Thymic localization of DC precursors. (A) Thymic tissue sections were incubated with anti-IL3R MoAb and counterstained with hematoxylin. IL-3R^high cells (red), which comprise DC precursors, are predominantly located in the medulla and a few are scattered through the cortex. (B) Thymic tissue sections were double-stained for IL-3R and CD1a. IL-3R^high cells (blue) are often found adjacent to CD1a⁺ thymocytes (red) (see inset). Original magnification (A) ×200, (B) ×400.