Ontogeny of immunoreactive calcitonin gene-related peptide in thyroid C cells from dogs, rabbits, and guinea pigs.

Ontogeny of immunoreactive calcitonin gene-related peptide (CGRP) in thyroid C cells of dogs, rabbits, and guinea pigs from early fetuses to adults was investigated by an immunoperoxidase method, in comparison with the development of immunoreactive calcitonin and somatostatin. The presence of immunoreactive CGRP in mature C cells was different from species to species. Dog and rabbit C cells revealed intense immunoreactivity for CGRP, whereas guinea pig C cells revealed very weak immunoreactivity or none. In dog fetuses, the appearance of immunoreactive CGRP was early. At around 35 days of gestation, when the follicular cells were not yet organized into follicles, immunoreactivities for three peptides—calcitonin, somatostatin, and CGRP—began to appear in C cells. While the highest population of somatostatin-positive cells was attained when the primordial follicles were vigorously formed throughout whole thyroid parenchyma and their frequency progressively declined thereafter, CGRP-positive cells as well as calcitonin-positive cells gradually increased in number and intensity with gestational age. The developmental pattern of immunoreactive CGRP coincided with that of immunoreactive calcitonin in dog C cells. In rabbit fetuses, at 25 days of gestation, when thyroid follicles stored large amounts of colloid and C cells already exhibited intense immunoreactivity for calcitonin, CGRP immunoreactivity as well as somatostatin immunoreactivity began to appear. Subsequently, immunoreactivities for the three peptides gradually increased with age, although calcitonin immunoreactivity was outstandingly intense among them. In guinea pig C cells, intense immunoreactivity for CGRP was not observed in any stages or development. These results indicate that there are developmental profiles of CGRP characteristic for each animal, and the ratio of CGRP and calcitonin produced from calcitonin genes in C cells seems to be fixed for life.