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The Thyroid Cells

In addition to the stromal component, the thyroid gland is composed of three epithelial cell populations (the thyroid parenchyma) of different embryologic origin: (1) the follicular cells, the largest population, which surround the follicular lumen and are responsible for thyroid hormone synthesis; (2) the parafollicular C cells, devoted to calcitonin production; and (3) the epithelial cells, vestiges of the ultimobranchial body.

THYROID FOLLICULAR CELLS

When observed under the light microscope, thyroid follicular cells (TFCs) show a neutrophilic cytoplasm, a basal nucleus, and para-aminosalicylic acid (PAS)-positive vacuoles (phagosomes).[51] Follicular cells appear as cuboidal epithelial cells whose height is approximately 15 Ám. Cells become flatter (squamous) or higher (columnar) depending on whether or not they undergo TSH stimulation.

Electron microscopy (Fig. 72-2) reveals the characteristic features of cells actively engaged in protein synthesis.[52]The rough …