Neuroligin 1 expression in horizontal cell subsets. Double labeling of embryonic day 21 chick retinas (ED21) with NLGN1 and trkA (A–D), GABA (E–H) or calretinin (I–L) showed colocalization with markers for different retinal horizontal cell subgroups. Arrows indicate NLGN1-positive dendritic tips of horizontal cells that are also positive for trkA(C), GABA(G), and calretinin(K). onl, outer nuclear layer; opl, outer plexiform; inl, inner nuclear layer; ipl, inner plexiform layer; gcl, ganglion cell layer. Scale bars = 15 μm in A; 5 μm in B.