TEM of the follicular epithelium shows pseudopodia and microvilli extending from the follicular cells (F) into the colloid of the lumen (L). The cells have apical junctional complexes, much RER, well-developed Golgi complexes, and many lysosomes. Inside the basement membrane (BM) of the follicle, but often not contacting the colloid in the lumen, are occasional C or parafollicular cells (C). To the left and right of the two C cells seen here are capillaries intimately associated with the follicular cells, but outside the basement membrane
A TEM of a C cell, with its large Golgi apparatus (G), extensive RER, and cytoplasm filled with small secretory granules containing calcitonin.
THYROID GLAND Located in the anterior neck region adjacent to the larynx and trachea Consists of two lateral lobes united by an isthmus Develops from the endodermal lining of the floor of the primitive pharynx Composed of connective tissue septa and follicles
THYROID FOLLICLE The lumen (L) of each follicle is surrounded by a simple epithelium in which the cell height ranges from squamous to low columnar. Also present are large pale-staining parafollicular or C cells (C).
FOLLICULAR CELLS Principal cells Most numerous cell type Cuboidal in shape with spherical nucleus Periods of increased activity – cells are more columnar and colloid is less abundant
FOLLICULAR CELLS Periods of decreased activity – cells are squamous and colloid content increases Exhibit organelles
active protein synthesis and secretion, phagocytosis and digestion
Synthesize, release, and store their product outside their cytoplasm
FOLLICULAR CELLS RED– Follicle with colloid BLUE – Follicular cells PINK - Connective tissue septa
PARAFOLLICULAR CELLS C (clear) cells Larger than follicular cells Contain secretory granules Can occur either alone or in clumps Synthesize and secrete calcitonin
PARAFOLLICULAR CELLS RED– Follicle with colloid BLUE – Parafollicular cells PINK – Retracted or distorted colloid
FOLLICULAR AND C CELLS C cells may be part of the follicular epithelium or present singly or in groups outside of follicles. Follicular cells (F) can usually be distinguished from C cells (C) by the smaller size and darker staining properties. Unlike follicular cells, C cells seldom vary in their size or pale staining characteristics.