Histogenesis of medullary carcinoma of the thyroid.

Thirty-one dog thyroid tumours and 28 spontaneous rat thyroid tumours were studied histologically and the findings compared with those of a study of 67 cases of medullary carcinoma of the human thyroid. Five of the dog tumours and 24 of the rat tumours were considered to belong to the same group of tumours as medullary carcinoma, a group characterized by solid sheets or lobules of uniform cells with granular cytoplasm and without papillary or follicular differentiation. In the rat tumours it was shown that the cell of origin was the parafollicular cell and not the thyroid follicle epithelial cell. It is suggested that medullary carcinoma is also derived from a parafollicular cell and that this origin would resolve the discrepancy between the relatively good prognosis and the apparently undifferentiated structure of this tumour. It is also concluded that the whole spectrum of clinical and pathological features of medullary carcinoma makes more sense if it is considered as a parafollicular cell tumour.