Electron microscopic observations on the contribution of the ultimobranchial bodies to thyroid histogenesis in the rat

The fine structure of ultimobranchial bodies in the rat has been studied from the fifteenth to the twentieth day of gestation. At 15 days, the ultimobranchial bodies were still separated from the median thyroid primordium by connective tissue. Their lumina were filled with cellular debris and their thick walls were made up of one type of cell in which rough‐surfaced endoplasmic reticulum was scanty. Early in the sixteenth day, cell cords from the median thyroid primordium came in direct contact with the ultimobranchial bodies; no basal lamina separated them. At 17 days the walls of the ultimobranchial bodies were composed of C cells, glycogen‐rich cells and cells that were poor in rough‐surfaced membranes and glycogen. At 18 days three types of cells were present in cell cords in direct contact with the ultimobranchial bodies: C cells, glycogen‐rich cells and follicular cells. The first two cell types had migrated from the ultimobranchial bodies while the follicular cells had originated from the median primordium. In the same cell cords, follicular cells containing glycogen granules were interpreted as being an intermediate stage between glycogen‐rich cells and true follicular cells. In follicular cells of the isthmus region, which presumably originate from the median primordium alone, glycogen granules were not observed after the fifteenth day of development. It is concluded that C cells and few follicular cells are of ultimobranchial origin.