THYROID GLAND

Slide #195 - The bi-lobed thyroid gland is an epithelial organ derived from pharyngeal pouch endoderm.   At low power, the most striking histological features of the thyroid gland are the thousands of thyroid follicles filled with varying amounts of an acidophilic substance called colloid .   Colloid is largely composed of thyroglobulin secreted by thyroid epithelial cells.   Observe differences in the height of the epithelium in various follicles.   Some are cuboidal; some squamous-like.   What is the functional signicance of this appearance?   Clear areas or gaps can be seen in the colloid of some follicles, indicating reuptake of thyroglobulin by the epithelial cells.   Be sure to review the steps involved in the production and release of thyroid hormone, and relate the process to the anatomical features you observe.   Processing and release of thyroid hormone is controlled by TSH, which binds to the follicular cells.

Each follicle is surrounded by connective tissue carrying a rich vascular supply of fenestrated capillaries (with diaphragms).   Another type of epithelial cell is located at the periphery of the thyroid follicles, within the connective tissue.   These are the parafollicular cells or A C @ (clear) cells .   Try to find them on your slide.   Look for cells with dark, round nuclei surrounded by a relatively clear cytoplasm.   Parafollicular cells do not actually contact the follicular lumen or colloid.   These cells produce calcitonin, a hormone that inhibits bone resorption by osteoclasts - thus lowering blood calcium levels.