Residual Cornea and the Degenerate Eye of the Cryptophthalmic Typhlotriton spelaeus

Abstract

The cave-dwelling Typhlotriton spelaeus larvas live in daylight. The larvas undergo a metamorphosis when the thyroxin level (T3 and T4) increases. They leave aquatic life for terrestrial and subterranean aphotic life. The larval eyes are normal and show good vision response to tests. On the contrary, in the oldest larva and the adult, the eyes are small with poor vision or no vision at all. The lens can disappear but in any case the eyelids grow over the eye. The retinal degeneration takes place during and after metamorphosis. The admitted statement that "after metamorphosis . . . the developing lids close over the eye and invade the cornea" is not confirmed concerning the invasion, because it is now evident that the eyelids separate spatially from the residual cornea. Consequently, the fibroblasts of the eyelids are not able to invade the corneal stroma. In fact, the periocular tissues do not invade the cornea but invade the eyelids' tissues. We are able to confirm that the development of the larval eyes is complete and apparently normal. After metamorphosis, the eyes decrease in size. The cornea "sinks" with the eye into orbit tissues. The eyelids cover the eye and are themselves replaced by a supraocular skin. This fits the description of the human cryptophthalmia. We also demonstrate that even in the degenerate eye the residual cornea retains the main structure of a normal salamander cornea. The preservation of the endothelium and of the Descemet membrane is exceptional in blind cave-dwelling vertebrates because in the other degenerate corneas the edema of the stroma depends on the spatial disjunction of the underepithelial stroma and the supraendothelial stroma. Perhaps, the very advanced development of the eye and the late start of the degenerative processes could explain the relative preservation of the cornea. Thus, the arguments put forward tend to prove that the thyroxinic metamorphosis sets off the growth and fusion of the eyelids but that the eye degeneration is related to hereditary processes.