Pancreas,Gonads and Placenta – The partly endocrine gland of Human

Heterocrine Glands are both in dual in function, i.e., Endocrine and Exocrine.eg., Pancreas, Gonads, and placenta.

Pancreas

The pancreas is second largest gland of the body located in the loop of the duodenum. It is a heterocrine gland and formed of about 1 to 2 million groups of epithelial cells, called islets of Langerhans.

These cells are scattered in the connective tissue between acini (exocrine part) of the pancreas. All the pancreatic hormone are polypeptides.

An islet of Langerhans is formed of three types of cells:

  1. (Alpha) α cells
  2. (Beta) β cells
  3. (Gamma) cells

Alpha cells are also called Oxyphils and secrete glucagon hormone. Beta cells secrete insulin hormone, and Gamma cells are the precursor of alpha and beta cells.

Function of pancreatic hormone

Insulin

Insulin is the earliest known hormone. It is also called Hypoglycaemic or antidiabetic factor, as it decreases glucose level in blood and prevents the occurrence of diabetes.
It is secreted by β-cells of the islets of Langerhans. Secretion of insulin is stimulated by higher glucose level in blood than the normal level(60-100 mg/100ml of blood) or rise in amino acid concentration. It performs following functions:
  1. Insulin stimulates utilization of glucose as respiratory fuel in body cells.
  2. It stimulates glycogenesis (conversion of glucose into glycogen) in muscles and liver cells.
  3. It prevents gluconeogenesis (conversion of amino acids into sugar).
  4. It spares amino acids for protein synthesis.
  5. It involves the conversion of the excess of glucose into fats.
  6. It prevents the formation of ketone bodies in blood as reduces the respiratory breakdown of fats.

Hypoactivity or degeneration of β-cells leads to the deficiency of insulin which causes insulin-dependent Diabetes mellitus.

Diabetes mellitus is characterized by:

  • Hyperglycaemia – rise in sugar level in the blood about three times and is more than 150 mg/100 ml of blood.
  • Diuresis – 10 times more urine output
  • Polydipsia – increased thirst
  • Glycosuria – sugar in urine (100 mg/24 hours)
  • Ketonaemia – ketone bodies like acetoacetic acid, butyric acid in the blood due to increased oxidation of fats.
  • Ketonuria – Ketones in urine
  • Acidosis – pH of blood decreased from7.4 to 6.8
  • Reduced glycogen in liver and muscles, increased cholesterol level in the blood, delayed healing, blurred vision.
  • Diabetic coma and death

The controlled dose of insulin and sugar-free diet are best preventive measures to diabetes mellitus. There is another more common type called insulin-independent diabetes in which insulin level is normal, but it cannot move glucose for storage, or it’s used as respiratory fuel.

Glycogen

Glycogen is a hyperglycemic or diabetogenic polypeptide hormone secreted by α-cells of islets of Langerhans. Its secretion is stimulated bu low blood sugar level.

Glucagon is antagonistic to insulin. Antagonistic hormones are those which work against each other. Glucagon increases the sugar level in blood by promoting glycogenolysis in liver cells, increasing gluconeogenesis and inhibiting the conversion of glucose to lactic acid.

Islets of Langerhans also have another type called delta cells which secrete somatostatin (SS).

Gonads

Gonads ( In males-Testes and In female-Ovaries ) are both exocrine (produce gametes) as well as endocrine in function. Their hormones control reproductive activities and are secreted from the attainment of puberty.

Testes

The endocrine part of testes is formed of groups of cells, called interstitial cells or Leydig’s cells; they are scattered in the connective tissue between the sperm-producing seminiferous tubules of the testes.

Stimulated interstitial cell secretes male sex hormone, called androgens, by ICSH of anterior pituitary by a feedback mechanism. The main types of androgens are Testosterone, Androsterone, Epiandrosterone, and Dehydro-Epiandrosterone. Out of these, Testosterone is the main androgen and is a steroid hormone.

Functions of Testosterone

  1. Control the growth and development of male secondary sex organs like epididymis, prostate glands, seminal vesicles, and penis.
  2. Testosterone promotes the development of male secondary sexual characters like beard, mustaches, deepening of the voice, broadening of shoulders, increased height due to elongation of bones and increased development of limbs.
  3. It is responsible for sex-urge for a female partner.
  4. Testosterone stimulates erythropoiesis.
  5. Testosterone stimulates the spermatogenesis and maturation of sperms. It also increases the fertilizing power of the sperms.

The failure of testosterone secretion causes eunuchoidism and person suffering from eunuchoidism is called eunuch or neuter. It is characterized by reduced secondary sex organs and failure of spermatogenesis.

Ovaries

Just like testes, the ovaries are cytogenic as well as endocrine in function. Ovaries secrete three types of female hormones:

Estrogens

Estrogens are a group of steroid hormones mainly secreted by follicular epithelial cells of membrane granulosa of Graafian follicle, but these are also produced by the adrenal cortex and placenta. The include β-estradiol, esterone, estriol, etc. Out of which most important estrogen is β-estradiol. Secretion of estrogens is stimulated by LH (Luteinizing hormone) of the anterior lobe of pituitary gland.

Functions of Estrogens

  • It stimulates the growth and normal functiioning of female secondary sex organs like fallopian tubes, iterus, vagina etc.
  • Control the development of female secondary sexual characters like breast-enlargement, broadening of the pelvis, growth of pubic and axillary hair, and beginning of the menstrual cycle.
  • In the development of sexual desire in the female.
  • Decrease the secretion of FSH while increasing the secretion of LH.

Progesterone

It is also a steroid hormone secreted by a yellow endocrine gland called corpus luteum. During the pregnancy, the corpus luteum is formed from the empty Graafian follicle. Adrenal cortex and placenta also secrete a small amount of progesterone. LH stimulates secretion of progesterone.

Functions of Progesterone

  • It stimulates proliferation of endometrium of the uterus and prepares it for implantation.
  • It desensitizes the uterine muscles to the action of oxytocin.
  • Progesterone supports in implantation, the formation of the placenta and normal development of the fetus in the uterus.
  • It suspends ovulation during pregnancy as decreases the secretion of LH.
  • It stimulates proliferation of mammary glands and enlargement of breasts.

Relaxin

It is a proteinous hormone secreted by the corpus albicans formed from the corpus luteum at the end of the gestation period. It softens the pubic symphysis so helps in parturition(childbirth).

Placenta

The placenta is a mechanical and physiological connection between the developing embryo and the maternal tissue True placenta is found in eutherians. its primary function is support and nourishment of the developing embryo but also acts as endocrine gland during pregnancy. The chorionic villi of placenta secrete several gonadotrophins like estrogens, progesterone, and HCG(Human Chorionic Gonadotrophin).

HCG is a glycoproteionous hormone whose primary function is to maintain the corpus luteum for continued secretion of progesterone so as to maintain the pregnancy. It also controls the normal development of the fetus in the mother’s womb.

 

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