Retinal cells were isolated as previously described [33 (link)]. Briefly, mouse retinas were prepared in sterile 0.9% saline. Approximately 200 mg of retinal tissue was incubated with saline containing 0.4 mg/ml papain for 30 min at 37 °C. The tissues were triturated with a siliconized pipette and dissociated single cells were centrifuged at 500 × g for 10 min. The cells were fixed in 4% paraformaldehyde for 10 min, centrifuged, and washed with saline. The cells were then placed onto a Superfrost Plus slide (Fisher Scientific) treated with Vectabond (Vector Labs) using a Cytospin4 (Thermoscientific) and stored at −80 °C until used for immunocytochemistry. For immunocytochemistry, the cells were permeabilized in methanol, incubated with 1% SDS in 0.01M PBS, and washed in 1X PBS. The slides were then incubated with 1% sodium borohydride, blocked using the mouse on mouse kit (Vector Labs), and colabeled with GS and the BMP receptor antibodies (BMPR1A, BMPRIB, and activin-like kinase receptor 2 [ALK2]). Following overnight incubation with the primary antibodies, the slides were incubated with the appropriate secondary antibodies with biotin streptavidin amplification used for the receptor antibodies. The slides were then washed, counterstained with Hoechst solution, and mounted with Aqua polymount.