Fine structure of a second kind of thyroid follicle in the c3h mouse.

In addition to the usual functional follicles, thyroid glands of normal C 3H mice contain a second kind of follicle distinguished by heterogeneous, “foamy” colloid, by ciliated epithelial cells, and by previously unreported epithelial cells, termed AR cells, with distinctive abundant agranular reticulum. Flecks of immiscible substance of low density impart the foamy appearance to the colloid and often adhere to the surfaces of the epithelial cells. The ciliated cells possess a uniform array of apical cilia and a scattering of long, branched microvilli; their cytoplasm contains numerous mitochondria but is otherwise sparsely populated with agranular reticulum, nondilated granular reticulum, and free ribosomes. The AR cells, in addition to their prominent agranular reticulum, contain numerous large mitochondria and occasionally glycogen, presumed secretory globules, and nondilated granular reticulum. Approximately half of the epithelial cells in this second class of follicles are indistinguishable from typi...