The Endocrine System Part II

The Endocrine System Part II

Thyroid gland Parathyroid glands Regulation of blood Calcium level Adrenal gland Exocrine part of pancreas (Islets of Langerhans)

Thyroid Gland Located in the anterior neck region adjacent to the larynx and trachea. Two lobes connected by isthmus Connected tissue capsule that also forms septa inside the gland

Thyroid gland consist of two types of cells: - Follicular cells - Parafollicular Cells (C cells)

The thyroid follicle is the structural and functional unit of the thyroid gland.

Follicular Cells Make the wall of thyroid follicles Produce hormones: - Triiodothyronine (T3) - Tetraiodothyronine or Tyroxine (T4) Hormones are stored in the form of thyroglobulin in the colloid inside the follicle TSH stimulate the release of T3 and T4 into the bloodstream Can be flat, cuboidal or cylindrical depending on functional activity

Thyroid hormones synthesis

Thyroid Hormones Triiodothyronine (T3) more active, but less abundant, short half-life (18 hours) Tyroxine (T4) less active, represent 90% of secreted thyroid hormones, the period of halflife is 5-7days Stimulate the secretion Thyroid-stimulating hormone (TSH) produced in adenohypophysis Inhibit the secretion high levels of T3 and T4 (negative feed back)

Thyrotropin releasing hormone (TRH) of hypothalamus stimulates basophilic cells of adenohypophysis to secrete Thyroid-stimulating hormone (TSH) which signals the thyroid to produce Thyroid hormones

Effects of thyroid hormones: Body growth stimulation Brain development Increase heart rate Stimulation of metabolism Protein synthesis Synthesis and mobilization of fats Glucose storage

Grave s disease (hyperthyroidism)

HYPOTHYROIDISM Myxedema (adults) Cretinism (kids) Mental retardation if not treated

Located among follicular cells or make clusters between follicles Poorly stained Secrete Calcitonin in response to high blood calcium Takes part in calcium homeostasis Parafollicular cells (C Cells)

High mag of thyroid follicles note Parafollicular or C-Cells (arrows)

Parathyroid glands 4 small glands located on the posterior surface of the thyroid gland 2 major cell types: - chief cells - oxyphil cells

Numerous, small cells with prominent nuclei and pale cytoplasm Chief Cells Produce Parathyroid Hormone (PTH) in response to low calcium blood level

Oxyphil cells Less numerous, appear in groups Cytoplasm is brightly stained Function unknown (possibly inactive chief cells)

Calcium Homeostasis

Adrenal Gland Located at the upper pole of each kidney (suprarenal gland) Consist of: - Cortex (mesoderm origin) - Medulla (neural crest)

The Adrenal Cortex The cortex is the steroid-secreting portion. Lies beneath the capsule and constitutes nearly 90% of the gland by weight. Divided into three zones: - Zona glomerulosa - Zona fasciculata - Zona reticularis

Zona glomerulosa Outermost cortical layer Columnar epithelial cells with scant cytoplasm appear as ovoid clumps on cross-sections The cells of the zona glomerulosa secrete mineralocorticoids The main hormone is aldosterone Aldosterone stimulates reabsorption of Na and water in the distal tubules of the nephron in the kidney The renin angiotensin aldosterone system provides feedback control of the zona glomerulosa.

Juxtaglomerular Apparatus Regulates blood pressure by activating the renin angiotensin aldosterone system Located near the vascular pole of each renal corpuscle Includes: - macula densa - extraglomerular mesangial cells - juxtaglomerular cells

Renin Angiotensin Aldosterone System

Zona fasciculata Brodest, lightest-staining zone Cells with large poorly stained cytoplasm are arranged into vertical columns Cells of the zona fasciculata secrete glucocorticoids The main hormone secreted is cortisol Zona fasciculata is under the control of pituitary ACTH

Adrenocorticotropic hormone Adrenocorticotropic hormone stimulates the adrenal cortex to secrete corticosteroids (long term stress).

Glucocorticoids Increase gluconeogenesis in the liver (production of glucose from non-sugar sources, such as amino acids) Increase the rate of fat tissue break down into fatty acids immediate sources of energy Depress the immune and inflammatory responses and, as a result of the latter, inhibit wound healing

Hypersecretion of glucocorticoids Cushing s syndrome

Hyposecretion of glucocorticoids

Zona reticularis Cells are arranged in irregular, anastomosing cords and clumps Secretory cells are small with darkly stained cytoplasm Cells of the zona reticularis secrete small amounts of adrenal androgens (less masculinizing and anabolic effect than testosterone)

Adrenal Medulla Derived from the neural crest Two types of cells: - Chromaffin cells - large, palestaining epithelial cells - Ganglion cells - morphological characteristics of autonomic ganglion cells (nerve cells) Produce catecholamins in response to sympathetic stimulation (stress): - epinephrin - norepinephrin

Catecholamins Fight or Flight Epinephrin Increased blood flow to the heart Increased heart rate Increased blood flow to the skeletal muscles Dilated bronchioles Norepinephrin Constricted blood vessels in nonessential organs (skin, GI tract) Increased blood flow to the heart, brain, skeletal muscle

Blood Supply of the Adrenal gland

Pancreas The pancreas contains both endocrine and exocrine cells. The exocrine cells are acinar cells that secrete digestive enzymes. The endocrine cells are located in the Pancreatic islets (or the Islets of Langerhans).

Low magnification of the Pancreas with Islets of Langerhans (arrows)

Cells of the Islets B-cells - insulin A-cells - glucagon D-cells -somatostatin F-cells (PP-cells) - pancreatic polypeptide

Insulin Released in response to high blood glucose Increases glucose uptake by all cells Increases storage of glucose in the liver (glycogen) Increases triglyceride synthesis by adipocyte

Glucagon In response to low glucose level in blood Glucogenolysis release of glucose from glycogen Gluconeogenesis synthesis of glucose from fat and protein - break down of fat - break down of protein

Immunohistochemistry of a pancrearic Islet of Langerhans Glucagon Insulin

Other hormones of Islets of Langerhans Somatostatin Decreased secretion of growth hormone in the pituitary gland Suppression of insulin and glucagon synthesis in the Islets Pancreatic Polypeptide Inhibits exocrine function of pancreas (enzymes and bicarbonate)

Diabetes Hyperglycemia increased blood glucose levels Polyuria increased frequency of micturition and urine volume Polydipsia sensation of thirst and increased liquid uptake

(insulin-dependent)

(non-insulin dependent)

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