Figure 3.

Remnant nonfunctional β-gal ⁺ cells associated with muscle fibers in Pax7 mutant mice. Expression of β-gal, knocked into the first coding exon of Pax7 gene to generate the Pax7 knockouts, in Pax7 ^{+ /lacZ} (A) and Pax7 ^lacZ/lacZ (B) muscle fibers isolated from 1-mo-old mice. In Pax7 ^{+ /lacZ} muscle fibers, expression of β-gal is always colocalized with Pax7 expression. In Pax7 ^lacZ/lacZ fibers, the β-gal⁺ cells display large cell bodies and extensive filopodia-like processes (B), which are in contrast to the smaller, round morphologies of the β-gal⁺ cells in the Pax7 ^{+ /lacZ} fibers (A). (C–F) After 6 d of suspended culture of single muscle fibers from Pax7 ^{+ /lacZ} mice, the Pax7⁺ cells proliferate and differentiate to form aggregates of cells that express Pax7, MyoD, or both (C and E). Under identical culture conditions, the β-gal⁺ cells associated with muscle fibers of Pax7 ^lacZ/lacZ mice are unable to proliferate or differentiate (D and F). Bar, 15 μm.