Effect of calcium on embryonic rat thyroid C cells in vitro

The alterations in the ultrastructure of rat embryonic thyroid C cells in vitro produced by variation in the ambient calcium of the medium were studied. The thyroid C cells from 19-day-old rat fetuses were cultured for 48 hours in calcium concentrations of 2, 4, 6, 8 and 10 mg%. The C cells show a cyclic change in the ultrastructure. The resting cells have a sparse, dispersed granular endoplasmic reticulum, small Golgi apparatus and numerous secretory granules. The granules are emptied into the extracellular space followed by aggregation of the granular endoplasmic reticulum, enlargement of the Golgi apparatus, and reaccumulation of secretory products in granules. Varying the calcium concentration failed to change the proportion of cells in the various stages of the secretory cycle, though the mean number of secretory granules per unit area of cytoplasm fell proportionally with increasing ambient calcium concentration (y = -0.1675x +3.93). This decrease is largely due to a reduction in the number of cells with higher densities of secretory granules and corresponds to the direct increase in calcitonin content of the medium. This could indicate that secretion is directly stimulated in cells rich in secretory granules by a high calcium concentration. In contrast, these ultrastructural studies also indicate that, unlike the situation in the parathyroid chief cell, alteration of the ambient calcium concentration has little effect on the rate of synthesis of thyrocalcitonin by thyroid C cells.