Figure 1 Pathology from skeletal muscle biopsy of the left rectus femoris taken 1 month after disease onset. (A, B) Hematoxylin-eosin–stained sections showing (A) internalized nuclei (arrowheads), scattered necrotic (black arrows) and regenerating (blue arrows) muscle fibers. Some muscle fibers are in early stage of necrosis (A, black arrows; B, arrowhead) while other necrotic fibers are fully invaded by macrophages (B, arrow). (C) Acid phophatase stained section highlighting in red macrophages invading necrotic fibers (examples indicated by the arrows). (D) Section immunoreacted for major histocompatibility complex class I (MHC-I) demonstrating non-necrotic muscle fibers with MHC-I sarcolemmal positivity (asterisks) which is absent in control muscle (G). (E) Section immunoreacted for C5b9 showing 2 non-necrotic muscle fibers (asterisks) with granular complement deposition over the sarcolemma (green rim). (F) Fluorescence co-localization of capillaries labelled with biotinylated Ulex Europaeus Agglutinin I in the same section and visualized with rhodamine-streptavidin showing no capillary depletion (asterisks indicate the same 2 fibers with C5b9 sarcolemmal deposition in E). Magnification: 10× (A), 20× (B, C), and 40× (D–G). Courtesy of Dr. Margherita Milone, Muscle Pathology Laboratory, Department of Neurology, Mayo Clinic.