Fig. 6. The dynamics of dissociation of DM into dermis and the contribution of the lateral half of the dorsal somite to dermis formation. (A-C) Hematoxylin-stained sections illustrate successive stages of dermis dissociation from the DM. At flank levels of the axis, dissociation begins by E3 from the central region of the epithelium (A) and progresses medially and laterally (B) until E4, when only the DML and VLL remain epithelial (C). (D) Dorsal view of a living embryo fragment shortly after CM-DiI labeling within the lateral half of a flank-level epithelial somite. (E) The same embryo was further incubated until T3 (E4), when a mesenchymal dermis (D) is already apparent. Representative transverse section shows that lateral injection gave rise to mesenchymal cells localized in a lateral domain of the dorsal dermis (red cells). The arrow indicates the border between somite and lateral plate mesoderm-derived dermis. Because the ectodermal indentation in this section is not significant, the precise localization of the border was determined from adjacent sections. At this stage, the myotome (green as revealed by desmin immunostaining) has already invaded the somatopleura. (F) DiI labeling of the dorsolateral portion of an epithelial somite performed at a cervical level of the axis (somite 10) and fixed at E4.5. Note labeling of the lateralmost dermis located dorsal to the ectodermal notch. Note as well in E and F the presence of CM-DiI+ cells within the desmin-immunoreactive myotome (M, see text for details). (G,H) A GFP-encoding vector was electroporated into the dorsolateral portion of an epithelial somite. Seven hours after transfection, GFP+ cells are apparent in the lateral domain of two successive segments (G). (H) At E4, GFP-positive cells (green) are encountered in the lateral domain of the dorsal dermis, in the VLL and in the lateral part of the desmin-positive myotome (red). Note that the VLL and accompanying myotome have entered the somatopleura, and the lateral dermis containing GFP+ cells remains dorsal, distinctly apart from the somatopleural-derived mesenchyme. Note as well the presence of a few fluorescent cells in the ventralmost sclerotome, likely to derive from transfection of the appropriate progenitors located slightly ventral to the dorsolateral epithelium fated to become DM. D, dermis, DML, dorsomedial lip, K, kidney primordium, NT, neural tube, Scl, sclerotome, VLL, ventrolateral lip. Scale bar: 45 µm in A-C; 30 µm in E; 45 µm in F; 70 µm in H.

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