Fine structure of a third epithelial component of the thyroid gland of the rat

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Abstract
Duct‐like structures made of cells different from follicular or light cells were observed with the electron microscope in the thyroid glands of 16 rats. The wall of most of the ducts was made of two types of cells: (1) the lumen‐bordering cells, mostly cuboidal or columnar, with short microvilli, and (2) the subjacent squamous cells, arranged in one or several layers of cells displaying numerous desmosomes and dense bundles of tonofilaments. Both cell types showed intricate folding of the plasma membrane at sites of cell‐to‐cell contact, an abundance of free ribosomes and nuclear indentations. In one occasion mucous‐like cells were found. The lumen contained a finely granular homogeneous material as well as cell remnants. The ducts were located among the follicles, and were ususally discrete from them; but on four occasions the epithelium of the duct was seen in direct continuity with that of a thyroid follicle. Only one case of a light cell associated with a duct was observed while many of these cells were associated with thyroid follicles.