Postembryonic eye degeneration in the troglobitic salamander Typhlotriton spelaeus

Comparative histological observations of the eyes of Typhlotriton spelaeus and several epigean, plethodontid species indicate that the principal postembryonic degenerative changes in the eyes of T. spelaeus involve the eyelids and cornea, visual cells, outer plexiform layer, and the pigment epithelium. Ordinarily these changes were initiated after metamorphosis, before attainment of sexual maturity, but a few larvae had degenerating retinae. The corneal epithelium becomes irregular and thin as eyelids develop during and after metamorphosis, but retains its larval structure in animals in which eyelid overlap is incomplete. Disruption and vacuolation of the lens sometimes occurs in postmetamorphic animals with degenerating visual cells. Retinal degeneration involves reduction of the inner and outer segments of visual cells, loss of the outer plexiform layer, and retraction of apical processes of the pigment epithelium. In its earliest stage, retinal reduction is first apparent at the retinal margin where visual cells are normally less well-differentiated, but in its terminal stage reduction has gone to completion over the entire retina. Extent of retinal degeneration in adults is directly related to postmetamorphic age but there is variability in each age group. Females generally have smaller eyes, and more extensive degeneration of visual cells than males. The loss of visual function in adults is correlated with extensive visual cell degeneration.