Organization and regeneration ability of spontaneous supernumerary eyes inplanarians - 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 the ir regeneration abilities are expected to be highly relevant to the questio n of where and how the field to regenerate eyes is determined. In this stud y, spontaneously generated supernumerary eyes were classified into various types. In all cases, they were formed in the anterior part of the head. Enu cleation of a normal eye elicited regeneration of a new eye; however, enucl eation of a supernumerary eye did not. The supernumerary eyes were morpholo gically and functionally indistinguishable from the normal eyes, revealed b y the studies of immunohistology and photophobic response, respectively. Fr om the obtained results, we proposed a model of the eye regeneration field that changes its distribution spatiotemporally during regeneration. Immunoh istological studies also showed that the optic nerves from the normal and s upernumerary eyes ran independently, which might have implication about the nature of guidance cues for the optic nerves.