Fine structure of the ultimobranchial cysts in the thyroid of the adult guinea pig.

The ultimobranchial cysts in the thyroid of the adult guinea pig are identified by a cytochemical method. The ultimobranchial tissue has the shape of an irregular cyst, with non-specific esterase activity that is resistent to HgCl2 inhibition. The TEM reveals five different types of epithelial cell in the cyst wall: 1. cells with deeply invaginated nuclei and of varying shape, from flat to cylindrical but most of them cuboidal. 2. Mucous cells that are tall and similar to goblet cells. 3. Tall cells with big granules that have a dense core. 4. Ciliated cells similar to those in the respiratory tract. 5. C-cells crowded with the small dense granules typical for the cell type. The cyst is enveloped by rather coarse collagen fibers and many wide sinusoids.