An analysis of urodelian retinal regeneration: IV. Studies of the cellular source of retinal regeneration in Triturus cristatus carnifex using 3H‐thymidine

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Abstract
The process of retinal regeneration in adult Triturus cristatus carnifex was analyzed autoradiographically following tritiated thymidine administration at closely spaced postoperative intervals. In unoperated control animals given five daily doses of labeled thymidine, a continuous input of two types of cells was observed from the ora serrata into the ganglion cell and inner nuclear layers. One cell type appeared to represent retinal neurons with dispersed nuclear chromatin while the second cell type, containing a nucleus with dense heterochromatic rim, was displaced into all retinal layers with time, ultimately appearing within the layer of photoreceptor outer segments approximating the pigment epithelium. In operated animals, both control and operated (enucleated) eyes showed a loss of proliferative capacity from the ora serrata by two days postoperative.Retinal degeneration and regeneration in this species differed from that observed earlier for Notophthalmus viridescens in that: (1) invading pigment epithelia dissected the degenerating retina sequestering the outer nuclear layer between two epithelial sheets for phagocytosis by day 8; (2) degenerating inner retinal cells demonstrated labeling with thymidine and mitotic division before degeneration; (3) the process of removal of retinal and lens debris was assisted by vascularly derived phagocytes. The restoration of the retina involves both cellular addition from the anterior pars ciliaris retina and ora serrata, forming the anterior half of the new retina, and the posterior pigment epithelium, forming the posterior half of the retina from the innermost lamina. Sites of regional cellular proliferation at the vortex veins do not develop in this species. The basal pigment epithelium, upon Bruch's membrane, does not contribute to the retinal regenerate.The species differences presented in the process of retinal regeneration and the implications for the understanding of the generation of retinotectal specificity are discussed. It is concluded that the specificity engendered during both development and regeneration in this system must: (1) be derived from the ganglion cells; (2) be responsible for not only tectal connections but intra‐retinal position and connectivity; and, (3) not be temporally related.