[An immunohistochemical study of the primordium and primary differentiation of the parafollicular cells in the human thyroid].

Formation and differentiation of calcitonin-secreting cells has been studied in human embryos. The cells form from the entodermal part of the rudiment of the last pharyngeal cavity. They do not secrete calcitonin before incorporation into the thyroid gland. The first calcitonin-secreting cells appear in the thyroid gland as a diffuse net. The properties and origin of this net have been described.