Ontogenesis of calcitonin mRNA in the rabbit.

Since plasma calcium levels are higher in the fetus than in the mother at the end of gestation, it has been suggested that calcitonin (CT) biosynthesis would be very active in the fetus. This hypothesis was tested in rabbit fetuses and newborns by measuring the amount of CT mRNAs found in the thyroid glands and the thyroidal CT stores. Dot-blot and Northern hybridizations with a specific CT cDNA probe (a BglII-NsiI fragment of the human CT cDNA) were used to determine the CT mRNA level. In fetuses, newborns, and mothers, only one molecular species of mRNA around 1 kb was detected by Northern hybridization with the specific CT cDNA probe. By dot-blot, CT mRNAs could be detected at 20 days of gestation on pooled fetal thyroid glands as a weak positive signal. The amount of CT mRNAs increased on day 24; at this stage they were also observed by Northern hybridization. During the last 6 days of gestation a 3-fold increase in CT mRNAs occurred in rabbit fetuses; concomitantly a 5-fold rise in the total thyroidal CT content was observed. Fetal plasma concentrations of both CT and calcium increased slightly between 24 and 30 days of gestation. After birth, the CT mRNA level was 10-fold increased between 2 and 30 days; these changes were not reflected in the plasma CT level but were probably accounted for by a rise in the number of C cells of the thyroid gland.