Physiology of Endocrine Abdelaziz M Hussein Assist prof

Physiology of Endocrine Abdelaziz M. Hussein Assist prof. of Medical Physiology

Pituitary Gland

Pituitary Gland (Hypophysis Cerebri) • 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

Anterior Pituitary Gland (adenohypophysis) • 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.

Anterior Pituitary Gland 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.

Anterior Pituitary Gland – 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

Control of Ant. Pituitary Gland Secretion

Control of Ant. Pituitary Gland Secretion 1. Hypothalamic Control 2. Feedback mechanism 3. Other Factors e. g. stress

1. Hypothalamic Control – Hypothalamus controls the synthesis and the release of the ant pituitary hormones through; • Hypothalamo-hypophyseal portal circulation. – Internal Carotid Artery ---> 2 Superior Hypophyseal Arteries – 1 st set of capillaries (In Median Eminence & Neural Stalk) ---> Portal Veins ---> – 2 nd set of capillaries (Sinusoids) (In Anterior Pituitary)

Hypothalamohypophyseal portal circulation

1. Hypothalamic Control

1. Hypothalamic Control 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.

2. Feedback Control • The activity of the ant pituitary is affected by target glands hormones e. g. Thyroxin, Cortisol and Gonadal steroids.

2. Feedback Control 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. Feedback Control 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.

3. Other Mechanisms of Control such as: 1. Physical and emotional stress, 2. Coitus and 3. Suckling.

Growth Hormone

Growth Hormone

Growth Hormone or Somatotropin 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 3 ng/ml.

Plasma Concentration

Functions of Growth Hormone 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.

Functions of Growth Hormone 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.

Functions of Growth Hormone II. On metabolism: 1. Protein metabolism: • GH stimulates protein synthesis by: 1. Increase amino acid transport through the cell membranes. 2. Increase formation of m. RNA. 3. Increase proteins synthesis by ribosomes. • Inhibit of protein catabolism

Functions of Growth Hormone 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.

Functions of Growth Hormone 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 HPO 4++ by kidneys.

Mechanism of GH action

Mechanism of GH action • 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.

Mechanism of GH action 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.

Control of GH Secretion

Control of GH Secretion 1. Hypothalamic control and feedback 2. Other factors (stimulate or inhibit)

1. Hypothalamic control and feedback

2. Other Factors