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Endocrine System SummaryENDOCRINE GLANDSThe endocrine system can be divided into three parts:  The major endocrine organs – in which the sole or major function of the organ is the synthesis, storage and secretion of hormones.  Endocrine components within other solid organs – for example, the endoc...

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Endocrine System Summary ENDOCRINE GLANDS The endocrine system can be divided into three parts:  The major endocrine organs – in which the sole or major function of the organ is the synthesis, storage and secretion of hormones.  Endocrine components within other solid organs – for example, the endocrine components of the pancreas, ovary, testis and kidney, in the form of clusters of endocrine cells within other tissues.  The diffuse endocrine system – scattered individual hormone cells (or small clumps), usually within an extensive epithelium, e.g. the gastrointestinal and respiratory tract  The endocrine glands are abundantly and richly vascularized so that their secretory product may be released into slender connective tissue spaces between cells and the capillary beds from which they enter the bloodstream.  Endocrine glands secrete their products (hormones) into capillaries where they are carried away by blood.  Exocrine glands, by contrast (e.g. Sudoriferous, sebaceous, and digestive) secrete their products through ducts into body cavities or onto body surfaces.  The major glands that make up the endocrine system are the hypothalamus, pituitary, thyroid, parathyroid, adrenals, pineal body, and the reproductive glands, which include the ovaries and testes. The pancreas is also part of this hormone-secreting system, even though it is also associated with the digestive system because it also produces and secretes digestive enzymes. Although the endocrine glands are the body's main hormone producers, some non-endocrine organs - such as the brain, heart, lungs, kidneys, liver, thymus, skin, and placenta - also produce and release hormone.

This unit will discuss the microanatomy of the thyroid, parathyroid, adrenal and pancreas.

I.

Thyroid gland Anatomy  Two lobes and isthmus  Cervical region  Anterior, inferior to Larynx  Extremely vascular Capsule

  Lobules  Follicles    

Narrow layer of dense connective tissue Wide external layer of loose areolar tissue. Scanty fibrous septa o Lobules of follicles.

Round, oval, or irregular in outline, completely closed Simple cuboidal epithelium with a hollow centers May be enlarged with epithelium stretched thin. (hyperactivity of the gland) Follicular cells  Cuboidal ( depending upon activity)  Thin basement membrane  Surrounded by fenestrated capillary networks  Colloid (thyroglobulin)  Clear, acidophilic material (stored secretion) Parafollicular cells or C (clear) Cells  Small isolated clusters outside of follicles  Pale staining cells(Calcitonin)  PTH antagonist, blocks osteoclast activity ++  Promotes both Ca & PO4- loss from the kidney  Inhibits Vit D activation in the kidney Thyroid gland functions (T 3 & T4)  Metabolic rate of all cells (body heat production, Catabolism)  Increases Glucose Utilization & carbohydrate absorption from the gut  Maintains growth hormone secretion  Affects CNS development, RBC production, muscle tone  Maintains cardiac rate, force, output  Disorders - Hypothyroidism & Hyperthyroidism, Goiters

Thyroid (TG and F – follicle) Parathyroid Gland (PG)

Parathyroid Gland  The parathyroid glands are four or more small glands, about the size of a grain of rice, located on the posterior surface (back side) of the thyroid gland. The parathyroid glands usually weigh between 25 mg and 40 mg.  Separated from thyroid by a separate membrane  Under the microscope the parathyroid glands are quite easily recognizable from the thyroid as they have densely packed cells, in contrast with the follicle structure of the thyroid. However, at surgery, they are harder to differentiate from the thyroid or fat.  They are essential for life Histology   

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Each parathyroid gland is surrounded by a thin connective tissue capsule. The thin fibrous capsule of the parathyroid gland gives rise to delicate thin septa which divide the parenchyma into nodules of secretory cells. The septa carry blood vessels, lymphatics and nerves. Parenchymal cells are arranged in anastomosing chords surrounded by delicate connective tissue septa. Capillaries are abundant. A considerable number of fat cells infiltrate the gland (beginning around puberty) and may account for about half the weight of the parathyroid glands in adults. Two cell types can be distinguished in the parathyroid glands:  Chief cells (also called principle cells) are the most numerous type. They are rather small, a round, light and centrally placed nucleus and a very weakly acidophilic (pale staining) cytoplasm. They synthesise parathyroid hormone (PTH) which is of pivotal importance for normal calcium concentrations in the fluids and tissues of the body. The effect is mediated by a stimulation of osteoclastic bone resorption, intestinal calcium uptake and calcium resorption in the kidneys. If the parathyroid glands are removed

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completely, calcium concentrations decrease rapidly, leading to tetany within 2-3 days and eventually death if left untreated. Oxyphilic cells are less frequently seen. Their cytoplasm is strongly acidophilic, the nucleus is small and uniformly intense basophilic. They contain large amounts of mitochondria. Their function is unknown by some believe they are the inactive chief cells  Oxyphilic cells are larger than chief cells Also present  Adipose Cells, increase with age (older 50% of all cells)

Parathyroid hormone o maintains blood level of calcium o absorbtion of calcium from intestines o mobilizes calcium from bones (osteoclast) o decreases renal reabsorbtiom of phosphate o activates Vit D3 in kidney

Adrenal Glands  Two triangular glands lying on top of each kidneys  Surrounded by a capsule of dense fibrous connective tissue  The connective tissue capsule extends septae into the substance of the gland.  The organ is richly vascularized and capsular blood vessels, nerves and lymphatics penetrate along with the connective tissue septae.

Paired adrenals on top of kidneys

The most distinctive feature of the adrenal is its partitioning into cortex and medulla. The medulla is fairly homogeneous, but even when viewed a low power, three concentric zones can be distinguished in the cortex: zona glomerulosa - thin, outermost zone, zona fasiculata - thick, middle zone and zona reticularis - thin, inner zone

Cortex produces corticosteroids that influence or regular salt and water balance that influences body’s response to stress, and affects metabolism, immune system , and sexual development and function. The Medulla – produces catecholamines such as epinephrine (increases blood pressure and heart rate when the body experiences stress). Histology of adrenals Cortex -Cord-like arrangement of 3 concentric layers of cells 1. Zona glomerulosa (outer cortex) a. Narrow layer just under the capsule b. Cells within this zone tend to be columnar in shape and are arranged in irregular cords 2. Zona fasiculata (middle) a. Thickest layer b. Straight columns, 2 cells thick, separated by septa which contain capillaries. c. Cells in the fasiculata are polyhedral and usually have a foamy appearance due to abundant lipid droplets. They also are arranged in distinctively straight cords that radiate toward the medulla 3. Zona reticularis a. Cells smaller than in other layers b. Irregular network of cords of cells rich in capillaries. c. Cells similar, smaller, than those in the other zones, differ ultrastructurally

d.

4.

Some irregular shaped cells with pyknotic nuclei (degenerating cells!)

Hormones of the Adrenal Cortex 1. Glucocorticoids - Increases blood glucose a. Stimulates Gluconeogenesis (use amino acids), glycogenolysis b. Stimulates hepatic enzymes synthesis for Gluconeogenesis c. Stimulates protein catabolism d. Increases circulation of aminio acids, fatty acids, ketone bodies, glucose e. Lipolysis & Ketogenesis f. Anti inflammatory 2. Mineralcorticoids (Adrenal Cortex) - Aldosterone 3. Sex Hormones(Adrenal Cortex) - Estrogens, Progesterones, Androgens, Testosterone

Medulla 1.

Histology a. Medulla irregular arrangement of masses and network of cords of cells. b. Between and around Blood sinusoids and venules c. few Sympathetic gangilon cells d. "chromaffin cells" stain for Catecholamines

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Stores secretion in granules, Continuously secretes small amounts Large quantities in response to stress Hormones of the Adrenal Medulla

Pancreas – endocrine function o

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Endocrine glandular tissue. o Islets of Langerhans (clusters of 5 to 50 cells), randomly scattered o Surrounded by Reticular fibers o cords of polygonal cells surrounded by capillaries o Stain lighter than acinar cells o 3 types of cells  The islet cells are indistinguishable from each other in sections, but in fact three secretory cells types are present: 1. alpha - secrete glucagon, 2. beta - secrete insulin 3,. delta secrete somatostatin The islets are supplied by up to three arterioles, which form a branching network of fenestrated capillaries, into which the hormones are secreted. The islet is drained by about six venules, which pass between the exocrine acini to the interlobular veins. o Insulin (Pancreas, Islets of Langerhans, Beta cells) o Promotes cellular uptake of glucose and synthesis of glycogen o Promotes cellular uptake of amino acids & protein synthesis o Stimulates fat storage, Promotes lipogenesis o Stimulates lipoprotein lipase activity (Depressed in Obesity)  Glucagon (Pancreas, Ilets of Longerhans, Alpha cells) o Stimulates Glucogenolysis o Stimulates Gluconeogenesis (use amino acids)  Pancreatic (Secretion)Enzymes o Pancreatic Secretion #1 Increase of pH Stimulated by hormone, CCK (cholecystokinen) o Pancreatic Secretion #2 Enzymatic Stimulated by hormone, Secretin  Trypsin and Chymotrypsin secreted as Trypsinogen and Chymotrypsinogen (Inactive)  Trypsin activated by Enterokinase from intestinal Glands  Chymotrypsin activated by Trypsin

Islet of Langerhans...