(C) Lateroventral view of a whole-mount stain of a ~12.5 dpc heterozygous embryo for PLAP and β-gal (inset picture) activities. Although PLAP was broadly expressed (e.g., somites, limbs, and CNS), the PLAP and β-Gal (inset picture) expression was particularly high in the optic vesicle (arrow). X-gal single (D) and combined staining with PLAP (E) of a retinal section of a 3-mo-old RanBP2+/− mouse. Consistent with previous immunocytochemistry studies, β-Gal activity is detected in the neuroretinal bodies and inner segment compartment of photoreceptors with conspicuously strong expression in ganglion cells. PLAP expression is found throughout the plexiform/synaptic layers and outer segment of photoreceptors (E). GC, ganglion cell; PLAP, human placental alkaline phophatase; ROS, rod outer segment; RIS, rod inner segment; ONL, outer nuclear layer; OPL, outer plexiform (synaptic) layer; INL, inner nuclear layer; IPL, inner plexiform (synaptic) layer; GC, ganglion cell layer.