Response: Contributions of the Myf5-independent lineage to myogenesis

Abstract

In this issue, Comai et al. present evidence that previous support for Myf5-independent myogenic cell lineages was confounded by inefficiencies in lineage marking and ablation. Here, Haldar et al. discuss other possible explanations for the inconsistencies between different data sets and reiterate their views on Myf5-independent myogenesis.

Figure 1. Generation and Analysis of the MyoDCre and MyoDMyf5ki Mice

(A) MyoD^Cre mice were generated by placing an IRES-Cre-FRT-Neo-FRT construct in the 3′ UTR of the MyoD gene. (B) X-gal staining of embryonic day 11.5 (E11.5) MyoD^Cre∷R26^lacz embryo demonstrates the expected pattern of MyoD lineage. (C) Absence of myofibers in cross-sections of forelimb from a 14.5 days postcoitum (dpc) MyoD^Cre:R26^DTA embryo, demonstrated by anti-laminin (green) and myosin heavy chain (red). (D) MyoD^Myf5ki mice were generated by replacing 451 base pairs downstream of the translational start site of the MyoD gene with a Myf5-FRT-IRES-EGFP-FRT construct. (E) Costaining of forelimbs of 11.5 dpc embryos (cryosections) with anti-MYOD and anti-MYF5 antibodies. Several Myf5⁺ MyoD⁻ single positive cells were observed in Myf5^+/− embryos (yellow arrowheads). In contrast, Myf5 is present only in MyoD⁺ cells of Myf5^−/−∷MyoD^Myf5ki embryo (white arrows). No expression of Myf5 is discernible in Myf5^−/− embryos. Scale bar, 25 mm. (F) Costaining of forelimbs of 12.5 dpc embryos (cryosections) with anti-Pax7 and anti-MyHC antibodies. MyHC⁺ muscle cells and Pax7⁺ muscle-precursor cells are abundantly present in limb (white arrows) and somite (yellow arrowheads) of heterozygous embryos. Note a severe reduction of MyHC⁺ and Pax7⁺ cells in limbs of Myf5^−/−∷MyoD^−/− and Myf5^−/−∷MyoD^{Myf5ki/Myf5ki} mutants (indicated by **). Scale bar, 200 mm. FL, forelimb.