Distribution of calcitonin-containing cells in the normal neonatal human thyroid gland: a correlation of morphology with peptide content.

C-cells have been mapped in the thyroid glands of 6 human neonates by means of immunoperoxidase localization of calcitonin and tissue calcitonin content as measured by radioimmunoassay. The C-cells were concentrated in a zone in the upper two-thirds of the lateral lobes bilaterally, where they were identified individually and in small groups in both an intrafollicular and parafollicular distribution. In contrast to findings in the adult, C-cells were predominantly intrafollicular in the neonate. The relative numbers of C-cells counted per unit area of thyroid tissue correlated strongly with the calcitonin content of immediately adjacent tissue sections. In areas rich in C-cells, as many as 75 immunoperoxidase-stained cells per low-power field were counted, and the concentration of calcitonin was as high as 540 to 2100 mU/g fresh weight, values that were as great as 10 times those observed in the normal adult thyroid gland. The prominence of the C-cell population and increased tissue calcitonin content in the human neonatal thyroid gland may reflect an as yet undefined physiologic role for calcitonin in the newborn.