Figure 7.

Horizontal cells are not required for the initial stratification and connectivity associated with both the rod and cone pathways. Despite the absence of horizontal cells before the onset of OPL formation, ribbon formation and rod bipolar cell dendritic outgrowth develop normally. By P9, in regions devoid of a cell sparse OPL (a, a′) lacking horizontal cells and their processes (b, b′), a continuous stratum of CtBP2-positive puncta defines an emerging OPL (c, c′); cone pedicles are properly stratified at this age (d, d′), but do not account for all of the CtBP2-positive profiles (f, f′). At this age, PKC-positive rod bipolar cells have differentiated dendrites that are confined to this stratum (e, e′,g, g′). Scale bars, 10 μm.