Endocrine System -I For Medical Students PDF

ENDOCRINE SYSTEM:I Prof.Dr./ Kawther M.Abdel Hamied General Structure. Pituitary Gland. Pineal Gland.  ENDOCRINE SYSTEM  Def.: The endocrine system is one of the 2 major control systems that regulate the body functions, the other being the nervous system.  It is the...

ENDOCRINE SYSTEM:I Prof.Dr./ Kawther M.Abdel Hamied General Structure. Pituitary Gland. Pineal Gland.  ENDOCRINE SYSTEM  Def.: The endocrine system is one of the 2 major control systems that regulate the body functions, the other being the nervous system.  It is the site of synthesis and secretion of hormones, in small amounts, which are disseminated throughout the body by the blood stream where they act on target organs, both slowly (minutes to days) and at low concentrations.  The endocrine system include the following:  Ductless glands: Pituitary, thyroid &parathyroid, adrenal glands and pineal body.  Isolated cells within certain organs of the body: Islets of langerhans, gonads, placenta, thymus, and kidney.  Diffuse neuro-endocrine system cells (DNES-C)..  General histological features of endocrine glands and cells:  Endocrine glands convey secretion directly to blood; they have no ducts (ductless).  Parenchymal cells are organized in the form of columns (fascicles), glomeruli, follicles or groups with at least one surface of these cells facing a blood capillary.  They are richly supplied with fenestrated blood capillaries with extraordinary large diameter.  Microscopic appearance of hormone-producing cells differ according to the hormones they secrete; peptides, amines or steroids.  1- Peptides/glycopeptides are synthesized by rough endoplasmic reticulum, transported within the Golgi apparatus and stored in intracellular granules (secretory vesicles) e.g. hormone-producing cells of pars distalis of the pituitary gland.  2- Amines are amino acid derivatives. Examples of amine hormones include thyroid hormone, epinephrine, and norepinephrine. Epinephrine and norepinephrine are synthesized and stored in a manner similar to peptidehormones.  3-Steroids are synthesized on demand from cholesterol via enzymes in the mitochondria and smooth ER of steroid-secreting cells (not stored) e.g. hormoneproducing cells of the adrenal cortex.   PITUITARY GLAND (HYPOPHSIS CEREBRI) -Pituitary gland is located in the base of the skull in a bony fossa called the Sella Tursica.  Has anterior and posterior lobes separated by a cleft. I) The Anterior lobe (adenohypophysis) consists of: A- large Pars Distalis. B- Pars Tuberalis which wraps around the infundibulum stalk. C- Pars intermedia is a thin zone separated from pars distalis by the hypophyseal cleft and lies adjacent to pars nervosa. -Pars intermedia is formed of basophils and contains colloid-filled cysts. II) The Posterior lobe (neurohypophysis) consists of: A-pars nervosa B-Infundibulum stalk which connects the pituitary gland to the hypothalamus PITUITARY GLAND (HYPOPHSIS CEREBRI)  a) Pars distalis:  Account for 75% of the mass of the pituitary gland.  L.M: it is surrounded by a thin connective tissue capsule. The cells are arranged in the form of branching cords separated by wide fenestrated blood capillaries and supported by a network of reticular fibers.  According to staining affinity the cells are of 2 types, chromophils and chromophobes when stained with common histological stains. Pars distalis with L.M. Cells of Pars Distalis Chromophobes Chromophils More numerous (52%) Less numerous (48%) not take histological stain take histological stain smaller Larger non granular cytoplasm Have granular cytoplasm  Chromophils Acidophil cells Basophil cells (37%) (11%) Moderate in size Larger in size Have acidophilic granules Have basophilic granules Somatotrophs: Growth Thyrotrophs: Thyroid Hormone (GH) Stimulating Hormone (TSH) Lactotrophs: prolactin Gonadotrophs: Follicle Stimulating Hormone (FSH) & Luteinizing Hormone (LH) Corticotrophs: Adrenocorticotrophic Hormone (ACTH) Pars Intermedia  A narrow zone lying between the pars distalis and the pars nervosa.  It contains basophils(corticotrophs), chromophobes, and small, colloid-filled cysts derived from the lumen of the embryonic hypophyseal pouch.  Corticotrophs of the pars intermedia synthesize a large –M.W. polypeptide & cleave it producing mainly smaller peptide hormones, including two forms of melanocyte-stimulating hormone (MSH), γ-LPH, and β- endorphin.  MSH increases melanocyte activity.  The overall functional significance of the pars intermedia remains uncertain.  II. Neurohypophysis (Pars Nervosa)  It is formed of fibers, cells and a rich blood capillary plexus.  -The fibers are the unmyelinated axons of magnocellular neurosecretory cells present in the supraoptic and paraventricular nuclei of the hypothalamus.  These fibers carry the neurosecretory material from the hypothalamic nuclei to the pars nervosa, where they terminate adjacent to the capillary plexus.  The neurosecretions are stored in dilatations along the length of the fibers as faintly basophilic small bodies known as Herring bodies.  - The cells of the pars nervosa are called pituicytes which are neuroglial cells. Microscopic Structure of Neurohypophysis(Pars Nervosa) Functioal histology of Pars Nervosa  Transported axonally into the pars nervosa, these hormones accumulate in axonal dilations called neurosecretory bodies or Herring bodies, visible in the light microscope as faintly eosinophilic structures.  The neurosecretory bodies contain membrane- enclosed granules with either oxytocin or ADH bound to carrier proteins called neurophysin I and II, respectively.  The hormoneneurophysin complex is synthesized as a single protein and then cleaved to produce the peptide hormone and its binding protein.  Nerve impulses along the axons trigger the release of the peptides from the neurosecretory bodies for uptake by the fenestrated capillaries of the pars nervosa, and the hormones are then distributed to the general circulation.  Axons from the supraoptic and paraventricular nuclei mingle in the neurohypophysis but are mainly concerned with ADH and oxytocin secretion, respectively.  ADH is released in response to increased blood tonicity, sensed by osmoreceptor cells in the hypothalamus, which then stimulate ADH synthesis in supraoptic neurons. ADH increases the permeability of the renal collecting ducts to water. So that, more water is reabsorbed from the filtrate in these tubules and osmotic balance of body fluids is restored.  Blood Supply of The Pituitary Gland  The pituitary gland’s neural connection to the brain and its blood supply are both of key importance for its function.Embryologically, anatomically, and functionally, the pituitary gland is connected to the hypothalamus at the base of the brain.  The blood supply derives from two groups of vessels coming off the internal carotid artery and drained by the hypophyseal vein.  The superior hypophyseal arteries supply the median eminence and the infundibular stalk; the inferior hypophyseal arteries provide blood mainly for the neurohypophysis. The superior arteries divide into a primary plexus of fenestrated capillaries that irrigate the stalk and median eminence.  These capillaries then rejoin to form venules that branch again as a larger secondary capillary plexus in the adenohypophysis.  This Hypothalamic-hypophyseal vascular portal system has a great importance because it carries neuropeptides, small regulatory peptides, from the hypothalamus (Median eminence) the short distance to the adenohypophysis where they either stimulate or inhibit hormone release by the endocrine cells there.  The Hypothalamic-Hypophyseal Portal system&Tract  The hypothalamic-hypophyseal tract courses into the neurohypophysis from two important hypothalamic nuclei. The peptide hormones ADH (antidiuretic hormone) and oxytocin are synthesized by large neurons in the supraoptic and the paraventricular nuclei, respectively.  Then undergo axonal transport and accumulate temporarily in the axons of the hypothalamic-hypophyseal tract before their release and uptake by capillaries branching from the inferior arteries. Control of Hormone Secretion in the Anterior Pituitary  The activities of the cells of the anterior pituitary are controlled primarily by peptide-related hypothalamic hormones produced by small neurons near the third ventricle, discharged from axons in the median eminence, and transported by capillaries of the portal system throughout the anterior pituitary.  Most of these hormones are releasing hormones that stimulate secretion by specific anterior pituitary cells.  Two of the hypothalamic factors, however, are inhibiting hormones that block hormone secretion in specific cells of the adenohypophysis.  Choose the correct answer:  1. Growth hormone is produced by: a- Lactotrophs. b- Corticotrophs. c- Gonadotrophs. d- Thyrotrophs. e- Somaotrophs.  2. Growth hormone-producing cells: a- Lack affinity for stains. b- Have basophilic cytoplasm. c- Store their secretion in Herring bodies. d- Have no secretory granules. e- Have acidophilic cytoplasm.  3. Growth hormone-producing cells represent: a- Degranulated chromophils. b- Stem and progenitor cells. c- Basophils of pars distalis. d- Half the chromophils of pars distalis. e- Diffuse neuroendocrine cells. PINEAL GLAND Structure of pineal gland ❑ Also known as the epiphysis cerebri, is a small, pine cone- shaped organ, approximately 5-8 mm by 3-5 mm. ❑ The pineal gland develops from neuroectoderm in the posterior wall of the third ventricle and remains attached to the brain by a short stalk. ❑ The pineal gland is covered by connective tissue of the pia mater, from which septa containing small blood vessels emerge and subdividing it into variously sized lobules. ❑ Prominent and abundant secretory cells called pinealocytes have slightly basophilic cytoplasm and irregular euchromatic nuclei.  A characteristic feature of the pineal gland is the presence of variously sized concretions of calcium and magnesium salts called corpora arenacea, or brain sand, formed by mineralization of extracellular protein deposits.  Such concretions appear during childhood and gradually increase in number and size with age, with no apparent effect on the gland’s function  With E.M. :  Pinealocytes are seen to have secretory vesicles, many mitochondria, and long cytoplasmic processes extending to the vascularized septa, where they end in dilatations near capillaries, indicating an endocrine function. These cells produce melatonin, a low-molecular-weight tryptophan derivative.  Unmyelinated sympathetic nerve fibers enter the pineal gland and end among pinealocytes, with some forming synapses.  The pineal gland also has numerous interstitial glial cells (modified astrocytes) staining positively for glial fibrillary acidic protein. Silver carbonate impregnation permits a more detailed analysis of their distribution and activity.  These have elongated nuclei more heavily stained than those of pinealocytes and are usually found in perivascular areas and between the groups of pinealocytes.  Function of Pineal Gland Melatonin release from pinealocytes is promoted by darkness and inhibited by daylight. The resulting diurnal fluctuation in blood melatonin levels induces rhythmic changes in the activity of the hypothalamus, pituitary gland, and other endocrine tissues that characterize the circadian (24 hours, day/ night) rhythm of physiological functions and behaviors. In humans and other mammals, the cycle of light and darkness is detected within the retinas and transmitted to the pineal via the retinohypothalamic tract, the suprachiasmatic nucleus, and the tracts of sympathetic fibers entering the pineal. The pineal gland acts, therefore, as a neuroendocrine transducer, converting sensory input regarding light and darkness into variations in many hormonal functions.