Fig. 4. Demonstration of two types of Merkel innervation: one trkA_K and the other trkC_K dependent. (A,B) Double labeling with anti-trkA (green) and anti-trkC (orange) of normal developing vibrissa follicles from central portions of mystacial pads in E15.5 mice and E16.5 rats. In both cases, trkA (green arrows) and trkC (orange arrows) is expressed primarily, if not entirely, on different axons. Sites of overlapping fluorophores (yellow) are due to axon intermingling. Merkel endings (open arrowheads in inset) expressed either trkA or trkC immunoreactivity, not both. Note trkC-labeling on follicle cells. (C-F) Impact of trkA_K deletion, trkC_K deletion and trkA_K/trkC_K deletion on P0 littermates. (C) Merkel axons (arrows), endings and cells (solid arrowheads) are present in wild-type mice (see Fig. 1A). (D) TrkAK^–/– have reduced Merkel innervation that becomes compacted towards the upper end of the follicles (see Fig. 1B). (E) trkC_K^–/– have even fewer Merkel endings and cells. Many cells are uninnervated (see Fig. 1A). (F) trkA_K^–/–/trkC_K^–/– have some detectable Merkel cells but none is innervated. Scale bar: 50 µm.