Fig. 6. Morphologically and molecularly distinct cell monolayer coats, first, trunk then tail somites. Embryos at stage 22(A), stage 28 (B,C) and stage 35 (D-F) were plastic embedded, transversally sectioned and stained with violet fuchsin. NT, neural tube; Not, notochord; Epi, epidermal bilayer. Yolk droplets appear yellow. A distinct superficial layer of cells covers trunk somites prior to superficial slow fibre differentiation (A,B,D, arrows) and anterior tail somites after slow fibre formation is initiated (E, arrows, compare with Fig. 1D). Insets show superficial layer (arrows) in middle of somite (D) and at dorsomedial lip (E) at stage 35. Note the transient lack of this layer in the nascent posterior somites present at stage 28 (C) and stage 35 (F), when single cells can be observed elongated across the somite (arrowheads). (G,H) Electron micrographs show a distinct dermomyotome (arrows) in somite 8 (G) but only spindly cells in somite 18 (H) above a layer of well-differentiated muscle with basal myofibrils (arrowheads). Pax3 mRNA was detected by whole-mount in situ hybridisation of stage 29-38 embryos (I-K, upper panels) and En1 mRNA marked a subset of medial cells in the superficial somite level with the notochord (L, arrowheads). Serial transverse 100 µm vibratome sections revealed that signal is superficial within the somite (I-K, lower panels, arrowheads) and non-overlapping with 12/101, a marker of differentiated muscle. Dorsal and ventral groups of cells in the tailbud express highly (K, inset, arrows). Expression persists in a complex pattern in all somites, but is consistently stronger in trunk somites anterior to about somite 12 at stage 29 (G) and stage 33/34 (H). Subsequently, Pax3 increases in tail somites (I).