Mitotic activity of the endocrine cells in rat thyroid glands during postnatal life.

This paper presents the results of investigations into the mitotic rates of thyroid endocrine cells in normal postnatal rats, aged 1-120 days. Our study revealed considerable age-dependent shifts in the mean mitotic activity of follicular cells and C-cells. The maximum indices of endocrine cell renewal were reached during the first 10 days of life, decreasing gradually and significantly until 25 days. At 1 month, there was a significant recuperation of the division rate for both cell types, which declined in the adult rat. These results mean that the proliferation of C-cells and follicular cells is inversely proportional to age. The preferential zone of localization of mitoses of both cell types is the central region of the thyroid lobe. The present paper provides new evidence for the postnatal origin of C-cells and follicular cells from the preexisting endocrine cells.