C cell activity during the prenatal and postnatal periods in the rat

The high plasma calcitonin (CT) level found in suckling newborns and baby rats led us to hypothesize that thyroid C cells might be exhausted during the postnatal period. This prompted us to evaluate the CT concentration of the thyroid C cells during the prenatal and postnatal periods in the rat as the CT content to number ratio of C cells. The CT content of the thyroid gland increased exponentially from 17.5 (0.03 ng) to 21.5 (1.72 ng) days of gestation in the rat fetus; C cells were detected by immunofluorescence and counted from 19.5 days of gestation until term. A value of 600 + 90 C cells was obtained at 19.5 days, 1557 +/- 239 at 20.5 days, and 2602 +/- 536 at 21.5 days of gestation. Plasma CT concentrations were undetectable (less than 150 pg/ml) before 20.5 days of gestation, but increased to approximately 500 microgram/ml in 20.5- and 21.5-day-old fetuses. After birth, both the thyroid CT content and the number of C cells increased progressively. In 3-day-old suckling newborns, 4298 +/- 412 C cells were found; 9679 +/- 1114 were found 7 days after birth, and 12202 +/- 1280 were observed 15 days after birth; at the same stages, the CT contents of the thyroid gland were 6.54 +/- 0.18, 8.59 +/- 0.19, and 19.49 +/- 0.79 ng, respectively. Thus, the CT concentrations of the C cells (approximately 1.0 pg/cell) were roughly similar during the prenatal and postnatal periods in the rat. These results suggest the presence of active C cells during fetal life in the rat. They also indicate a capacity of C cells during the prenatal and postnatal periods to increase their secretion of CT while maintaining their hormone content, since the CT concentration of the C cell remains unaltered in spite of the high CT secretion in suckling rats.