused by Hormonal Irregularities

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