Fig. 3. Light microscopy and EM of the retina in BETA2/NeuroD1 mice. Light microscopy of retinal slices shows an area taken from the posterior retina of 2-month-old +/+ mice (A), 2-month-old −/− mice (B), and 18-month-old −/− mice (C). In 2-month-old null mice, the ONL thickness is reduced to four to five cells. Null mice also demonstrate decreased thickness of the outer segments (OS), inner segments (IS), and OPL. In 18-month-old −/− mice, the ONL has completely degenerated. Scale bar, D (forA–C), 20 μm. EM for 2-month-old +/+ (D) and −/− (E) mice focusing on the OS is shown. In null mice, both outer and inner segments are shorter. Scale bar, F (for Dand E), 10 μm. F, EM of a rod synapse in a +/+ mouse. Note the presence of a synaptic ribbon (arrow), two postsynaptic horizontal cell processes (h), and a bipolar cell dendrite (b) forming a classic synaptic triad.G, OPL taken from a −/− mouse. Note the presence of multiple ribbons in rod terminals (arrows).H, Higher magnification taken from a −/− mouse showing multiple synaptic ribbons. RPE, Retinal pigment epithelial. Scale bar, 0.5 μm forF–H.