Prenatal development of the cat thyroid: immunohistochemical demonstration of calcitonin in the "C" cells.

The presence of calcitonin in the cat thyroid was studied immunohistochemically in a series of gland development. The first positive cells are to be found on the 38th day of gestation, i.e. 1-2 days after level nine of ontogenetic development has been reached. The cytoplasm of these cells form only a narrow border round the nucleus. With advancing development the number of calcitonin-positive cells rises, the cytoplasm becomes intensely positive and its amount increases. From approximately the 50th day of prenatal development, the initially diffusely scattered, solitary calcitonin-positive cells are gradually replaced by groups of cells, which begin to occupy a characteristic position in relation to the follicular epithelium. The largest quantity of calcitonin-positive cells is found in foetuses about to be born. In non-pregnant adult cats, the incidence of immunohistochemically calcitonin-reactive cells is more sporadic and their distribution in the lobes of the thyroid is uneven.