Fig. 5. Growth of the myotome occurs by even cell intercalation all along its dorsoventral extent and newly added fibers localize medially abutting the sclerotome. (A) Transverse section showing the homogeneous entry of labeled progenitors (newly added postmitotic fibers) along the entire dorsoventral extent of the desmin-positive myotome. Note the localization of labeled nuclei to the medial (sclerotomal) side of the myotome (arrowheads). The left side of the section shows the result of a DML ablation performed 1 day earlier. Note that both the DML and the dorsalmost part of the myotome are missing (dorsal to the long arrow). Nevertheless, the remaining myotome is an active structure growing by a similar, homogeneous addition of new cells all along the dorsoventral extent. (B) Quantification of the distribution of second-wave fibers that entered the myotome during an approximate period of 6 hours (two pulses of radiolabeled thymidine, each available to the embryo for 3 hours) (Kahane et al., 1998a; Kahane et al., 1998b). The proportion of newly added cells was counted in serial frontal sections, as described in the Materials and Methods. Calculated values (mean±s.d., n=6 somites) for six contiguous regions per segment are: 17.5±3.17; 18.2±3.8; 18.7±8.6; 12.2±1.63; 19.8±6.7; and 13.5±4.0. D, dermis; DA, dorsal aorta; DML, dorsomedial lip; M, myotome; NT, neural tube; NO, notochord; Scl, sclerotome. Scale bar: 30 µm.