Fig. 2. Slow muscle fibres mature in a posterior to anterior wave. Embryos at successive developmental stages were serially sectioned and stained for slow (EB165, red) and all (BA-D5, green) fibres. Approximate somite number in each row of sections is indicated on the right, counting from anterior. Thus, temporal development of a somite can be followed left to right. (A,B) Only fast fibres (green) are present in the 10 somites formed at stage 22. (C) Summary scale diagram showing the stages described with anteroposterior extent of expression of MyHC markers highlighted [modified, with permission, from Nieuwkoop and Faber (Nieuwkoop and Faber, 1967)]. (D-F) Fast expression continues anteriorly in stage 28 embryos, but in the most posterior region, the fast marker is less apparent. A few cells reactive with slow muscle marker (red) are detected medially only in the most posterior region of this 20 somite embryo (F, inset, brackets). (G-J) By stage 35, cells in post-anal tail express slow markers in a monolayer of superficial cells (I, arrows), which is not apparent in trunk somites (G,H). In the most posterior somites of these 36-somite embryos, slow markers appear to span the somite (J, brackets). (K-N) Fast fibres fill the somite at stage 48, as occurs earlier, and slow fibres form a monolayer at the dorsal and ventral extremes of the lateral myotome (arrows). Note that several antibodies show weak and variable crossreactivity to epidermis. not, notochord. Scale bars: in J, 75 µm for A,B,D-J; in K, 75 µm for K-N; in insets, 86 µm.