[Thyroid pigments in Gunn rats].

The structural and histochemical characteristics of the thyroid pigment in homozygous Gunn rats were examined. The pigment occurs in the form of numerous yellow granules in the cytoplasm of the follicular cells. The focal depositions of the pigment were also seen in the central part of the luminal colloid. However, the pigment granules were not present in the light or parafollicular cells. The main histochemical properties of the pigment are that it is basophilic, PAS-positive, acid-fast and reducing toward alcaline silver nitrate and ferricyanide. Besides, it was easily bleached with oxidizing agents. It was deduced that the pigment was a lipofuscine. Ultrastructurally, pigment bodies are characterised by an electron dense content, and a smooth surface single limiting membrane. They resembled lysosomes or peroxisomes. It was concluded that the pigment granules visible at the light microscopic level resulted frim the accumulation of the pigment substance in the preexisting lysosomes.