Consequences of gene disruption. Sections of muscle from normal (A) and myogenin-knockout (B) mice, as viewed under the light microscope. The unlabeled arrows in both panels identify comparable sections of the pelvic bone, indicating that similar anatomical regions are depicted. Muscles do not develop properly in mice having both myogenin genes disrupted. A poorly formed muscle fiber in the knockout strain is indicated by the M arrow. (C) The development of mature skeletal muscle from progenitor cells is a highly regulated process involving a number of intermediate cell types and multiple transcription factors. Through the gene-disruption studies in (A) and (B), myogenin was identified as an essential component of this pathway.

[(A) and (B) From P. Hasty, et al., Nature 364:501–506, 1993; (C) Information from S. Hettmer and A. J. Wagers, Nat. Med. 16:171–173, 2010, Fig. 1]