THE EMBRYOLOGICAL DEVELOPMENT AND REGENERATION OF THE RETINA IN THE AXOLOTL, AMBYSTOMA MEXICANUM

Abstract

Development and regeneration of the axolotl (Ambystoma mexicanum) retina were compared using both light and electron microscopes. The embryonic development of the retina is basically similar to that described by others in amphibians, particularly urodeles, but the development of photoreceptor disks is somewhat different than that which occurs in tadpoles: the early outer segment disks in the axolotl embryo are very irregular and do not become organized until after many disks are formed. The first appearance of myeloid bodies was noted as the last event in the development of pigment epithelial cells. Regeneration of the retina following surgical neuralretinectomy was confirmed with autoradiography. Cellular differentiation in the regeneration of neural tissue is similar to that in the embryo, but the direction of tissue differentiation, from peripheral to center, is not only opposite to that of the embryo, but also opposite to that described in regeneration by others. The source of neuronal cells in regeneration as well as differences in surgical approach to the retina was discussed as possible reasons. The simultaneity of certain developmental events in the embryo, such as photoreceptor development and outer plexiform layer development, compared to the lack of coincidence of these events in the regenerate implies that there is no causal relationship between them. Apposition of pigment epithelium and neural retina appears to be a necessary condition for proper development of the photoreceptors, since where no such apposition exists in the regenerating retina, outer segments do not differentiate.