Most GnRH neurons fail to migrate from the VNO in CXCR4 mutant mice. A, At E12 in CXCR4+/− mice GnRH neurons are detected in the VNO and in the nasal mesenchyme (arrows) caudal to the VNO on their way toward the cribriform plate (cp; dashed line). B, In CXCR4−/− mice at E12 nearly all GnRH neurons are detected in the VNO, and most are clustered (arrowhead) in the ventral and rostral quadrant of the VNO, although a small percentage of GnRH neurons migrates from the VNO into the mesenchyme of CXCR4−/− mice (arrow). C, At E13 in CXCR4+/− mice most GnRH neurons have migrated from the VNO, and >50% have crossed the cp and turned (arrow) toward the ventral forebrain (fb). D, In CXCR4−/− mice at E13 only a few GnRH neurons migrate across the nasal septum and cross the cp (arrows). Many GnRH neurons reside in the rostroventral quadrant (dashed circle; arrowhead) of the VNO. Rostral is left, and dorsal is up. Scale bar: (in D) A, B, 50 μm; C, D, 100 μm.