Organization and regeneration ability of spontaneous supernumerary eyes in planarians - Eye regeneration field and pathway selection by optic nerves

Planarians can propagate asexually by fission and successive regeneration. During head regeneration, they again form a new pair of eyes, and sometimes supernumerary eyes. The positions of normal and supernumerary eyes and their regeneration abilities are expected to be highly relevant to the question of where and how the field to regenerate eyes is determined. In this study, spontaneously generated supernumerary eyes were classified into various types. In all cases, they were formed in the anterior part of the head. Enucleation of a normal eye elicited regeneration of a new eye; however, enucleation of a supernumerary eye did not. The supernumerary eyes were morphologically and functionally indistinguishable from the normal eyes, revealed by the studies of immunohistology and photophobic response, respectively. From the obtained results, we proposed a model of the eye regeneration field that changes its distribution spatiotemporally during regeneration. Immunohistological studies also showed that the optic nerves from the normal and supernumerary eyes ran independently, which might have implication about the nature of guidance cues for the optic nerves.