Electron microscopic studies of the developing amphibian thymus.

Electron-microscopic studies of larval Xenopus laevis thymus at three different stages of development are presented. Studies of thymus from 5 and 8 day old larvae (ages when early thymectomy is performed) reveal a relatively undifferentiated rudiment composed of epithelial cells and lymphoid cell precursors. These findings are in marked contrast to ultrastructural observations of 30 day old larval thymus, where there exists a well defined corticomedullary differentiation with an abundance of small lymphocytes in the cortical region.