Posterior eyecups from light-exposed or dark-adapted mice were washed with ice-cold PBS buffer. The RPE/choroid was scraped from the scleral wall of eyecups aseptically with the aid of a dissecting microscope. The dissected RPE/choroid was pooled from eight eyes and digested in 1 mL collagenase type I (2 mg/mL; Roche Diagnostics) and elastase (Grand II; 0.25 mg/mL; Roche Diagnostics) in Dulbecco’s modified Eagle’s medium (DMEM, Cellgro; Mediatech, Herndon, VA) supplemented with 1% fetal calf serum (Invitrogen-Gibco, Grand Island, NY) for 1 hour at 37°C. After digestion, the cellular digests were filtered through a 40-μm-nylon mesh (BD Falcon, Bedford, MA). The filtered cellular digests were then isolated by using immunomagnetic beads (Miltenyi Biotec, Auburn, CA) according to the manufacturer’s instructions. Briefly, the digested RPE/choroid was incubated with a biotin-conjugated CD31 monoclonal antibody (MEC13.3; BD PharMingen, San Diego, CA) for 1 hour at 4°C. The cells were washed once with PBS buffer containing 2 mM EDTA and 0.5% BSA (PBS/EDTA/BSA) and resuspended in 0.5 mL of PBS/EDTA/BSA buffer. Ten microliters of anti-biotin microbeads (Miltennyi Biotec) were added to the cellular digests. After a 0.5-hour incubation, the cellular digests were washed with PBS/EDTA/BSA buffer and then passed over a separation column placed in the magnetic field of a separator (MACS; Miltinyi Biotec). After the column was washed with PBS/EDTA/BSA buffer, the column was removed from the magnet and retained CD31 positively selected cells in the column were flushed with a plunger. Isolated cells were quantified by counting CD31-positive cells per 200× field by fluorescence microscopy. This number was divided by the total number of cells, determined by counting DAPI-stained nuclei in the same field, to yield the percentage of CD31-positive cells within each observed field.