Figure 2. Genetic visualization of PrV2-origin lemniscal fibers. A, Strategy for genetic visualization of PrV2-origin (maxillary Pr5-origin) lemniscal fibers. Top, Alleles of Krox20-Cre and Ai14 mice. These lines were crossed. Bottom, Schematic drawing of selective labeling of PrV2-origin lemniscal fibers with tdTomato. Selective expression of Cre recombinase in the V2 subregion of the Pr5 drives tdTomato in the double heterozygote (Krox20-Ai14 mice). B, Krox20-Ai14 mice have strong tdTomato expression in almost all neurons in the PrV2 that project to the maxillary (V2) VPM. The layer-six neurons in the somatosensory cortex of Krox20-Ai14 mice that project to the V2 VPM do not colocalize with tdTomato expression. CTB was injected in the V2 VPM on P28. Scale bars: Left, 400 μm; Right, 50 μm. C, Krox20-Ai14 mice have strong tdTomato expression in >85% of neurons in the PrV2, but not in the mandibular Pr5 (PrV3). Such expression was observed as early as P2. Scale bars: Left, 200 μm; Right, 50 μm. D, tdTomato-labeled lemniscal fibers from PrV2 to the VPM in the P28 Krox20-Ai14 mouse. Top, tdTomato signals throughout lemniscal fibers. Scale bar, 200 μm. Bottom: Terminals of tdTomato-labeled lemniscal in the V2 VPM. Terminals were clustered in barreloid hollows surrounded ring-shaped alignment of thalamic neurons. Broken lines indicate barreloid units. Scale bar, 20 μm. E, Serial thalamic sections of P28 Krox20-Ai14 mice were subjected to CO staining, tdTomato immunostaining, and acetylcholine esterase (AChE) staining. Cutting plane was perpendicular to the longest axis of barreloid units. tdTomato-immunoreactive puncta were confined to the most caudal region of the dorsolateral part of the VPM (V2 VPM). Scale bar, 200 μm. VPL, ventral posterolateral thalamic nucleus. F, Time course of barreloid establishment in the Krox20-Ai14 mice. Scale bar, 200 μm. Top and Middle: Adjacent CO and fluorescent Nissl stained sections are shown. A–E, Barreloid rows; 1-3, barreloid arcs. Bottom, Eight-bit signals in the tdTomato channel of confocal images shown in middle. R, rostral direction; VM, ventromedial direction.