Ultrastructure of developing Xenopus retina before and after ganglion cell specification

The neuroretina of Xenopus laevis was examined at embryonic stages 28 and 32 to determine whether the specification of the ganglion cells is accompanied by morphological changes detectable with the light and electron microscopes. Two changes were noted: an apparent increase in the length of polysome helices, and an apparent reduction in the number of pigment granules. The significance of these changes is discussed. In addition to the morphological features common to undifferentiated cells generally, the cells of the neuroretina also exhibit subsurface cisternae, granulo‐filamentous inclusions, and membranous organelles resembling the “spine apparatus.” These structures may be useful in identifying presumptive nerve cells in the proliferative state.