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.

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.

Sella Turcica

Diaphragma Sellae

• 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

– 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.