Distribution of MDC in the human thymus. (A) Selective MDC expression in the medullary areas. The section was immunostained with anti-MDC monoclonal antibody using the avidin-biotin-peroxidase method and the AEC substrate (red color; original magnification × 40). (B) MDC immunostaining in the medulla is clearly visible in cells of the outer layer of a Hassal’s corpuscle and in the other cells scattered throughout the medulla (red color; original magnification × 100). (C) High power magnification of cells scattered in the medulla (shown by arrows) showing MDC immunostaining (red color; original magnification × 250). (D) High power magnification of a Hassal’s corpuscle showing strong MDC immunoreactivity (original magnification × 1,000). (E) Double immunostaining for MDC (blue-gray) and CD3 (red); individual cells staining for MDC but not CD3 are shown by arrows (original magnification × 250). (F) Double immunostaining for MDC (blue-gray) and PAM-1 (red) showing clear cut separation between MDC-positive Hassal’s corpuscles or single cells (arrows) and macrophage dendritic cells (original magnification × 250). (G) Double immunostaining for MDC (red) and cytokeratin (blue-gray). Hassal’s corpuscles and some cells staining for both MDC and cytokeratin (purple-brown), as well as many cells staining for cytokeratin alone (blue-gray) are visible (original magnification × 250). (H) Higher magnification of medullary cells showing double immunostaining for MDC and cytokeratin (purple-brown; indicated by arrows) or cytokeratin alone (blue-gray; original magnification × 1,000). (A) through (D) were counterstained with Gill’s hematoxylin, whereas no counterstain was applied in (E) through (H).