Endocrine glands histology (1)-1.pptx
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Lecture : Endocrine Glands Histology By : Rakhshindah Qureshi Email : [email protected] Learning Objectives : •1.Describe the thyroid gland histology . •2.Describe the major cell types of the thyroid gland and explain their functions . •3.Briefly explain the basis of thyroid secretory mechanism....
Lecture : Endocrine Glands Histology By : Rakhshindah Qureshi Email : [email protected] Learning Objectives : •1.Describe the thyroid gland histology . •2.Describe the major cell types of the thyroid gland and explain their functions . •3.Briefly explain the basis of thyroid secretory mechanism. •4.Describe the histology of Parathyroid gland . •5.Describe the cell types of the parathyroid gland and explain their function . •6.Describe the histology of adrenal gland. •7.Describe the histology of adrenal cortex . •8.Name the major hormones secreted in each zone of the adrenal cortex . •9.Describe the histology of adrenal medulla . •10.Describe the cell types and major secretions of adrenal medulla. •11.Describe the blood supply of the adrenal gland . Reference : AT Lecture Notes Color atlas of Histology by Gartner & Hiatt 8th edition •LAS OF HISTOLOGY 6TH EDI •LAS OF HISTOLOGY 6TH EDI •Notes • CHAPTER 3 CONNECTIVE TISSUE G&H ATLAS OF HISTOLOGY 6 TH EDI THYROID GLAND • The thyroid is a highly vascular butterfly shaped gland surrounding the anterior surface of trachea just below the larynx . • It consists of right and left lobes that are connected by a narrow isthmus across the thyroid cartilage and upper trachea . • It is enveloped by connective tissue capsule whose septa penetrate the substance of gland, forming not only its supporting framework but also a conduit for its rich blood supply . THYROID GLAND • It synthesizes the thyroid hormones, thyroxine ( tetra – iodothyronine or T4) and triiodothyronine (T3) . • These hormones help control the basal metabolic rate in cells throughout the body . • The hormone bind to nuclear receptors in target organs , stimulating RNA transcription . • Thyroid gland also synthesizes a hormone , Calcitonin . THYROID GLAND • The parenchyma of thyroid is composed of millions of rounded thyroid follicles of variable diameter , each with simple epithelium and a central lumen filled with gelatinous acidophilic colloid . The colloid secreted and resorbed by the follicular cells is composed of a large glycoprotein, thyroglobulin, the thyroid hormone that is bound to a large protein. • The follicular cells or thyrocytes range in shape from low cuboidal to high cuboidal or columnar , dependent on their activity that is controlled by TSH from the anterior pituitary . • The follicular cells have organelles like Golgi complexes, rough ER, numerous phagosomes and lysosomes , indicating protein synthesis , secretory function , phagocytosis and digestion . • The cells have a central , rounded nucleus . • Thyrocytes rest on basal lamina . • Thyrocytes have apical junctional complexes THYROID GLAND • The EM of follicular cells shows a single layer of low columnar follicular cells with apical microvilli (Mv) in contact with the colloid , whereas basal surfaces of follicular cells rest on a basal lamina (FBL) . • A narrow extracellular connective tissue space separates the follicular cells from the lumen of the capillary . • The fenestrated endothelial cells (En) rest on the basal lamina (EBL). THYROID GLAND • Another endocrine cell type , the parafollicular cell or C cell is also found as isolated clusters of cells between the follicles, occasionally found dispersed among the follicular cells . They secrete Calcitonin which is released into neighboring capillaries . • Secretion of calcitonin is triggered by elevated blood calcium levels, and it inhibits the osteoclast activity . • Calcitonin helps decrease blood calcium level by inhibiting bone resorption by osteoclasts . • The release of calcitonin by the parafollicular cells depends on increased blood calcium and is independent of the pituitary gland hormones . THYROID GLAND • Hormone secretions of Follicular cells : Thyroxine ( T₄) Triiodothyronine ( T₃) • Both T₃ & T₄ are involved in regulation of cell and tissue metabolism . • T₄ is secreted about 10 times the rate of T₃, is much less potent than T₃. • Much of the T₄ is converted to T₃ in various organs . • These hormones induce increased protein synthesis and carbohydrate metabolism . They are important in body growth and nervous system development . • The effects result from the hormone binding to nuclear receptors in target organs , stimulating RNA transcription . SYNTHESIS OF THYROID HORMONE BY FOLLICULAR CELLS • The production of thyroid hormones thyroxine (T₄) and T₃ takes place in the thyroid follicle in a series of discrete steps : • 1. The synthesis of thyroglobulin • 2. Uptake of iodide • 3. Iodination of thyroglobulin with either one or two atoms of iodine . • 4. Formation of T3 & T4. • 5. Resorption of colloid • 6. Release of T3 &T4. CONTROL OF THYROID HORMONE The major regulator of the anatomic and functional state of thyroid follicle is TSH from the anterior pituitary . CLINICAL CORELATES • The most common symptom of thyroid disease is a goiter , the enlargement of thyroid gland. • Hypothyroidism : can be caused by insufficient dietary iodine or because of inherited autoimmune diseases . • Hyperthyroidism : (Graves’ disease) excessive amounts of thyroid hormones are released into circulation . PARATHYROID GLANDS • Are 4 small ovoid masses located on the back of the thyroid gland , usually embedded in the capsule of the thyroid gland . • The endocrine cells of the parathyroid gland are called principal ( chief) cells. They are small, polygonal cells with round nuclei and a pale acidophilic staining cytoplasm . • Principal cells contain irregularly shaped cytoplasmic granules , Parathyroid hormone (PTH) polypeptides . • Principal cells decrease with age and are replaced by adipose cells . • PTH is an important regulator of blood calcium levels . PARATHYROID GLANDS • Much smaller population of oxyphil cells are also seen in the parathyroid gland , more commonly in older people . • These are much larger than the principal cells and are characterized by a highly acidophilic cytoplasm filled with mitochondria . PARATHYROID GLANDS • Parathyroid hormone (PTH) has 3 major targets : • 1. Osteoblasts respond to PTH by producing an osteoclast – stimulating factor that increases the number and activity of osteoclasts . The resulting resorption of calcified bone matrix & release of calcium increases the concentration of circulating calcium , which suppresses PTH production . The effect of PTH on blood calcium levels is thus opposite to that of calcitonin . • 2. In the distal convoluted tubules of the renal cortex, PTH stimulates calcium reabsorption (and inhibits phosphate reabsorption in the proximal convoluted tubules ) • 3. PTH also indirectly increases the calcium absorption in the small intestines by stimulating vitamin D activation . AGE CHANGES IN PARATHYROID GLAND • Parathyroid glands begin to accumulate fat after puberty . • In older people the adipose tissue makes up to 50-60 % of the gland . • Absence of parathyroid glands is not compatible with life . CLINICAL CORELATE : • HYPOPARATHYROIDISM : can cause bones to become more mineralized & denser , skeletal muscle to exhibit abnormal contractions due to inadequate calcium concentration . • HYPERPARATHYROIDISM: stimulates osteoclasts number and activity leading to increased levels of blood calcium that can be deposited pathologically in cartilage , arteries or kidneys . ADRENAL GLANDS • The adrenal ( or suprarenal) glands are paired organs lying near superior pole of each kidney embedded in the pararenal adipose tissue and fascia . • Adrenal glands are covered by a dense connective tissue capsule that sends in septa into the gland's parenchyma . • The stroma consists of reticular fibers that support the secretory cells and microvasculature . ADRENAL GLANDS • ADRENAL CORTEX : • Cells of adrenal cortex have characteristic feature of steroid secreting cells : acidophilic cytoplasm rich in lipid droplets , with central nuclei . • TEM shows that cytoplasm of the cells of adrenal cortex contains abundance of smooth ER which contains enzymes for cholesterol synthesis and conversion of steroid prohormone pregnenolone into specific active steroid hormones . • These cells also have numerous mitochondria which not only synthesize ATP but also have enzymes for converting cholesterol to pregnenolone . ADRENAL GLANDS • ADRENAL CORTEX : • The adrenal cortex has 3 concentric zones in which the cords of epithelial steroid – producing cells are arranged in a specific manner and these cells synthesize different classes of steroid hormones . • 1. ZONA GLOMERULOSA : • Immediately inside the capsule and comprises about 15% of the cortex. This zone consists of closely packed , rounded or arched cords of columnar cells with many capillaries . The steroids synthesized by these cells are called mineralocorticoids because they affect uptake of sodium , potassium and water by cells of renal tubules . • The principal product is Aldosterone , the major regulator of salt balance . It stimulates sodium reabsorption in the distal convoluted tubule . • Aldosterone secretion is stimulated primarily by angiotensin II and by an increase in plasma potassium concentration , but only weakly by ACTH . ADRENAL GLANDS •ADRENAL CORTEX : 2. ZONA FASICULATA : is the middle zone and occupies about 65%-80% of the cortex and consists of long cords of large polyhedral cells , one or two cells thick separated by fenestrated sinusoidal capillaries The cells are filled with lipid droplet and appear vacuolated on routine histologic preparations. These cells secrete glucocorticoids especially cortisol, which affects carbohydrate metabolism by stimulating gluconeogenesis in many cells and glycogen synthesis by the liver Cortisol also suppresses many immune functions and can induce fat mobilization and muscle proteolysis . Secretion is controlled by ACTH , with negative feedback proportional to the concentration of circulating glucocorticoids. Small amounts of weak androgens are also produced here . ADRENAL GLANDS • ADRENAL CORTEX : • 3. ZONA RETICULARIS : is the inner most layer , comprises of about 10% of the cortex and consists of smaller cells in a network of irregular cords interspersed with wide capillaries .These stains deeply than those of the other layers because they contain fewer lipid droplets and more lipofuscin pigment. • Cells of zona reticularis also produce cortisol but primarily secrete weak androgens including dehrepiandrosterone ( DHEA) , that is converted to testosterone in both men and women . • Secretion of these cells is also stimulated by ACTH with regulatory feedback . ADRENAL GLANDS • ADRENAL CORTEX CLINICAL CORELATE : • Addison disease or adrenal cortical insufficiency is a disorder usually autoimmune in origin , which causes degeneration of any layer of adrenal cortex , with concomitant loss of glucocorticoids , mineralocorticoids or androgens . ADRENAL GLANDS • ADRENAL MEDULLA : • Is composed of large pale staining cells arranged in cords or clumps and surrounded by reticular fiber network. There is a profuse supply of sinusoidal capillaries in between these cells . Medullary parenchymal cells are called chromaffin cells, and they arise from neural crest cells . (as do postganglionic neurons of sympathetic and parasympathetic ganglia ) . • Chromaffin cells are considered modified sympathetic postganglionic neurons lacking axons and dendrites and specialized as secretory cells . ADRENAL GLANDS •ADRENAL MEDULLA : •Chromaffin cells contain many electron – dense granules for storage and secretion of catecholamines either epinephrine or nor epinephrine •About 80% of catecholamines secreted by adrenal medulla is epinephrine . •Medullary chromaffin cells are innervated by preganglionic sympathetic neurons which trigger epinephrine and norepinephrine release during stress and intense emotional reactions. ADRENAL GLANDS • ADRENAL MEDULLA : • Epinephrine : increases heart rate , dilates bronchioles & dilates arteries of cardiac and skeletal muscles . • Norepinephrine : constricts vessels of the digestive system and skin , increasing blood flow to the heart , muscles and brain . • Both hormones stimulate glycogen breakdown and elevating blood sugar levels. Thus, augmenting the capability for defensive reactions . • During normal activities, the adrenal medulla continuously secrets small quantities of these hormones . ADRENAL GLANDS • Adrenal gland blood supply : • The adrenal gland receives one of the richest blood supplies in the body . • The adrenal gland lack a hilum , so the blood vessels penetrate the capsule independently and branch immediately to form a subcapsular arterial plexus . ADRENAL GLANDS • Adrenal gland blood supply : • It receives blood from superior , middle and inferior suprarenal arteries which arise from 3 different sources . • From this plexus arterioles for the adrenal cortex and medulla emerge separately to form rich networks of fenestrated capillaries and sinusoids . • Cortical capillaries supply endocrine cells of the cortex , then drain into the microvasculature of adrenal medulla . ADRENAL GLANDS • Adrenal gland blood supply : • The medulla has a dual blood supply . • Arterial blood from the medullary arterioles and venous blood from the capillaries of the cortex . • Venous drainage from the gland occurs via the suprarenal veins .
