PCs are distant from APRIL-producing cells. Serial BM sections were immunostained with CD138 and Stalk-1. Inset shows high magnification of a CD138⁺ BM PC. Scale bars represent 80 μm, inset 3 μm. (Bottom) Estimated distances between single PCs and their most proximal Stalk-1⁺ cell, for a total of 6 BM sections.