Subchondral bone marrow lymphocytic follicles in rheumatoid arthritis. A representative example of the inflammatory infiltrate in the subchondral bone marrow of a patient with rheumatoid arthritis undergoing joint replacement surgery of the hip. (A) Hematoxylin and eosin-stained section showing a mononuclear cell aggregate in the subchondral bone marrow adjacent to synovial tissue invasion into the marrow cavity through established cortical erosions. Note that the synovial tissue at the synovium-marrow boundary is mainly fibrous and acellular, whilst dense cellular infiltration can be observed within the marrow space. st, synovial tissue; e, erosion; b, bone; m, marrow. (B, C) Serial histologic sections of a mononuclear cell aggregate in the subchondral bone marrow stained for the B-cell marker CD20 (B) and the T-cell marker CD3 (C). Note the abundance of CD20 expression in the subchondral bone marrow aggregate. (A) Original magnification ×10; (B, C) original magnification ×40.