Identification of Photoreceptor Precursors in the Pars Plana during Ocular Development and after Retinal Injury

Abstract

To study the distribution and differentiation of photoreceptor precursors in the ciliary epithelium in mice. Proliferating cells in flat-mount specimens of the ciliary body and retina were studied by bromodeoxyuridine (BRDU; 150 mg/kg) labeling in young C57Bl mice. Immunoreactivity to anti-recoverin, rhodopsin, and Pax6 antibodies and binding to peanut agglutinin were analyzed histologically to assess the distribution and differentiation of photoreceptor progenitors or precursors. Mice injected intraperitoneally with N-methyl-N-nitrosourea (MNU; 60 mg/kg) were also examined. Part of the neuroblast layer composed of BrdU-positive retinal progenitor cells was identified within the ciliary epithelium of the pars plana in continuation of the layer of the peripheral retina during ocular development. In both the ciliary epithelium and the retina, the layer size decreased rapidly and disappeared mostly by postnatal day (P)9. Within the ciliary epithelium of the pars plana, numerous postmitotic rod and cone photoreceptor precursors were identified that rapidly differentiated morphologically and decreased in number with ocular development. Rod precursors were no longer seen in the pars plana at P12, whereas rare presumptive cone precursors persisted even at P120. An increase in the number of presumptive cone precursors (approximately 16-fold) was identified in the pars plana of adult mice with MNU-induced photoreceptor degeneration. Rod and cone precursors were identified in the ciliary epithelium of the murine pars plana during ocular development but nearly disappeared after the completion of histogenesis. However, in response to retinal injury, an increased number of presumptive cone precursors was found even in the adult pars plana.