[Tissue of ultimobranchial origin in normal and pathologically altered thyroid gland].

Heterogeneity of the thyroid parenchyma and the sources of development of its main components are considered. The least studied are components originating from the ultimobranchial body to which "the second" system of the ultimobranchial follicles and solid epithelial cell nests belong. The data on the incidence, topography and age dynamics of these nests are presented. Their histochemical and immunocytochemical characteristics are given. Hypothesis on a possible role of solid cell nests as a source of various cell types in the thyroid as well as on their probable participation in the genesis of mixed thyroid tumours is discussed.