Ontogeny and ultrastructure of somatostatin and calcitonin cells in the thyroid gland of the rat.

Calcitonin cells are relatively numerous in the thyroid gland of the rat. In contrast, somatostatin cells are very scarce except at the time of birth and a few days thereafter, when they are conspicuously numerous. Somatostatin cells of the thyroid gland, which are ultrastructurally similar to somatostatin cells in gut and pancreas, also contain immunoreactive calcitonin. It is not clear whether somatostatin cells in the rat thyroid gland produce calcitonin or accumulate calcitonin from the environment.