Functional Morphology of the Thyroid

Abstract

Publisher Summary This chapter discusses the characteristic ultrastructural and cytochemical functions of the follicular epithelial cell. The relationship between the uniqueness of the secreted hormones and that of the functional morphology of the thyroid gland and the follicular epithelial cell is also discussed. Thyroxine (tetraiodothyronine, T4) and triiodothyronine (T3), secreted from the thyroid follicular epithelial cell, are characteristic hormones because they are special amino acid derivatives (diphenyl ethers) derived from only two molecules of iodotyrosine linked by an ether bond. The follicular epithelial cell morphologically shows typical exocrine gland cell polarity, synthesizes high-molecular-weight glycoprotein (thyroglobulin) by the rough endoplasmic reticula (RER) and Golgi apparatus system, and releases secretory granules containing thyroglobulin into the follicular lumen in exocrine cell manner. Luminal thyroglobulin is usually reabsorbed into the cell by pinocytosis and in stimulated conditions, by phagocytosis as well as pinocytosis. The thyroid is innervated by three kinds of nerve fibers, sympathetic, cholinergic, and peptidergic, although only a few fibers actually enter the organ. The main route for stimulating the follicular epithelial cell to secrete T4 and T3 is humoral, through the thyrotropin-releasing hormone (TRH)- thyroid-stimulating hormone (TSH) system, rather than neuronal.