Small retinas in masterblind fish fail to differentiate. Compared with wild-type (WT) fish at 72 hours post fertilization (hpf) (A), masterblind (mbl) fish typically lack forebrain and eyes (B), although some fish develop with a small eye (B’). Cryosections of wild-type retinas at 72 hpf (C) reveal lamination of the nuclei, labeled with DAPI in blue, into three layers: the ganglion cell layer (GCL) nearest the lens (L), the inner nuclear layer (INL), and the outer nuclear layer (ONL). Calretinin-labeled neurons have differentiated in the GCL and INL. Proliferative cells, marked with proliferating cell nuclear antigen (PCNA) are found only in the periphery of the retina in the ciliary marginal zone (CMZ). In contrast, cryosections of mbl retinas at 72 hpf (D) show a lack of nuclear lamination, a failure of calretinin-positive neurons to differentiate, and all cells within the retina remain PCNA-positive. In these and all following figures, the eye is outlined with a dotted line to distinguish it from adjacent tissues. Scale bar: A, B, 200 μm; C, D, 100 μm.