Transcript pituitary gland 1

Assist prof. of Medical Physiology
• Is an ovoid structure weighing 500 to 600 mg in an adult
(0.5 gm).
• Is located at the base of the brain in a small cavity
called ‘Pituitary Fossa' or ‘Sella Turcica',
• Covered by extension of the dura mater (diaphragma
sellae) through which passes the pituitary stalk
connecting the gland to the hypothalamus.
Diaphragma Sellae
Sella Turcica
• 75% of the weight of the pituitary gland.
• Dark red colour (due to blood sinusoids in between the
secretory cells)
• Staining techniques show two cell types; each form
about 50% of the cell :
1. Chromophils 50% :
•
Acidophils (35-40%)
•
Basophils (10-15%).
2. Chromophobes 50%, small cells, devoid of
granules and have poor affinity for dyes.
Chromophils :
– through specific immunostaining may be either:
• a) Acidophil cells
– Somatotrop cells: secrete GH.
– Mammotrop cells: secrete prolactin hormone.
• b) Basophil cells :
– Thyrotrop cells: secrete TSH.
– Gonadotrop cells: secrete FSH & LH also called
gonadotrophic hormones.
– Corticotrop cells: secrete ACTH hormone, B-lipotropeins and
gamma MSH.
– So, in man, it secretes 8 hormones:
1. GH (also called somatotropic hormone or somatotropin).
2. Prolactin (also called lactogenic hormone or mammotropin)
3. MSH (also called melanotropin or intermedin).
4. TSH (thyrotropin or thyrotropic hormone).
5. ACTH (or corticotrophin).
6. FSH.
7. LH (in male called interstitial cell stimulating hormone).
8. Beta lipotropins.
N.B.: Adenohypophysis controls all other endocrine
glands except PTG, Pancreas and adrenal medulla
1. Hypothalamic Control
2. Feedback mechanism
3. Other Factors e.g. stress
– Hypothalamus controls the synthesis and the release of the
ant pituitary hormones through;
• Hypothalamo-hypophyseal portal circulation.
– Internal Carotid Artery ---> 2 Superior Hypophyseal Arteries
– 1st set of capillaries (In Median Eminence & Neural Stalk) --> Portal Veins --->
– 2nd set of capillaries (Sinusoids) (In Anterior Pituitary)
Hypothalamohypophyseal portal circulation
Evidence:
• a) Cutting of the pituitary stalk
– Causes atrophy of the adrenal cortex, the thyroid and the
gonads
– These glands recover after regeneration of the portal
vessels.
• b) Transplantation of the ant pituitary under capsule of kidney
also
– leads to atrophy of the target glands although the
transplanted pituitary tissue survives.
So the pituitary portal system is essential for the ant pituitary
function.
• The activity of the ant pituitary is affected by target
glands hormones e.g. Thyroxin, Cortisol and Gonadal
steroids.
1) Application of thyroxin to the:
• ant pituitary reduces TSH secretion
• anterior hypothalamus (site of release of TRH) reduces
TSH output but the degree of reduction is less;
• The feedback mechanism controlling thyroid gland activity act
mainly on the ant pituitary.
2) Application of oestrogen or cortisol to:
– the posterior hypothalamus is much more effective in
reducing gonadotropin or ACTH release than direct
application to the ant pituitary.
The hypothalamus is important in the feed-back
control of the gonads and adrenal cortex.
such as:
1. Physical and emotional stress,
2. Coitus and
3. Suckling.
Source:
• Somatotrop acidophil cells (30-40% of anterior
pituitary cells)
Chemistry:
• GH is a protein hormone formed of a single chain of
amino acids (about 191).
• Its basal blood concentration level is less than 3ng/ml.
Plasma Concentration
1. On growth: growth promoting factor.
– GH responsible for about 50% of linear growth of the body.
a) It has a protein anabolic effect in soft tissues:
– increase of weight and bulk of soft tissues except:
• Gonads
• Adrenals
• Thyroid
These are controlled by specific ant pituitary trophic H.
1. On growth:
b) Increases the length of bones by:
1. Stimulate the proliferation of the epiphyseal cartilage.
2. Formation of more protein bone matrix.
3. Increases the precipitation of minerals in bones.
• Anabolic effect of GH is potentiated by normal level of insulin
• By its effect on glucose metabolism, to supply the energy
needed for building up proteins.
II. On metabolism:
1. Protein metabolism:
• GH stimulates protein synthesis by:
1. Increase amino acid transport through the cell
membranes.
2. Increase formation of mRNA.
3. Increase proteins synthesis by ribosomes.
• Inhibit of protein catabolism
II. On metabolism:
2. Carbohydrate metabolism: GH has anti-insulin action:
1. Inhibits: the hexokinase enzyme and decreases
glucose uptake by tissues.
2. Stimulates:
• Glucagon secretion by the pancreas that increase in
glycogenolysis in the liver.
• Gluconeogenesis in the liver with more production of
glucose.
II. On metabolism:
3. Fat metabolism:
 Has powerful
lipolytic effect & increase the blood
FFA level
 To provides energy during stress conditions as:
a. Hypoglycaemia
b. Starvation.
Functions of growth hormone:
II. On metabolism:
4. Electrolyte metabolism:
– Increase absorption of Ca++ from GIT.
– Decrease excretion of Na+, K+ and HPO4++ by kidneys.
• GH has no direct anabolic effects.
• Growth promoting actions mediated by a group of
intermediary polypeptide called Somatomedins.
1) Formed in the liver, in bone cells, and some other
tissues.
2) Structurally similar to proinsulin. So, called:
– Insulin-like growth factors (IGF), 2 types: IGF-I &
IGF-II.
Somatomedins:
• Particularly IGF-I (known as Somatomedin-C),
1) Interact with target organs to induce growth as in growing
cartilage.
2) They also feedback on the pituitary to inhibit GH secretion.
3) Bind to specific cell membrane receptors.
• That can bind insulin and pro-insulin but with less
affinities.
1. Hypothalamic control and
feedback
2.Other factors (stimulate or
inhibit)