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Electron microscopic studies on the thyroid gland of larval salamanders Hynobius nebulosus during metamorphosis.

The fine structure of the thyroid gland of Hynobius nebulosus larvae during metamorphosis was observed. The thyroid gland was most active at the stage of the beginning of gill resorption. According to both the size and distribution of the cisterns of granular endoplasmic reticulum and to the electron density of cytoplasmic matrix, the follicular epithelial cells were classified into an“ordinary type”having small-sized cisterns of the granular endoplasmic reticulum and electron lucent cytoplasmic matrix and an“unusual type”which was divided into four subtypes: honeycomb-like cells with dilated cisternae of granular endoplasmic reticulum, dark cells with flattened cisternae, dark cells filled with colloid particles, and altered and destroyed cells. The ordinary type cells have previously been described in the thyroid during metamorphosis, whereas the unusual type has been overlooked.It was suggested that the two types of dark cells correspond to Langendorff's colloid cells and that all the unusual type cells are derived from the ordinary type cells. The possible functional state of these unusual type cells was also discussed.