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Fig. 1 The dynamic expression of cxcr4a and sdf1a during segmentation. Dorsal views (A,C-G,J,K) and lateral views (B,H-I). (A,B) Expression of cxcr4a in posterior trunk. (D-I) RNA in situ hybridization with sdf1a riboprobe (blue). (A,B) High level of cxcr4a transcript in newly formed and posterior somites, 13.5 h and 16 h respectively. In somites, cxcr4a expression is restricted to anterior part. Expression becomes increasingly restricted to anterior part within each somite over time, black arrows. (C) Overlapping expression domain of cxcr4a with myoD (red) is observed, 14 h. (J) Schematic representation of cxcr4a expression (blue) in posterior somites. (D) sdf1a staining covers almost the entire three anterior-most somites indicated by white bracket, while in posterior somites expression is restricted to posterior part, 13 h. (E) Expression in early somites, 11 h. (F,G) Overlap of expression of sdf1a and myoD (red), 14 h. (H) Expression is restricted to the posterior part of each somite, 16.5 h, white arrowheads. (I) Faint expression is detected in forming and newly formed somite, 21 h. (K) Schematic representation of sdf1a expression (blue) in posterior somites. (L) Reverse transcription (RT)-PCR detects continuous presence of transcript of sdf1a during early development. sdf1a transcript is present at low levels before mid-blastula transition (MBT). To confirm results, the PCR products were sequenced. β-actin was used as a positive control. -RT control using β-actin primers without addition of reverse transcriptase, no band was detected (data not shown). Black dashed lines indicate boundary between somite and newly formed somite (B,I). White lines demarcate the somite boundaries (C,H). Abbreviations: a – anterior; p – posterior; psm – presomitic mesoderm; r – rhombomere; s – somite; S0 – forming somite; S1 – newly formed somite; tb – tailbud; ys – yolk sac. Scale bars = 50 μm.