Fig. 8. Even growth of the myotome by progressive cell intercalation as opposed to incremental cell addition. (A,B) Simultaneous expansion of the myotome in the dorsoventral and transverse planes is accounted for by two successive myoblast waves first wave of pioneers in red and second wave in green). (A) Even pattern of myotome growth in the DM to VL orientation by progressive cell intercalation. Green arrows illustrate the contribution of the second wave stemming from all four lips of the DM. Fibers are directly generated from all along the rostral and caudal lips. By contrast, cells from along the DML and VLL first delaminate into the sub-lip domain (1), mesenchymal myoblasts then migrate longitudinally to one of the extreme edges (2) and generate myofibers (3) upon joining their rostral and caudal counterparts. Myofiber elongation occurs in a direction that is parallel to pre-existing pioneer fibers (red) and newly added cells intercalate among adjacent pioneers. This results in an even pattern of dorsoventral growth (arrows below the scheme) with young and old fibers evenly spread. (B) Growth in the transverse plane occurs in a superficial (DM) to deep (Scl) direction. Black arrows indicate the direction of myotome growth, leaving the pioneer fibers (red dots) superficially localized apposed to the DM. (C) Incremental growth of the myotome driven by the DML and VLL (small arrows) as proposed by Ordahl et al. (Ordahl et al., 2000). The oldest fibers are of unit length (dark green) and localize to the center of the myotome. Further growth occurs both medially and laterally (large arrows in which the green gradient represents relative myofiber age). Accordingly, cells from the DML and VLL delaminate into an intermediate domain where they begin differentiating bi-directionally in situ with no prior longitudinal relocation. This growth domain is therefore assumed to contain partial-length fibers in a staggered configuration subjacent to both medial and lateral edges, respectively.