Adult ciliary epithelial cells, previously identified as retinal stem cells with potential for retinal repair, fail to differentiate into new rod photoreceptors. 
The ciliary margin in lower vertebrates is a site of continual retinal neurogenesis and a stem cell niche.  By contrast, the human eye ceases retinal neuron production before birth and loss of photoreceptors during life is permanent and a major cause of blindness.  The discovery of a proliferative cell population in the ciliary epithelium (CE) of the adult mammalian eye, designated retinal stem cells, raised the possibility that these cells could help to restore sight by replacing lost photoreceptors.  We previously demonstrated the feasibility of photoreceptor transplantation using cells from the developing retina.  CE cells could provide a renewable source of photoreceptors for transplantation.  Several laboratories reported that these cells generate new photoreceptors, whereas a recent report questioned the existence of retinal stem cells.  We used Nrl.gfp transgenic mice that express green fluorescent protein in rod photoreceptors to assess definitively the ability of CE cells to generate new photoreceptors.  We report that CE cells expanded in monolayer cultures, lose pigmentation, and express a subset of eye field and retinal progenitor cell markers.  Simultaneously, they continue to express some markers characteristic of differentiated CE and typically lack a neuronal morphology.  Previously reported photoreceptor differentiation conditions used for CE cells, as well as conditions used to differentiate embryonic retinal progenitor cells (RPCs) and embryonic stem cell-derived RPCs, do not effectively activate the Nrl-regulated photoreceptor differentiation program.  Therefore, we conclude that CE cells lack potential for photoreceptor differentiation and would require reprogramming to be useful as a source of new photoreceptors.