Calcitonin cell population and distribution in the thyroid gland of the rat.

Calcitonin-containing cells (C cells) were identified in male Wistar white rats using an immunoperoxidase technique. They occupied a central position within the thyroid; very few were found peripherally, inferiorly, and superiorly; and none were present in the isthmus. The number of calcitonin-containing cells present per gram of body weight increased with age up to 70 days and had declined by 100 days. Determining the true total C-cell count through the entire thyroid is a very laborious procedure. However, a simple estimate of this total count can be made; the total number of C cells in every tenth section (6 microns) of thyroid was found to be highly correlated with the weight of the animal expressed as an allometric function. A better estimate can be derived from counts of just three sections: the tenth, twentieth, and thirtieth after the section of greatest cross sectional area.