Cytological changes induced by low temperature in the thyroid glands of larvae of the salamander Hynobius retardatus.

In order to study the environmental conditions and the endocrine mechanism which induce neotenous characteristics, the fine structure of the thyroid glands of larvae of the salamander Hynobius retardatus were examined. The thyroid glands of larvae exposed to a low temperature (4 degrees) were compared with those of larvae exposed to a higher temperature (20 degrees). At 4 degrees, the follicular lumen was large and its contents of low electron density; the epithelial cells were flat or cuboidal. In contrast, the 20 degrees larvae showed a small dense lumen and tall cylindrical epithelial cells, which may indicate a higher level of activity. No apical vesicles were found in the epithelial cells of the 4 degrees larvae, although they were present in 20 degrees larvae. On the other hand, in 4 degrees larvae the chromatin in the nuclei was scattered, while in 20 degrees larvae it was condensed. In addition, large dense granules occupied 2.4% of the epithelial cytoplasm in 4 degrees larvae and as much as 7.3% in 20 degrees larvae. The structural features observed in larvae exposed to the cold resembled those reported for thyroid glands made inactive by a lack of thyrotropin.