Development and differentiation of lateral thyroid.

Development and differentiation of lateral thyroid were investigated histologically upon the serial sections of the rat embryos. On the 11th day of gestation, the "third pharyngobranchial duct" (3. PBD) develops from the third pouch and elongates rapidly downward. The "fourth pharyngobranchial duct" (4. PBD) develops transversally from the 4th pouch on the same day. Median thyroid forms the diverticulum. On the 13th day, median thyroid elongated perpendicularly. On the 14th day, the short 4. PBD is separated from the pharyngeal pouch and approximates the dorso-medial surfaces of the median thyroid. The primordial parathyroid III is set free from the 3. PBD. On the 15th day, the peripheral parts of. PBD develop into the numerous cords provided with the characteristic features of primordial thyroid follicles. In this manner, the 4. PBD constitutes the lateral thyroid. On the 15th day, in the central region of the primordial median thyroid, mitotic division of nuclei is frequently observed. This area designated as "proliferating area" comes in contact, on the 16th day, with the thickened wall of 4. PBD. On the 17th day, however, the proliferating area becomes obscure with the promoted peripheral outgrowth of numerous cords. Thus, it has been clearly demonstrated that a large number of thyroid follicles are derived from the lateral thyroid of 4. PBD origin. The primordium of parathyroid IV can be seen transiently from 4. PBD on the 17th day, but becomes soon undetectable.