Eyeballs were fixed via trans-cardial perfusion with 2% paraformaldehyde, 2% glutaraldehyde, and 0.05% calcium chloride in 50 mM MOPS, pH 7.4. Tissue processing was done according to a previously described procedure (Ding et al., 2015 (link)) as follows. Retinal vibratome sections were treated with 1% tannic acid (Electron Microscopy Sciences) and 1% uranyl acetate, dehydrated with graded ethanol, and infiltrated and embedded in Spurr’s resin. For the developmental study in Fig. 3, eyeballs were fixed by immersion into a solution containing 2% paraformaldehyde and 2% glutaraldehyde in 0.1 M sodium cacodylate buffer. Eyeballs were then treated with 2% osmium tetroxide, dehydrated, and embedded. For experiments with cell culture, cells were fixed in a solution containing 2% paraformaldehyde and 2% glutaraldehyde in 0.1 M sodium cacodylate buffer, treated with 2% osmium tetroxide, dehydrated, and embedded in Spurr’s resin. Thin sections of 60–80 nm were collected on copper grids, counterstained with uranyl acetate and Sato’s lead, and examined using an electron microscope (JEM-1400; JEOL) at 60 kV. Images were collected using a charge-coupled device camera (Orius; Gatan). The Feret diameter, which is the maximum diameter of an ovoid object, was measured using ImageJ software (National Institutes of Health).