An overview of Pituitary Gland
It is called master gland (master of endocrine orchestra) as it regulates the functioning of other endocrine glands like thyroid, Adrenal cortex, testes and ovaries etc. However, the secretions of pancreas, thymus and pineal body are not regulated by the hormones of the pituitary.
Pituitary is a single or unpaired gland weighing about 0.5 gm.
It is present in the ‘Sella tursica’ cavity of Basi-sphenoid bone and is attached to the ﬂoor of Diencephalon (hypothalamus) of forebrain through a stalk called Infundibulum.
As it is present below diencephalon of fore-brain it is also known as Hypophysis cerebri.
In mammals, it is ectodermal in origin.
It consists of two-parts – (1) Adenohypophysis, and (2) Neurohypophysis
It develops from Rathke’s pouch, a diverticulum of buccal cavity and includes
- Anterior lobe (pars distalis)
- Middle lobe (pars intermedia )
It is directly connected to the Hypothalamus (diencephalon) part of the forebrain. It includes posterior lobe (pars nervosa)
(A) Anterior lobe - 6 hormones
- Growth hormone or somatotrophic hormone (STH)
- Prolactin (PRL) or mammotropin or LTH (Leuteotropic Hormone)
- Adreno corticotropic hormone (ACTH)
- Thyroid-stimulating hormone (TSH)
- Follicle-stimulating hormone (FSH)
- leutinizing hormone (LH)
STH & PRL are secreted from Acidophilic (40%) cells; ACTH is secreted from chromophobic cells, while rest three, TSH, FSH & LH are secreted from Basophilic (10%) cells.
Last three hormones, TSH, FSH and LH, are glycoproteins while the ﬁrst three are peptides/ proteins.
Except prolactin, all other hormones of the anterior pituitary are Tropic hormones because they stimulate other endocrine glands. Prolactin hormone stimulates mammary glands which are not endocrine but exocrine in nature.
Growth Hormone (STH)
It is an anabolic hormone and stimulates protein synthesis. The catabolism of proteins and amino acids is decreased.
It stimulates lipolysis, i.e. breaks down of lipids for energy production.
It decreases the use of glucose for energy production and conserves carbohydrates as glycogen.
It reserves calcium and phosphate and causes the elongation of bones.
Hyposecretion of STH
In children – Dwarﬁsm, producing ‘Midgets’ with stunted growth
In adults – Simmond’s disease (protein synthesis is decreased and lactation, in females, is reduced)
Hypersecretion of STH
In children – Gigantism (overgrowth of the body)
In adults – Acromegaly, i.e. gorilla appearance (free ends or extremities like ﬁngers, toes, chin etc. are elongated). The affected person develops Osteoarthritis, Gynecomastia and Hirsutism (showing the presence of coarse & pigmented hair on chest back etc.
Growth hormone is considered to be a tropic hormone though it does not aﬀect the endocrine organs directly. It stimulates liver for the secretion of a hormone-like chemical, Somatomedin. This hormone (somatomedin) then stimulates the growth of bones, muscles or body organs.
It stimulates mammary glands for the synthesis of milk.
It is a non-tropic hormone.
Adreno corticotropic hormone (ACTH)
It stimulates the secretion of corticoids from Adrenal cortex.
It also regulates melanin pigment in human skin.
Thyroid-stimulating hormone (TSH)
It stimulates the growth of thyroid follicles.
It also promotes the utilization of iodine by the gland, for the synthesis of thyroxine.
Follicle stimulating hormone (FSH)
It is commonly called Gametogenic hormone as it regulates Spermatogenesis in males and Oogenesis in females.
In females, it also stimulates ovarian follicles for the secretion of estrogen hormone.
Leutinizing hormone (LH)
It is responsible for ovulation, i.e. release of female gamete from the ovary.
It also helps in the formation of Corpus luteum.
It stimulates corpus luteum for the synthesis of mainly progesterone hormone.
In males, this hormone stimulates interstitial cells or Leydig cells, for the synthesis of ‘testosterone’ and is, therefore, called Interstitial cells stimulating hormone (ICSH).
Hypothalamic control over Anterior Pituitary
The chemical factors, released from hypothalamus affect the secretions of anterior pituitary through the hypophyseal portal system. Following are the six releasing factors or hormones.
- Growth hormone-releasing factor (hormone) –
- GRH It stimulates the secretion of growth hormone from the anterior pituitary.
- Growth hormone release inhibiting hormone – GIH
- It is also known as Somatostatin and inhibits the secretion of growth hormone from the anterior pituitary.
- Thyrotropin-releasing hormone – TRH
- It stimulates the secretion of thyroid stimulating hormone (TSH or thyrotropin)
- Corticotropin-releasing hormone – CRH
- It stimulates the secretion of ACTH.
- Gonadotropin-releasing hormone – Gn RH
- It stimulates anterior pituitary for the secretion of FSH and LH
- Prolactin inhibitory hormone – PIH
- The production of a prolactin-releasing hormone (PRH) is doubtful in human.
It is secreted in ﬁ shes, amphibians and reptiles.
It causes darkening of skin by dispersing pigmented cells (Chromatophores).
Melanocyte stimulating hormone – MSH
It is secreted in birds and mammals.
It also causes darkening of skin by synthesizing more melanin and by causing dispersal of melanocytes.
In human the middle lobe is rudimentary and MSH is either vestigial or is not secreted in adults. The dispersal of melanocytes or the skin pigmentation in human is regulated by ACTH
The hormones of posterior lobe of the pituitary are actually produced from the axons of neurons of the hypothalamus. These hormones are, therefore, called Neurohormones. There are two neurohormones secreted (released) from the posterior pituitary.
It is a small peptide of 9-amino acids.
It causes contraction of smooth muscles, mainly of mammary glands and uterus. • The contraction of mammary glands causes the release of milk and the hormone is, therefore, called as ‘Milk-let-down factor’.
The contraction of smooth muscles of the uterus during matured pregnancy causes expulsion of the child (parturition). This hormone is, therefore, also known as ‘Birthhormone’.
It causes constriction of blood vessels and raises blood pressure.
It also helps in the absorption of water from glomerular ﬁltrate in uriniferous tubules of the kidney. Thus, it reduces the amount of water in the urine and hence concentrates it.
This hormone is commonly called as Anti-diuretic hormone (ADH)
Hyposecretion of ADH
In such a condition the water absorption is reduced and the amount of water in urine is increased. This condition of diluted urine (Diuresis), without sugar or glucose, is called Diabetes insipidus(It is different from Diabetes mellitus in which sugar/glucose passes with urine).
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