Immunocytochemical studies on thyroid parafollicular cells in postnatal development of the rat.

Studies were performed on Wistar strain rats aged 1-720 days. Immunocytochemical reactions were used to detect calcitonin, somatostatin, calcitonin gene-related peptide (CGRP), cholecystokinin, serotonin, neuron-specific enolase (NSE), secretory protein-I, chromogranin and Ca-binding protein. In the parafollicular cells of the rat, the presence of calcitonin, somatostatin, CGRP, NSE and secretory protein-I could be demonstrated. The number of parafollicular cells increased with the age of animals, and the increase was particularly pronounced in the early postnatal period and after the first year of age. The number of somatostatin-immunoreactive cells decreased after birth and increased again after the first year of age. The number of calcitonin-immunoreactive cells increased in the early postnatal period independently of the increase in parafollicular cell number, forming frequently tumor-like outgrowths in 2-year-old animals. A small proportion of these outgrowths contained no calcitonin even if they did contain somatostatin, CGRP and NSE immunoreactivity. Evident changes in immunoreactivity in the first days after birth may reflect the sudden change in environment and may be associated with growth and differentiation. In any period of life, CGRP- and NSE-immunoreactive cells have constituted the most numerous groups and, therefore, the respective antigens seem to represent the most suitable markers of parafollicular cells in the rat.