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The thyroid looks like an exocrine gland that has lost its ducts.  The secretory cells form follicles, which are hollow chambers rather like acini, but much larger.  These follicles have no outlet, so that thyroglobulin secreted into the lumen accumulates inside.

Follicle contents, called "colloid," should appear as a uniform material filling each follicle.

Two artefacts are quite common in histological specimens of thyroid.  Clear bubbles may appear around the edge of the colloid; and parallel bands may appear across multiple follicles (these bands are called "chatter," created during tissue preparation by vibration of the microtome knife's cutting edge).

The principal follicular cells form a simple cuboidal epithelial lining for the follicles.  Under the influence of TSH from anterior pituitary, thyroglobulin is absorbed by these cells from the follicular lumen, transformed to thyroxin, and secreted basally.

The height of the follicular epithelium (whether low or tall cuboidal) and the size of the follicular lumen depend on the metabolic activity of the follicular cells.

Calcitonin-secreting parafollicular cells, or C cells, form a relatively small and scattered population, pale-staining with H&E, located along the basal surface of the follicular epithelium.  Calcitonin inhibits osteoclast activity in bone and thus reduces the blood calcium level.

Parafollicular cells are difficult to identify reliably on routine histological specimens.  For a nice image of C cells labelled with immunoperoxidase, see WebPath.

Unlike the follicular cells which are endodermal in origin, parafollicular cells derive from neural crest.

Parathyroid tissue may appear within specimens of thyroid; this tissue has a distinctly different texture of epithelial cords.

Comments and questions:

SIUC / School of Medicine / Anatomy / David King
Last updated:  14 July 2023 / dgk