- On the basis of development, pituitary gland is completely Ectodermal.
- It is situated in the sella–turcica of sphenoid bone.
- This gland is attached to the hypothalamus through a stalk which is called as infundibulum.
- The upper terminal end of infundibulum which is attached to the hypothalamus is called as tubercinerium/ Median eminence.
- The lower terminal end of infundibulum is bulging type which is called as posterior lobe or pars nervosa.
- Tubercinerium, infundibulum & pars nervosa are collectively called as Neurohypophysis.
- A small & fine tube is developed in the foetus which is called as gut. This gut is developed into alimentary canal. Anterior part of gut is called as stomodaeum (developed from ectoderm).
- A small projection is developed from dorsal surface of stomodaeum (the anterior part of gut) this part is separated from stomodaeum and forms the Rathke’s pouch.
- Upper surface of Rathke’s pouch is developed into mid–lobe which is also known as pars intermedia.
- The upper margin of R.pouch, which surrounds the infundibulum & forms a collar like structure is called as parstuberalis.
- Remaining part of R.pouch. is developed into anterior lobe which is also called as pars distalis.
- Pars distalis, pars tuberalis & pars intermedia are collectively called as adenohypophysis.
- Neurohypophysis + Adenohypophysis is called as hypophysis cerebri or pituitary body.
- A remaining & vestigeal cavity of R.pouch is found in the anterior lobe which is called as Hypophysis recess.
- Superior branch of hypophyseal artery supplies blood to the hypothalamus.
- Inferior branch of hypophyseal artery supplies blood to the pituitary gland.
Hypophyseal portal vein collects the blood from hypothalamus and supplies to the pituitary gland.
- Branch of hypophyseal artery & hypophyseal portal vein form a network of blood capillaries which is called as circle of Willis.
- In human, mid lobe is ill developed or membrane like, while mid lobe is absent in elephants & Whales.
- Three types of glandular cells are found in Adenohypophysis.
1. Basophils → They are large cells & found in the peripheral part of gland. They are more in number.
2. Acidophil → They are small in size & found in the central part of pituitary gland.
3. Neutrophils → They are smallest in size and are found in scattered form. They are minimum in number.
- STH & LTH are secreted by acidophils.
- Remaining hormone of adenohypophysis are secreted by basophils.
- All hormones of pituitary gland are protein Hormones.
HORMONES SECRETED BY ADENOHYPOPHYSIS
All these hormones are of proteinaceous nature.
1. GROWTH HORMONE OR SOMATOTROPIC OR SOMATOTROPIN HORMONE
[G.H. or S.T.H] :– By Somatotroph cells
- Major effect: On metabolism.
- Major effect: On growth.
Effect on Growth :–
(1) On bone : It increases the longitudinal length of bone.
(2) On soft tissue :
(i) GH promotes mitosis & increases number of cells in many organs & tissues. e.g. liver.
(ii) GH stimulates growth of muscle and cartilage due to increase synthesis of collagen, muscle grow in bulk.
Effect on metabolism –
Fat : Increases lipolysis so that free fatty acid level of plasma rises. Under the influence of growth hormone, fat is used for energy in preference to carbohydrate and protein, due to this protein deposition increases.
Carbohydrate:– Hyperglycemia develops due to decreased uptake of glucose in the cells, so it is also called diabetogenic hormone.
Protein : GH increases amino acid uptake by the cells of the liver & muscles & helps in protein synthesis.
- In the presence of thyroxine and insulin, growth hormones become more active and help in body growth.
In this way this hormone is important for the growth of body.
Diseases due to hyposecretion or hypersecretion of somatotropin hormone :– (a) Hyposecretion of STH :– Due to deficiency of STH in childhood or adolescence, dwarfism is observed.
- Dwarfism due to the defect of pituitary is called Ateliosis.
- Clowns of circus are such dwarfs, they are called midgets. This midget is physically & mentally normal while sexual maturation is delayed.
(b) Hypersecretion of STH :– Due to hypersecretion in childhood and adolescence, a higher quantity of aminoacids is supplied to the body cells.
(i) Epiphyseal cartilage present on the edges of bones does not convert into bone for a long time. Thus the bones of legs & hands become very long and height of that person increases very much. Body becomes imbalanced. This disease is called Gigantism.
(ii) Acromegaly :– Due to hypersecretion of STH in adulthood, jaw bones of the affected person become long, cheek bones buldge out, broad hands, legs & fingers of person becomes gorilla like. These symptoms are observed as a result of acromegaly.
2. THYROTROPHIC [T.T.H OR T.S.H.] OR THYROID STIMULATING HORMONE :–
- T.S.H is secreted by basophil cells. (Thyrotroph cell).
- It is glycoprotein in nature.
- T.S.H stimulates thyroid gland to secrete thyroxine. TSH helps in almost all step of the thyroid hormone synthesis & it causes growth of thyroid gland.
- Secretion of TSH is stimulated by Thyrotrophin releasing factor & inhibited by somatostatin of hypothalamus.
3. ADRENO CORTICO TROPHIC HORMONE OR CORTICOTROPINE [ACTH] :–
- ACTH is secreted by basophil (corticotroph) cells.
- In the ACTH of man, valine and tyrosine aminoacids are more in quantity.
- It accelerates the cortex part of adrenal gland to secretes hormones.
4. FOLLICLE STIMULATING HORMONE [FSH] :–
- It is also secreted by basophils. (Gonadotroph cell)
- It is a glycoprotein in nature.
- This is secreted in male and female both.
- In males, it stimulates spermatogenesis and normal functioning of seminiferous tubules.
- In females, it stimulates oogenesis and development of Graafian follicles in ovary.
- FSH is also known as Gametokinetic factor.
- Estrogen hormone that is secreted by Graafian follicles is also affected by FSH.
5. LUTEINIZING HORMONE [LH OR ICSH] OR INTERSTITIAL CELL STIMULATING `HORMONE :–
- It is also secreted by basophil (Gonadotroph) cells. It is also a glycoprotein in nature.
- It stimulates ovulation in female as a result of this corpus luteum is formed.
- Hormone progesterone which is secreted by corpus luteum is also stimulated by L.H.
- In men LH is called ICSH. It affects the Leydig's cells or Interstitial cells of testes and stimulates the secretion of male hormone " Testosterone".
- L.H. is also called " gamete releasing factor".
- FSH and LH both are called gonadotrophic hormone GTH.
- FSH and LH act in combined form so these are called synergesic hormone.
- Gonadotrophic hormones (FSH & LH) secretion start secreting during puberty. Their secretion is regulated by hypothalamus. It is supposed that there a biological clock present to control all this.
6. LUTEOTROPHIC OR PROLACTIN OR LACTOGENIC OR MAMMOTROPIC HORMONE (PRL) :
It is secreted by acidophil (Lactotroph) cells.
Function of prolactin hormone.
(1) Lactation (Galactopoiesis) :- Prolactin is responsible for lactation in postpartum (after delivery) women.
- Regulation of Prolactin secretion by Hypothalamus :- Hypothalamus mainly stimulate the production of all pituitary hormone, but it mainly inhibits prolactin production because normally hypothalamus prolactin inhibitory hormone (Dopamine) is greater than the releasing hormone.
- Breast is prepared for lactation by oestrogen (duct growth) and progesterone (lobule growth) but both of these hormone inhibit the actual secretion of milk.
- Dopamine is a catecholamine (Biologically active amine) and neurotransmitter in the hypothalamus. It inhibits lactation.
- During pregnancy :- Prolactin hormone steadily increase until term but due to high level of oestrogen and progesterone (secreted by placenta) lactation is inhibited.
- After expulsion of the placenta at parturition, there is an abrupt decline in circulating estrogen and progesterone.
- The drop of circulating estrogen initiate lactation.
- Nursing stimulates prolactin secretion.
- Prolactin inhibits the action of GnRH on the pituitary and antagonize the action of gonadotrophin on the ovaries. Ovulation is inhibited and the ovaries gets inactive.
- Nursing is important and effective method of birth control.
7. HORMONES SECRETED BY MIDDLE LOBE OF PITUITARY GLAND :–
- The part of adenohypophysis of pituitary gland, which is very close to neurohypophysis is a very thin portion, it is called pars intermedia or middle lobe of pituitary gland.
- In men, it is in the form of a thin membrane only. It is inactive in men.
MELANOCYTE STIMULATING HORMONE [MSH] :–
- It is secreted by middle lobe. (By corticotroph cell)
- MSH is also called Intermedin.
- In man, MSH is secreted by anterior lobe, because middle lobe is ill- developed.
- It stimulates the melanocytes to synthesize melanin in mammals.
- This hormone is related with change in the colour of skin in Amphibian and Reptiles. This phenomenon of colour changing is known as metachrosis.
- It darkens the complexion of skin by distributing melanin pigment evenly under the skin.
- Just opposite to it, melatonin secreted by pineal body, collects the melanin pigments at one place thus fairing the complexion of skin.
- MSH is found in all the vertebrates, but it is functional in poikilothermic animals e.g. fishes, amphibians, reptiles etc.
- The importance of MSH in man has not been evaluated.
HORMONE RELEASING FACTORS OF HYPOTHALAMUS :–
Hormones secreted by adenohypophysis of pituitary gland are under the control of neuro- secretory cells of hypothalamus. These neuro-secretory cells produce some substances in very small amount which are called releasing factors.
(1) Growth hormone releasing factor:- [GHRF]:- It stimulates the secretion of growth hormone.
(2) Thyrotropin releasing factor [TRF]:- It stimulates the secretion of TSH.
GnRH Follicle stimulating hormone releasing factor [FSHRF] :– It stimulates the secretion of FSH.
GnRH Luteinising hormone releasing factor [LHRF] :– It stimulates the secretion of LH.
(4) Growth hormone inhibitory factor [GHIF] (Somatostatin) :– It inhibits the secretion of GH.
(5) Prolactin release factor [PRF] :– It stimulates the secretion of Prolactin.
(6) Prolactin inhibitory factor [PIF] (Dopamine) :– It inhibits the secretion of Prolactin.
(7) Melanocyte release factor [MRF] :– It stimulates the secretion of MSH.
(8) Melanocyte inhibitory factor [MIF] :– It inhibits the secretion of MSH.
(9) Adrenocorticotropic releasing hormone (CRH) :– It stimulates secretion of Adrenal cortex hormone.