Figure 3

MD6.0-lacZ is active in slow and fast fibre types as each matures. Muscle cryosections from E18 (A) and P6 (B) showing MyoD protein, or P0 (C, E, G) and P9 (D, F, H) MD6.0-lacZ mice reacted with X-gal. Scale bar = 50 μm. A, B, Late embryonic (A, E18, longitudinal section) and P6 (B, transverse section) lower hindlimb muscle showing the decline in MyoD (brown) in fibre nuclei and presence in myogenic cells (arrowheads). Asterisks, cartilage. C, EDL/TA section stained for slow MyHC (brown) and X-gal (blue). Note correlation blue nuclei in slow fibres (arrows; brown), magnified in inset at top. Blue nuclei are more rarely associated with fast fibres (arrowheads). E, G, Plantaris and lateral gastrocnemius muscles stained for slow MyHC (black). At P0, the majority of βgal⁺ (blue) fibres express slow MyHC (arrows), although rare βgal⁺ associate with fast fibres (arrowheads). D, F, H, By P9, serial cross-sections of anterolateral region reacted for X-gal (D), slow (F) and fast IIb MyHC (H) show the majority of βgal⁺ fibres are within a fast IIb muscle area of TA (arrows) and little remains within muscle containing slow fibres. Line marks border of TA (left) and EDL (right).