Fig. 6. Analysis of reporter gene transcription. In situ hybridisation was performed on 9.5 dpc embryos bearing (A) pEEBZ, (B) y200E-Myf5-nlacZ and (C) y200-Myf5-nlacZ constructs. (A) Transverse sections of a 9.5 dpc (26 somite) pEEBZ transgenic embryo stained for ß-galactosidase. When using the criteria of Spörle and Schughart (Spörle and Schughart, 1997), this is a is 27 somite stage embryo. (PSM) No transgene expression can be seen in the presomitic mesoderm adjacent to the first somite. (So O) As soon as the somite is born, nlacZ transcripts are detected in cells in the dorsal half of the somite. (So I-IIa) Stronger epaxial expression is seen in the dermomyotome of the subsequent older somites. (So X-Brachial) Intensity of dorsal dermomyotomal expression decreases thereafter. (Brachial-Cervical) The expression of nlacZ transcripts is observed more hypaxially through the myotome as the somite matures. (B) The y200E-Myf5-nlacZ reporter transcripts are detected in the myotomes of all somites (red arrowhead) and in the hypaxial dermomyotome at the interlimb level. (C) The y200-Myf5-nlacZ transgene transcript pattern is the sum of those of pEEBZ and y200E-Myf5-nlacZ. The higher magnification picture of the youngest somites illustrates the switch from the transcriptional output of the EEE (white arrowhead) to that of the enhancers that operate in the myotome (red arrowhead). Dashed line indicates the level at which this switch is seen.

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