Endocrine Histology PDF

ENDOCRINE HISTOLOGY DR A. ABDULKADIR DEPT OF ANATOMY CHS FUBK OVERVIEW The endocrine system comprises glands and tissues composed of parenchymal cells, which synthesize and secrete products called hormones. The term hormone derives from a Greek word meaning to set in motion. The major endocrine g...

ENDOCRINE HISTOLOGY DR A. ABDULKADIR DEPT OF ANATOMY CHS FUBK OVERVIEW The endocrine system comprises glands and tissues composed of parenchymal cells, which synthesize and secrete products called hormones. The term hormone derives from a Greek word meaning to set in motion. The major endocrine glands are the pituitary, hypothalamus, thyroid, parathyroids, adrenals, islets of Langerhans, and pineal. PITUITARY  The adult pituitary gland is divided into adenohypophysis and neurohypophysis are each divided into three parts. The adenohypophysis, consists of the anterior lobe (pars distalis), the largest part; pars tuberalis, a thin collar of tissue surrounding the infundibular stalk; and pars intermedia (intermediate lobe). The neurohypophysis, comprises the posterior lobe, the main, most expanded part; median eminence, the upper part that attaches the gland to the hypothalamus; and connecting infundibular stalk. Functions of the pituitary gland The adenohypophysis synthesizes and secretes several polypeptide and glycoprotein hormones. The neurohypophysis (by way of modified neurons from the hypothalamus) secretes two peptide hormones. All hormones enter the systemic circulation and are taken to distant target tissues to regulate functions. Functions of the pituitary gland Growth hormone (GH) Oxytocin (OXY) Prolactin (PRL) Antidiuretic hormone (ADH) Corticotropin (ACTH) Follicle-stimulating hormone Intermediate lobe (FSH) Melanocyte-stimulating Luteinizing hormone (LH) hormone (MSH) Thyrotropin TSH beta-endorphin ANTERIOR LOBE OF THE PITUITARY It consists of clumps or cords of glandular epithelial cells in close relation to a network of sinusoidal capillaries with large and irregular lumina. Scant loose connective tissue is made of delicate reticular fiber stroma, which supports glandular cells and sinusoid walls. There are two distinct parenchymal cell types: Chromophils (large, have secretory granules, stain intensely) and Chromophobes (smaller, have few or no secretory granules, stain faintly). Chromophils can be acidophils or basophils on the basis of their cytoplasmic affinity for acid or basic dyes. POSTERIOR LOBE OF THE PITUITARY The two peptide hormones released in the neurohypophysis are oxytocin and antidiuretic hormone (ADH, also known as vasopressin). Most of the gland consists largely of bundles of unmyelinated axons of neurosecretory neurons whose cell bodies are in the hypothalamic paraventricular and supraoptic nuclei. Pituicytes mingled with about 100,000 axons of the hypothalamo-hypophyseal tract in a rich network of sinusoidal fenestrated capillaries. Pituicytes are irregularly shaped cells with oval nuclei. Pituicytes and their processes ensheath and support axons. Often difficult to see with the light microscope are Herring bodies, a unique feature of the neurohypophysis. These dilated terminal expansions of the axons contain aggregates of neurosecretory material, which are stored before release.  Main actions of ADH are water excretion in the kidney and arteriolar constriction. Oxytocin stimulates uterine contraction during late stages of pregnancy and contraction of myoepithelial cells in the breast for milk ejection. THYROID GLAND The thyroid lies in the lower part of the front of the neck in contact with the upper trachea. A connective tissue capsule derived from cervical fascia encloses the two lobes and a connecting isthmus. THYROID Trabeculae of the capsule penetrate the gland to provide internal support and a pathway for a large vascular and nervous supply. Glandular parenchyma consists of spherical follicles of various sizes (50-500 μm) whose total number may exceed 20 million. Follicle lumina are filled with gelatinous colloid made of thyroglobulin. Follicles are lined by simple cuboidal epithelium, which consists of thyroid follicular cells that rest on an inconspicuous basement membrane. The height of the epithelium varies with function: usually low cuboidal in an underactive gland and high in an overactive one. A large network of fenestrated capillaries is in delicate reticular connective tissue between follicles. Small numbers of larger and paler parafollicular (or C) cells lie, as single cells or small groups, between the basement membrane of the follicles and follicular cells, or in an interfollicular position. FUNCTION OF THYROID GLAND Thyroglobulin is the temporary storage form and precursor to main thyroid hormones before release as triiodothyronine (T3) and tetraiodothyronine (thyroxine; T4). They increase oxygen consumption and metabolic rates of most body tissues and are essential for normal growth, maturation, and mental activity. The parafollicular (C) cells secrete calcitonin, which lowers blood calcium levels and counterbalances actions of parathyroid hormone. PARATHYROID GLANDS Adults typically have four small ovoid parathyroid glands, each about the size of an apple seed—3 by 6 mm. They are embedded on the thyroid’s posterior surface. A thin fibrous connective tissue capsule surrounding each gland separates it from the thyroid. The outer fibrous capsule gives rise to delicate trabeculae that convey blood vessels, lymphatics, and nerves to the interior of the gland and divide it into poorly defined lobules. The parathyroid synthesizes and secretes parathyroid hormone (PTH), which maintains blood calcium levels by increasing the rate of osteoclastic activity, thus mobilizing calcium from bone.  The adult parenchyma consists of two types of cells—chief cells and oxyphils. The polyhedral, slightly eosinophilic chief (or “principal”) cells are more numerous and form irregular, anastomosing cords supported by delicate connective issue. Oxyphils, are larger, more acidophilic cells that are irregularly distributed and occur singly or in clumps. Fat cells may also be found in the stroma and increase in number with age. ADRENAL GLANDS The paired adrenal, or suprarenal, glands lie on the superior pole of each kidney. Each is an organ composed of two distinct parts—cortex and medulla—with separate functions, and all-enclosed in a common connective tissue capsule. The outer cortex and inner medulla of the adrenal differ structurally, functionally, and developmentally. The cortex is essential to life, but the medulla is not. The cortex contains secretory cells that produce three classes of steroid hormones. The medulla has secretory cells called chromaffin cells. The outer capsule is made of dense fibrous connective tissue, which consists mostly of collagen interspersed with fibroblasts. The capsule sends thin trabeculae into the gland interior; these give rise to a delicate stroma made mostly of reticular fibers and forming a supportive network for parenchymal cells in both cortex and medulla. Three concentric zones characterize the cortex. The zona glomerulosa (10%-15%): made of closely packed, rounded clusters of parenchymal cells that produce mineralocorticoids, mainly aldosterone. The middle zona fasciculata (75%): consists mainly of radially oriented cords of polyhedral cells called spongiocytes that produce steroid hormones such as cortisol, they also produce androgens. The thin innermost zona reticularis makes up (5%-10%): consist of acidophilic parenchymal cells arranged as an anastomosing network of short cords with intervening sinusoidal capillaries. These cells synthesize androgens. C ZG ZF Me ZR The medulla contains cords or nests of polyhedral chromaffin cells surrounded by fenestrated capillaries that produce two classes of catecholamines: epinephrine and norepinephrine, which are stored in secretory granules.  ZR Me ISLETS OF LANGERHANS The pancreas is a major exocrine gland of the digestive tract with a well-developed acinar and duct system. The endocrine parts of the pancreas are the islets of Langerhans. Individual islets are scattered throughout the pancreas, but they are twice as numerous in the tail of the gland as in other parts. About 1 million exist in a normal human pancreas. Islet cells make up compact, cord-like clusters and in H&E sections appear as closely packed, pale-stained polygonal cells. Distinguishing different types of islet cells requires special stains or immunocytochemistry. German pathologist Paul Langerhans first described the islets in the pancreas: Alpha cells – glucagon Beta cells – insulin Delta cells – somatostatin PP(F) cells – pancreatic polypeptide Epsilon cells – ghrelin PINEAL GLAND The pineal is a small, cone-shaped, richly vascularized neuroendocrine organ. It projects from the roof of the third ventricle in front of the midbrain and is supplied by both sympathetic and parasympathetic nerves. It is divided into poorly defined lobules by delicate connective tissue septa that extend inward from the capsule formed around the gland by pia mater. The pineal has a mostly glandular architecture and consists mainly of closely packed, pale cells— pinealocytes—forming cords or clusters. Pinealocytes are the source of the hormone melatonin. This hormone exerts powerful effects on circadian rhythms and in some species regulates reproduction. After puberty, mineralized extracellular concretions, called corpora aranacea (brain sand), are a salient feature of the gland.