THE “LIGHT CELLS” OF THE THYROID GLAND IN THE RAT1

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Abstract
Shortly after an injection of thyrotrophic hormone, all follicular cells of the thyroid gland of the rat contain numerous intracytoplasmic colloid droplets, whereas a few cells—referred to as light cells—do not contain any. In the young rat, this brief thyrotrophic hormone treatment is essential for the identification and counting of light cells. In the old rat, however, the presence of many colloid droplets and densely stained material in its follicular cells makes the use of the hormone superfluous. AVith most staining techniques, the light cells show a large, pale nucleus and a faintly stained cytoplasm. They are all located within follicles, but are not in contact with the colloid. They lie singly or in small groups between the follicular cells and the basement membrane, or among the follicular cells, but even in this case they are separated from the colloid by a layer of follicular cell cytoplasm. The light cells arc distributed in the two thyroid lobes, chiefly in the follicles of the central region, but arc absent from the isthmus. Their number is small, one or a few per cent of the total population of epithelial cells. This proportion does not change with age, so that the populations of follicular and light cells must increase at the same rate throughout life. Since light cells are within follicles, they are considered to be epithelial in nature and, therefore, are neither ganglionic cells, parathyroid cells nor embryonic cell rests, as assumed by some authors. The light cell may be looked upon as a cell species in its own right, although it is possible that it originates from the follicular cell.