Endocrine System - Part 3

ENDOCRINE SYSTEM

PART 3

PANCREAS
•    Pancreas is a composite gland which acts as both exocrine and endocrine gland.
•    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 p-cells. The a-cells secrete a hormone called glucagon, while the p-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 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, 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 glucagons.
•    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. Diabetic patients are successfully treated with insulin therapy.

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, maturation 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.
•    Androgens play a major stimulatory role in the process of spermatogenesis (formation of spermatozoa).
•    Androgens act on the central neural system and influence the male sexual behavior (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 follicles and stromal tissues. The estrogen is synthesized 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. Estrogens also regulate female sexual behavior.
•    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.

HORMONES OF HEART, KIDNEY AND GASTROINTESTINAL TRACT

•    As mentioned earlier, hormones are also secreted by some tissues which are not endocrine glands. For example, 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.
•    Several other non-endocrine tissues secrete hormones called growth factors. These factors are essential for the normal growth of tissues and their repairing/regeneration.

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.
•    Hormone receptors present on the cell membrane of the target cells are called membrane-bound receptors and the receptors present inside the target cell are called intracellular receptors, mostly nuclear receptors (present in the nucleus).
•    Binding of a hormone to its receptor leads to the formation of a hormone-receptor complex. 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.
•    Hormones which interact with membrane-bound receptors normally do not enter the target cell, but generate second messengers which in turn regulate cellular metabolism.
•    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. Cumulative biochemical actions result in physiological and developmental effects.

On the basis of their chemical nature, hormones can be divided into groups:
1.    peptide, polypeptide, protein hormones (e.g., insulin, glucagon, pituitary hormones, hypothalamic hormones, etc.)
2.    steroids (e.g., cortisol, testosterone, estradiol and progesterone)
3.    iodothyronines (thyroid hormones)
4.    amino-acid derivatives (e.g., epinephrine).

SUMMARY
•    There are special chemicals which act as hormones and provide chemical coordination, integration and regulation in the human body.
•    These hormones regulate metabolism, growth and development of our organs, the endocrine glands or certain cells.
•    The endocrine system is composed of hypothalamus, pituitary and pineal, thyroid, adrenal, pancreas, parathyroid, thymus and gonads (testis and ovary). In addition to these, some other organs, e.g., gastrointestinal tract, kidney, heart etc., also produce hormones.
•    The pituitary gland is divided into three major parts, which are called as pars distalis, pars intermedia and pars nervosa.
•    Pars distalis produces six trophic hormones. Pars intermedia secretes only one hormone, while pars nervosa (neurohypophysis) secretes two hormones.
•    The pituitary hormones regulate the growth and development of somatic tissues and activities of peripheral endocrine glands.
•    Pineal gland secretes melatonin, which plays a very important role in the regulation of 24-hour (diurnal) rhythms of our body (e.g., rhythms of sleep and state of being awake, body temperature, etc.).
•    The thyroid gland hormones play an important role in the regulation of the basal metabolic rate, development and maturation of the central neural system, erythropoiesis, metabolism of carbohydrates, proteins and fats, menstrual cycle.
•    Another thyroid hormone, i.e., thyrocalcitonin regulates calcium levels in our blood by decreasing it.
•    The parathyroid glands secrete parathyroid hormone (PTH) which increases the blood Ca2+ levels and plays a major role in calcium homeostasis.
•    Thyroid and adrenals secrete their hormones when they receive orders from the pituitary through its hormones.
•    Metamorphosis in insects is controlled by insect hormones. In a frog, it is controlled by thyroxine, the hormone produced by thyroid. Thyroxine production requires the presence of iodine in water. If the water in which the tadpoles are growing does not contain sufficient iodine, the tadpoles cannot become adults.
•    The thymus gland secretes thymosins which play a major role in the differentiation of T-lymphocytes, which provide cell-mediated immunity. In addition, thymosins also increase the production of antibodies to provide humoral immunity.
•    Adrenal glands secrete hormones which maintain the correct salt balance in the blood.
•    The adrenal gland is composed of the centrally located adrenal medulla and the outer adrenal cortex. The adrenal medulla secretes epinephrine and norepinephrine. These hormones increase alertness, pupilary dilation, piloerection, sweating, heart beat, strength of heart contraction, rate of respiration, glycogenolysis, lipolysis, proteolysis.
•    The adrenal cortex secretes glucocorticoids and mineralocorticoids. Glucocorticoids stimulate gluconeogenesis, lipolysis, proteolysis, erythropoiesis, cardio-vascular system, blood pressure, and glomerular filtration rate and inhibit inflammatory reactions by suppressing the immune response.
•    Mineralocorticoids regulate water and electrolyte contents of the body. The endocrine pancreas secretes glucagon and insulin.
•    Glucagon stimulates glycogenolysis and gluconeogenesis resulting in hyperglycemia. Insulin stimulates cellular glucose uptake and utilisation, and glycogenesis resulting in hypoglycemia. Insulin deficiency and/or insulin resistance result in a disease called diabetes mellitus.
•    The testis secretes androgens, which stimulate the development, maturation and functions of the male accessory sex organs, appearance of the male secondary sex characters, spermatogenesis, male sexual behaviour, anabolic pathways and erythropoiesis.
•    The ovary secretes estrogen and progesterone. Estrogen stimulates growth and development of female accessory sex organs and secondary sex characters. Progesterone plays a major role in the maintenance of pregnancy as well as in mammary gland development and lactation.
•    The atrial wall of the heart produces atrial natriuretic factor which decreases the blood pressure. Kidney produces erythropoietin which stimulates erythropoiesis.
•    The gastrointestinal tract secretes gastrin, secretin, cholecystokinin and gastric inhibitory peptide. These hormones regulate the secretion of digestive juices and help in digestion.