Relationship between local cell division and cell displacement during regeneration of embryonic Xenopus eye fragments

We examined the relationship between early healing modes and extra cell division (via tritiated thymidine incorporation) during embryonic retinal regeneration. Nasal (N) and dorsal (D) one-third sized eye fragments were surgically created in stage 32 Xenopus laevis embryos. Embryos were injected with tritiated thymidine two days postsurgery (stage 43), and then fixed and processed for autoradiography one day postinjection (stage 46). Histological analysis revealed that all nasal one-third sized fragments showed cell displacement in healing regions. These displaced cells were located in the ventral retinal region and showed heavy thymidine label incorporation. Alternatively, most dorsal one-third sized fragments showed little cell displacement during healing; in addition, no extra thymidine incorporation was evident. A minority of dorsal one-third sized fragments showed cell displacement during healing, and also showed significant local label ventrally through all regions of the eye. Therefore, in both dorsal and nasal one-third sized retinal fragments, when cell displacements were observed during early healing, associated mitosis was apparent in ventral retinal regions. Furthermore, by 60 hours postsurgery, all eyes which showed cell displacements during healing were greater in volume than those eyes which showed little cell displacement. Increases in volume appear to be derived from a combination of both cells migrating in from underlying optic stalk tissue and from related extra cell division during healing. These data further support a model which predicts that specific healing interactions which involve cell displacement during embryonic retinal regeneration and subsequent intercalary growth underlie visuotectal pattern formation.