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Endocrine Thryoid Notes

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THE THYROID GLAND... Foetal growth
-Thyroid divertiulum-forms from an epithelial downgrowth in the midline of the floor of the mouth between the 1st and 3rd branchial arch components of the developing tongue. It grows uadally over the developing larynx to the anterior aspect of the trachea
-As it descends it associates with the superior/inferior parathyroids which develop from the 3rd/4th pharyngeal pouches and with neural crest cells which forms parafolliular C cells (calcitonin cells)
-Two lateral lobes and a central isthmus form-tissue along the line of descen usually disappears. Ectopic thyroid tissue can be found in the tongue along the line of descent and in the thorax. Anatomy of the thyroid gland
-The isthmus lies anterior to the 2nd-4th collagenous costal rings
-lateral lobes are found on either side of the trachea and larynx
-thyroid gland is enveloped by a fibrous capsule- fine collagenous septa extend into the gland dividing it into lobules
-pretrachial fascia anchors it to the airway- moves on swallowing- this makes it easy to examine
-rich blood supply- superior thyroid artery-branch of the external carotid artery and the inferior thyroid artery branch of the thyrocervical trunk, subclavian artery. It also a rich venous drainage- superior thyroid vein-internal jugular vein, inferior thyroid veins-brachiocephalic veins Rich blood supply ensures an efficient transport of hormones stimulating it and transport of hormones secreted by the thyroid gland
-functional units of the thyroid gland are the thyroid follicles (secrete thyroid hormones)spheroid structures made of a single layer of cuboidal epithelial cells bounded by a basement membrane
-follicle lumen contain colloid material- stained pink in preparation using H&E stain- colloid consists of iodinated thryoglobulin- large extracellular storage protects against nutrient deficiency
-C cells (parafollicular cells) release peptide hormone calcitonin- located at the base of the follicle epithelium. Synthesis of T3 and T4
-Follicular cells make T3 and T4 in response to TSH thyroid stimulating hormone from the anterior pituitary. TSH binds to GPCR on the basolateral surface of the follicular cells. This results in an increased cyclic AMP concentration which stimulates the Na/K ATPase pump which maintains a high extracellular sodium concentration
-This allows follicular cells concentrate iodide from the blood by a sodium/iodide symporter in the basal plasma membrane- the symporter traps and pumps iodide from the plasma.

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-Within the cell iodide is oxidised to iodine via a thyroperoxidase enzyme on the apical plasmalemma and transported to the follicular plasma membrane where it is released into the follicular lumen via the transporter Pendrin
-glycoprotein thryoglobulin is synthesised in the RER, glycosylated and packaged by the golgi apparatus-then released across the apical surface into the lumen by excocytosis
-thyroid follicles store thryoglobulin within the colloid (store more than 100x than daily requirement: within the follicular lumen, iodine combines with tyrosine residues of thryoglobulin to form T3, T4 : iodinated glycoprotein- storage form of T4, T3.
-When active thyroid hormone is required- TSH stimulates endocytosis of the colloid- the epithelial cells remove segment of colloid and detach T3 (tri-iodothryonine) + T4
*tetraiodothryonine. - Pinocytosis of the thyroglobulin-hormone complex to form cytoplasmic vacuoles- the vacuoles fuse with the lysosomes of the follicular cell cytoplasm and hydrolytic enzymes cleave the hormone from thryoglobulin-These pass through the cell into the capillary

Structure of Follicular cells related to function EM
-The follicular cells have large amounts of RER which is required for synthesis of thryoglobulin
-There are also large, dark prominent lysosomes
-the nuclei consist of dilated chromatin- active synthesis is occurring.
-There are large number of synthetic vesciles which are important in trafficking hormones.
-active thyroids- epithelial cells are tall and colloid is reduced in size. In inactive glands the cells are low cuboidal and the follicles are filled with colloid
-stores large amounts of hormone in an inactive form within extracellular compartments in the centre of follicles- other endocrine glands only store small amounts of hormones

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