Replacement of FSHD muscle by non-myogenic cells. (A) Transverse section of the biceps muscle from subject 01A with facioscapulohumeral muscular dystrophy (FSHD), showing extensive fibrosis and large pockets of adipocytes; muscle fibers displayed variable diameter. By contrast, biceps muscle from his brother (subject 01U, inset) had a relatively uniform array of myogenic fibers with no endomysial fibrosis or fatty infiltration. Hematoxylin and eosin, original magnification 20 ×. (B) Percentage of CD56+ cells in primary cell cultures isolated from FSHD biceps muscle shown in (A) during in vitro cultivation. PD = population doubling. (C) Growth curves of sorted CD56+ (red squares) and CD56- (green squares) populations from the primary 01A biceps cell culture. The percentage of CD56+ cells in sorted, CD56+ and CD56- cultures over time are also displayed (circles outlined in red (CD56+) or green (CD56-)). (D) Desmin immunostaining (green) of sorted CD56+ and CD56- cells. Nuclei were counterstained with 4',6-diamidino-2-phenylindole (blue). (E) Sorted CD56+ and CD56- cells after 3 days in differentiation medium.