ULTRASTRUCTURAL OBSERVATIONS ON THYROID GLAND OF XENOPUS LAEVIS DAUDIN THROUGHOUT METAMORPHOSIS

The fine structural changes in thyroid follicular epithelial cells of Xenopus laevis larvae throughout metamorphosis are described. The peak of thyroidal activity as indicated by follicular cell height, amount of endoplasmic reticulum, size of Golgi apparatus, numbers of mitochondria and cytosomes occurs not at the start of metamorphic climax, but at the stage at which tail resorption begins. In young toads and adults the follicular epithelial cells assume an appearance of inactivity. The ultrastructural features of Xenopus thyroid are compared with those of other vertebrate thyroid glands and the significance of the various organelles, especially with regard to the effects of thyrotropic stimulation, is discussed. © 1968.