Fig. 1. Summary of impacts of various previous knockouts on innervation (black fibers) of mid-vibrissa follicle Merkel cells (dark gray ovals) (Fundin et al., 1997). An enlarged schematic of a Merkel ending and cell is shown in C. For illustrative purposes, the follicles are all shown at the same size and maturity for all ages. (A) In newborn Ntf3^–/–, only a few Merkel cells are present and only a few of these are innervated (Fig. 6A,B). Only a few uninnervated Merkel cells remain on P7 and these disappear by P14. Although reduced compared with normal, more Merkel cells and innervation are present in newborn trkC_K^–/– than in Ntf3^–/– (Fig. 6C,D, Fig. 7E). These Merkel cells and innervation are further reduced but still present by P7. Only a few uninnervated Merkel cells remain by P14. (B) In Ngf^–/– or trkA^–/– animals, Merkel cells and innervation from P0 to P14 are reduced and tightly packed to a level near the ICB (Fig. 7A). The impact was slightly less severe in the Ngf^–/–. (C) Merkel innervation seems normal from P0 to P14 in p75^–/– and gradually declines over the next few months.