Meanwhile, at P6, most of the recoverin-positive cells in the ciliary epithelium had a uniform appearance; one or two processes were typically shorter than those of rod photoreceptor precursors in the retina (91.8% ± 7.7%;
n = 5;
Figs. 4d 4e ). These cells were probably cone photoreceptor precursors. As we confirmed in some of the cells, one of the processes was positive for peanut agglutinin (PNA;
Fig. 4d ), a marker specific for the inner and outer segments and pedicles of cone photoreceptors.
18 19 This is consistent with previous reports indicating that in the developing eye, cone photoreceptor precursors express recoverin earlier than rod photoreceptor precursors.
13 14 Unfortunately, accurate evaluation of cone photoreceptor precursors was difficult because of the relatively weak signal and the high background of PNA staining, particularly during retinal development.
18 20 By P9, many recoverin-positive retinal precursors in the pars plana had developed a longer process (65.1% ± 17.1%;
n = 4) and aligned radially
(Figs. 4g 4h) . In addition to the appreciable inner and outer segment structures
(Figs. 4h 4i) , some of these long processes showed PNA-positive terminal budding consistent with the pedicles of cone photoreceptors
18 19 20 (Supplementary Fig. S3). The observed morphologic features of recoverin-positive cells are in close agreement with immature cone precursors found in the developing retina,
20 which is summarized in Supplementary Figure S4. Some of these cells bridged the ciliary body and the retina. At P12, many recoverin-positive cells persisted in the pars plana, most of which had a detectable PNA-positive process (68.1% ± 4.4%;
n = 6). Some areas of the pars plana still contained recoverin-positive cells at P18
(Fig. 5a) , but by P30 these cells were nearly absent from the ciliary epithelium
(Figs. 5b 5c) . However, rare cells persisted in the pars plana up to at least P120 (data not shown). Most of these persistent recoverin-positive cells had morphology consistent with early cone photoreceptor precursors.