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Endocrine Glands

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Endocrine Glands ENDOCRINE GLANDS The different activities of the body are regulated by 2 main systems:- 1. The nervous system. 2. The endocrine system. The nervous system: is concerned with regulation of metabolic activities and secretions of glands. It is a rapid control system. The endocrine system: is concerned with regulation of metabolic functions. It is a slow control system. What is meant by the endocrine system? It is a system of ductless glands, which differs from other types of glands such as the salivary glands in that their secretions which are called hormones enter the blood stream directly which carries them to different tissues to produce their effects. What is a hormone ? It is a chemical substance secreted into the blood stream by endocrine glands to act on distant organs called effector organs. Properties of hormones: They are secreted in very small amounts. So, their blood and tissue concentrations are very low. They do not act on the organs secreting them (endocrine glands) but act on distant organs (effector organs). Hormones may act on a specific effector organ e.g. thyrotrophic hormone (TSH) of the anterior pituitary acts specifically on the thyroid gland, or hormones may act on the body as a whole e. g. growth hormone (GH) of the anterior pituitary. Hormones act on the target organ by changing the rate of biochemical reactions in that organ. This effect persists for a longer period even after the hormone which produced it becomes inactivated. Difference between a hormone and an enzyme: Hormone Enzyme Some hormones are All enzymes are proteins. proteins, some are amino acids and some are steroids. Hormones act on distant Enzymes act in the same organs and not on those tissues forming them. secreting them. Enzymes catalyze Hormones are continually biochemical reactions without secreted, inactivated or being destroyed or excreted. excreted. Chemical nature of hormones: Hormones may be derived from: (1) Amino acids: e.g. Tryptophan serotonin and melatonin. Tyrosine T4 (thyroxine) and T3 (triiodothyronine). (Thyroid hormones) Catecholamines (e.g. Adrenaline and Noradrenaline.) (Hormones of the adrenal medulla) (2) Peptides and proteins: e.g. All pituitary hormones are either peptides or proteins. The pancreatic hormones insulin and glucagon (peptides). (3) Steroids in nature : e.g. Adrenocortical hormones (cortisol). Sex hormones (testosterone). The plasma level of a hormone depends upon 2 factors: 1- The rate of hormone secretion by the endocrine gland : rate of secretion plasma hormone level (and vice versa) . N.B. Some hormones may show daily variation in the rate of secretion known as the circadian rhythm . 2- The rate of hormone inactivation and excretion: rate of inactivation plasma hormone level (and vice versa). N.B. Inactivation may occur in the effector organ or in the liver, or may be excreted by the kidney. Mechanism of Action of Hormones: Hormones change the activity of the effector organs by changing the rate of enzymatically catalyzed reactions in them. This may be the result of: (1) lncreased rate of enzyme synthesis increased enzyme concentration. lt is a slow response. e.g. Steroid hormones. N.B. The rate of a biochemical reaction is to enzyme concentration. (2) lncreased activity of the enzyme without increase in its concentration accelerates a biochemical reaction rapid response. e.g. peptides and protein hormones . (3) Both effects. The response in this case is rapid and prolonged. How does activation of enzymes occur? (Fig.1) lt occurs through the following series of steps: (1) For non - steroidal hormones (proteins, peptides and amino acids) : . Hormones are carried by the blood stream to the cells of the effector organ. Hormones do not pass the cell membrane to the inside of the cells because of their large molecular size. Therefore, hormones bind to specific protein receptors on the outer surface of the cell membrane. This binding activates a protein enzyme on the inner surface of the cell membrane called adenyl cyclase. Adenyl cyclase catalyzes the conversion of ATP cAMP + PPi cAMP inside the cells of the effector organ. This cAMP diffuses through the cells of the effector organ acting as a second messenger to stimulate the enzyme activity . N.B. The hormone is the primary messenger. adenyl cyclase phosphodiesterase ATP ------------------- ppi + cAMP ------------------------ 5 – AMP (AC) (PDE) N.B. lnhibitors of PDE (e.g. methyl xanthines cGMP, cIMP) cAMP potentiate hormonal effects. Substances which potentiate the effect of PDE (e.g. ammonium ions ) cAMP and antagonize the hormonal effects. Other second messengers include cGMP and prostaglandins (PGs). (2) For steroidal and thyroid hormones : Hormones are carried by the blood stream to the cells of the effector organ. Being lipid soluble, they enter the cell and bind to specific receptors inside the cytoplasm forming a complex. The hormone-receptor complex moves towards the nucleus and enters through the nuclear membrane. ln the nucleus, it accelerates RNA formation from DNA (transcription). Newly formed RNA (mRNA) leaves the nucleus to the cytoplasmic ribosomes where the process of protein synthesis is stimulated according to the genetic code carried by mRNA (translation). Thus, the enzyme protein is increased. N.B. Thyroid hormones bind directly to nuclear receptors. (A) (B) Mechanism of hormonal action of (A) protein hormones and (B) steroid hormones Methods of Studying Endocrine Functions: 1) By measuring the concentration of a specific hormone in the plasma using a highly sensitive method as radio immunoassay (RIA). 2) By observing the effects of injury or removal of part or all of the gland on body functions. 3) By observing the effects of injection of glandular extracts. THE PITUTARY GLAND (HYPOPHYSIS) Anatomically: - lt is a small gland present at the base of the brain, occupying the sella turcica. - lt is 0.5 to 1 gm in weight; being slightly heavier in females. - lt is connected to the hypothalamus by the pituitary stalk (infundibulum) Physiologically : The pituitary gland is formed of 2 parts, which differ both embryologically and functionally and are considered as 2 separate glands: (1) The Anterior Pituitary (Adenohypophysis ): - lt develops from the roof of the buccal (Rathk's pouch ) and is of epithelioid nature. - lt is formed of 3 parts: 1. Pars distalis. 2. Pars tuberalis. 3. Pars intermedia (intermediate part, or sometimes called intermediate lobe.) (2) The posterior pituitary (Neurohypophysis ): - lt develops from the floor of the 3rd ventricle. - lt is divided into : 1. The pituitary stalk (infundibulum); connecting the pituitary to the hypothalamus. 2. The nervous part (pars nervosa . N.B. There is no intermediate lobe in the human pituitary gland. HORMONES OF THE PITUITARY GLAND (1) Anterior Lobe Or Anterior Pituitary Hormones : 1- Growth Hormone (GH, Somatotropin, STH). 2- Thyroid Stimulating Hormone (TSH, Thyrotropin). 3- Adrenocorticotrophic Hormone (ACTH, Corticotropin). 4- Follicle Stimulating Hormone (FSH). 5- Luteinizing Hormone or lnterstitial Cell Stimulating Hormone (LH, ICSH) N.B. FSH and LH are referred to as gonadotrophic hormones (GnH). 6- Prolactin (PL), Lactogenic Hormone, Mammotropin, Luteoptrophic Hormone (LTH). (2) lntermediate Lobe hormones : - Melanocyte stimulating Hormone (MSH) . N.B. This hormone does not appear to have function in man . (3) The Posterior Lobe Hormones : 1- Antidiuretic Hormone (ADH, or Vasopressin, AVP). 2- Oxytocin. THE ANTERIOR PITUITARY GLAND (ADENOHYPOPHYSIS) Histologically: With the use of acid - base stains the anterior pituitary cells are classified into 3 types: 1- Acidophil cells: Their cytoplasm is granular and the granules stain with acid dyes. They from 35% of the total cells. They secrete protein hormones : growth hormone (GH) and prolactin (PL) . 2- Basophil cells : Their cytoplasmic granules stain with basic dyes. They form 15 % of the cells. They secrete : - Polypeptide hormones (ACTH and MSH). - Glycoprotein hormones (FSH, LH and TSH). 3- Chromophobe cells : Their cytoplasm is non-granular. They form 50% of the cells. Their exact function is not fully determined . N.B. Recent classification using immunocytochemistry and electron microscopy: 5 types of secretory cells were distinguished: 1. Somatotropes, which secrete growth hormone. 2. Lactotropes (also called mammotropes), which secrete prolactin. 3. Thyrotropes, which secrete TSH. 4. Gonadotropes, which secrete both LH and FSH. 5. Corticotropes, which secrete both ACTH and B-LPH. Functions Of The Adenohypophysis: 1) lt stimulates growth of the body and skeleton. 2) lt stimulates growth and functions of target glands (thyroid, adrenal cortex , testes and ovaries). Adenohypophyseal hormones are called therefore trophic hormones. 3) lt stimulates reproduction and sexual activities. 4) lt stimulates general and specific metabolism. Regulation Of Adenohypophyseal Function The hormonal secretion by the anterior pituitary is regulated through 3 mechanisms : 1) The hypothalamic control. 2) Feed back control by the hormones of the effector organs (target glands). This is the long loop feed back . 3) Feed back control by the pituitary hormones themselves. This is the short loop feed back . 1- The Hypothalamic Control : . The hypothalamus is a large nervous center that receives many afferent nerve fibers carrying information about the changes that occur outside or inside the body. According to information, the hypothalamus secretes hypothalamic factors (neurohormones) that reach the
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