Fig. 3. Expression of FREK mRNA in developing myotomes. (A) Whole-mount in situ hybridization of E3.5 embryo, revealing that expression of FREK is enriched in both rostral and caudal regions of the myotome (arrowheads) as well as in discrete stripes subjacent the DML epithelium (between arrows). (B) Sagittal section of a similar embryo, further showing the distribution to the extreme regions of the myotomes. (C) Frontal section through a cervical region of a 30-somite stage embryo again revealing intense FREK signal at the edges of the myotomes (arrowheads), whereas the rest of the myotome and the overlying dermomyotome show a fainter intensity of labeling (see also D). (D) Transverse section showing that FREK signal is particularly intense in myotomal cells localized towards the sclerotomal portion of the myotome, a position adopted by ingrowing second-wave myofibers relative to pre-existing pioneers that always remain apposed to the dermal side of the myotome. C, caudal; DM, dermomyotome; M, myotome; NT, neural tube; NO, notochord, R, rostral; Scl, sclerotome. Scale bar, 50 µm.
