Ocular migration and the metamorphic and postmetamorphic maturation of the retinotectal system in Xenopus laevis: an autoradiographic and morphometric study.
The growth of the retina and tectum during larval life in Xenopus has previously been studied extensively. These two structures continue to grow in metamorphosing and postmetamorphic animals. During these later stages there are marked changes in eye position. We have used histogenetic and morphometric techniques to monitor retinal and tectal growth in Xenopus aged from stage 58 of larval life to ten years postmetamorphosis. The relative eye migration was quantified with optical techniques. An attempt has been made to relate modes of retinal growth to problems associated with changing interocular geometry. The predominant mode of growth during this period is hypertrophic, that is, it is due largely to an increase in size and complexity of existing elements, rather than the histogenetic creation of new elements. In the retina, however, tritiated thymidine autoradiography reveals that histogenesis does persist until at least six months after metamorphosis. The rate of histogenesis is much decreased compared with that of larval stages but, like that of larval stages, it takes place at the ciliary margin. It is asymmetrically distributed in the retina, most histogenesis occurring at the ventral retinal pole and very little at the dorsal retinal pole. Some histogenesis occurs at the nasal and temporal retinal poles. Thymidine autoradiography and cell counts indicated that little significant histogenesis occurs in the tectum after metamorphic climax. Two possible biological reasons for the asymmetry of retinal histogenesis have been examined. The first was the suggestion that the asymmetry was related to the emergence, during this period, of a binocular visual field secondary to the relative migration of the two eyes. It is shown that the asymmetric pattern of histogenesis is not closely related to those retinal areas that come to view binocular visual space. It was found, however, that the asymmetry of retinal growth from metamorphic climax was that required to stabilize retinal visual field positions with respect to body position. It is suggested that this minimizes the requirements for visuomotor readjustments that would otherwise be necessary to compensate for a situation in which eye position, relative to the body, is changing.
D000367:Age Factors; D000818:Animals; D001345:Autoradiography; D002452:Cell Count; D005123:Eye / Q000254:growth & development; D005728:Ganglia, Sympathetic / Q000033:anatomy & histology; D007814:Larva; D008675:Metamorphosis, Biological; D009900:Optic Nerve / Q000033:anatomy & histology; D012160:Retina / Q000033:anatomy & histology / Q000254:growth & development*; D003336:Tectum Mesencephali / Q000254:growth & development*; D013997:Time Factors; D014794:Visual Fields; D014982:Xenopus laevis