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G.Histiology(Endocrine system part 1) Instructor:Dr.Heba F. Hassan The Endocrine System Endocrine glands don't have ducts. Their secretions (hormones) are secreted into the blood stream. Because of this, the hormones can act over long distances, and reach any organ in the body to co-ordinate activity. The endocrine glands include the pituitary gland, the thyroid gland, the parathyroid glands, pineal gland, the suprarenal glands (the adrenals), and islets of Langerhans. Hormones Hormones are molecules that function in the body as chemical signals. They are liberated by specialized cells that are called endocrine cells. Many endocrine cells, however, produce hormones that act at a short distance. This is called paracrine secretion. Another method of secretion is the juxtacrine secretion, by which a molecule is released into the extracellular tissue, diffuses through the matrix, and acts on cells at a very short distance. In autocrine secretion, cells may produce molecules that act on themselves or on cells of the same type. Insulin-like growth factor (IGF) produced by several cell types may act on the same cells that produced it. The tissues and organs on which the hormones act are called target tissues or target organs. Endocrine glands are also target organs ,the body is able to control hormone secretion through a mechanism of feedback and to keep blood hormonal levels within strict limits. Pituitary Gland (Hypophysis) The pituitary (also known as the hypophysis) is found at the base of the brain, about 1cm in diameter, lying beneath the third ventricle in a bony cavity (sella turcica) in the base of the skull.The pituitary gland produces hormones that regulate growth, metabolism, and reproduction. Composed 2 part: 1 G.Histiology(Endocrine system part 1) 1-The posterior part (Neurohypophysis) of the pituitary has its embryological origins in nervous tissue. It is formed from a downgrowth of the diencephalon that forms the floor of the third ventricle. Instructor:Dr.Heba F. Hassan 2-The anterior part ( Adenohypophysis ) is derived from an upgrowth from the oral ectoderm of the primitive oral cavity called Rathke's pouch. Along the posterior part of the the anterior lobe there is a narrow region called the pars intermedia, which also has its embryological origin in Rathke's pounch. The part of the hypophysis that arises from oral ectoderm is known as the adenohypophysis (anterior part) and is subdivided into 3 portions: a large pars distalis, or anterior lobe; a cranial part, the pars tuberalis, which surrounds the neural stalk; and the pars intermedia. The neurohypophysis, the part of the hypophysis that develops from nerve tissue, consists of a large portion, the pars nervosa, and the smaller infundibulum or neural stalk. The neural stalk is composed of the stem and median eminence. Adenohypophysis (Anterior part) 1-Pars Distalis (Anterior lobe) The main components of the pars distalis are cords of epithelial cells interspersed with capillaries . Common stains allow the recognition of 3 cell types in the pars distalis: chromophobes and 2 types of chromophils called basophils and acidophils according to their affinity for basic and acid dyes, The hormones produced by the hypophysis have widespread physiologic activity ; they 2 G.Histiology(Endocrine system part 1) Instructor:Dr.Heba F. Hassan regulate almost all other endocrine glands, the secretion of milk, and the metabolism of muscle, bone and adipose tissue. 2-Pars Tuberalis The pars tuberalis is a funnel-shaped region surrounding the infundibulum (neural stalk) of the neurohypophysis . Most of the cells of the pars tuberalis secrete gonadotropins (follicle-stimulating hormone and luteinizing hormone) and are arranged in cords alongside the blood vessels. 3- Pars Intermedia The pars intermedia, which developed from the dorsal portion of Rathke's pouch , in humans, a rudimentary region made up of cords and follicles of weakly basophilic cells that contain small secretory granules. The function of these cells is not known. Neurohypophysis (posterior part) The neurohypophysis consists of the pars nervosa and the neural stalk. It is composed of some 100,000 unmyelinated axons of secretory neurons situated in the supraoptic and paraventricular nuclei . The secretory neurons have all the characteristics of typical neurons, including the ability to conduct an action potential, but have more developed Nissl bodies related to the production of the neurosecretory material. The neurosecretions are transported along the axons and accumulate at their endings in the 3 G.Histiology(Endocrine system part 1) pars nervosa. Here they form structures known as Herring bodies that are visible in the light microscope. Instructor:Dr.Heba F. Hassan The electron microscope reveals that the Herring bodies contain neurosecretory granules that have a diameter of 100200nm are surrounded by a membrane. The granules are released and enter the fenestrated capillaries that exist in large numbers in the pars nervosa; the hormones are then distributed to the general circulation. The neurosecretory material consists of 2 hormones, both cyclic peptides made up of 9 amino acids. The hormones have a slightly different amino acid composition, which results in very different functions. They are arginine vasopressin-also called antidiuretic hormone-and oxytocin. Each hormone is joined to a binding protein (neurophysin). Cells of the Neurohypophysis Although the neurohypophysis consists mainly of axons from hypothalamic neurons, about 25% of the volume of this structure consists of a specific type of highly branched glial cell called a pituicyte. Islets of Langerhans The islets of Langerhans are multihormonal endocrine micro organs of the pancreas; they appear as rounded clusters of cells embedded within exocrine pancreatic tissue. Although most islets are 100-200µm in diameter and contain several hundred cells, small islets of endocrine cells are also found interspersed among the pancreatic exocrine cells. There may be more than 1 million islets in the human pancreas, with a slight tendency for islets to be more abundant in the tail region. 4 G.Histiology(Endocrine system part 1) In sections, each islet consists of lightly stained polygonal or rounded cells, arranged in cords separated by a network of blood capillaries . In 3-dimensional reconstructions, islets of Langerhans are seen as round, compact masses of secretory epithelial cells pervaded by a labyrinthine network of blood capillaries. Both the endocrine cells and the blood vessels are innervated by autonomic nerve fibers. A fine capsule of reticular fibers surrounds each islet, separating it from the adjacent exocrine pancreatic tissue. Instructor:Dr.Heba F. Hassan Routine stains or trichrome stains allow the recognition of acidophils (alpha) and basophils (beta) .Using immunocytochemical methods four types of cells-A (glucagonproducing), B (insulin-producing), D, and F-have been located in the islets. Terminations of nerve fibers on islet cells can be observed by light or electron microscopy. Pineal Gland The pineal gland is known as the epiphysis cerebri, or pineal body. It is an endocrine gland whose secretions are influenced by the light and dark periods of the day. In the adult, it is a flattened conical organ measuring approximately 5-8mm in length and 3-5mm at its greatest width and weighing about 120mg. It is found in the posterior extremity of the third ventricle, above the roof of the diencephalon, to which it is connected by a short stalk. The pineal gland is covered by pia mater. Connective tissue septa (containing blood vessels and unmyelinated nerve fibers originate in the pia mater and penetrate the pineal tissue. Along with the capillaries, they surround the cellular cords and follicles, forming irregular lobules. 5 G.Histiology(Endocrine system part 1) Instructor:Dr.Heba F. Hassan The pineal gland consists of several types of cells, principally pinealocytes and astrocytes. Pinealocytes have a slightly basophilic cytoplasm with large irregular or lobate nuclei and sharply defined nucleoli. When impregnated with silver salts, the pinealocytes appear to have long and tortuous branches reaching out to the vascular connective tissue septa, where they end as flattened dilatations. These cells produce melatonin and several other substances (e.g., serotonin) that may influence reproduction. Melatonin is secreted at night, whereas serotonin is produced during the day. The astrocytes of the pineal gland are a specific type of cell characterized by elongated nuclei that stain more heavily than do those of parenchymal cells. They are observed between the cords of pinealocytes and in perivascular areas. These cells have long cytoplasmic processes that contain a large number of intermediate filaments 10nm in diameter. 6