Fig. 8. Schematic summaries for fine-tuning of CaP cell positioning to correspond to overlying somitic segments. The differentiation and subtype specification of CaPs (A), somite segmentation (B), spatial fine-tuning (C), axonogenesis (D), and numerical fine-tuning (E). CaP cell bodies initially do not correspond precisely in position to newly segmented somites (A,B). In wild type, by the time of axonogenesis, CaP cells are adjusted to the proper positions in relation to somitic segments (C, left). When two CaPs are overlaid by a single somite, they overlap their position with each other, thereby maintaining one axonal exit point per adjacent somite (D, left). Later, one of the two CaPs is eliminated as VaP and each ventral myotome comes to be innervated by only one CaP (E, left upper diagram). If the spatial fine-tuning is successful, secondary motor axons born later (purple, lower diagram) follow the single exit point in each segment. However, after knockdown of Nrp1a or Sema3ab, CaP cell bodies remain in the initial irregular positions (C, right). Abnormal CaP cell positioning brings about the double exit phenotype of CaP axons (D, right) and these are followed by secondary motor axons (purple; E, right lower).