The Significance and Fate of the Ultimobranchial Body in Man in Relation to the Development of the Thyroid Gland

Abstract

The present investigations on the ultimobranchial body of the human being elucidated the following:
The ultimobranchial body appears as a part of the so-called caudal pharyngeal complex, dorsal to the thyroid lobe, and separated from the primitive pharynx in the 12 mm CRL stage. From the 18 mm CRL stage, it comes to be completely or incompletely incorporated within the thyroid lobe. It soon becomes an independent body with a slit-like cavity and suggests a forerunner of the cysts.
The epithelial cells surrounding the cavity are stratified in earlier stages and later become partly a kind of reticular tissue with clear cytoplasm. In the reticular tissue a few vesicles are formed. Glycogen is found in the epithelial cells throughout fetal life. Mitotic and degenerative figures are often found in them.
The ultimobranchial body does not contribute to the formation of thyroid tissue, but rather remains relatively often as cysts and yesicles within the thyroid lobe even in late fetal life. The cysts are composed of stratified epithelium and partly single-or double-layered epithelium. These epithelia contain often ciliated cells.
The cysts contain faintly eosinophilic granular substance, unlike the thyroid follicles. Cell detrituses are often contained in them.
A thymic tissue islet appears rarely as a part of the parathyro (IV)-thymo-ultimobranchial complex in the dorsal zone of the thyroid lobe. From this fact, it is supposed that the thythic tissue islet found within the thyroid lobe is of the fourth pharyngeal pouch origin.