The Light Cell as Compared to the Follicular Cell in the Thyroid Gland of the Rat

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Abstract
Electron microscopic examination of the rat thyroid gland conclusively confirmed that, in addition to follicular cells, a second epithelial cell type is present. Because of the characteristically pale appearance of these cells in electron micrographs, the name “light cell,” proposed earlier, was found suitable. Light cells are always contained within the basement membrane of follicles, but do not make contact with the luminal colloid. They have no terminal bars and do not exhibit definite polarity. The cisternae of the ergastoplasm as well as the Golgi sacs usually show a virtually closed lumen, a feature indicating low activity. However, a few light cells show a moderate dilation of the lumina, indicative of moderate activity. A characteristic feature is the presence of empty looking, light vesicles bounded by a membrane which may be irregular and even disintegrate; these vesicles are seen around the Golgi zone and throughout the cytoplasm, chiefly in those cells considered to have low activity. On the other hand, vesicles with a fairly dense, homogeneous content, similar to those present in follicular cells, have been observed in some light cells, chiefly in those with moderate activity. Two types of granules are seen in light cells: 1) very dense, zymogen-like granules which are similar to those seen in follicular cells and may show signs of degeneration; 2) vacuole-enclosed dark granules, which are not seen in follicular cells. Thus, light cells may have vesicles and zymogen-like granules similar to those of follicular cells. Most of the differences between the 2 cell types can be related to the rather low activity of light cells. Hence, it is possible that light cells are derived from follicular cells which lose contact with the luminal colloid; the separation from the colloid would hinder release of secretory products and thus lead to a decline in activity.