Cells of monocyte/macrophage lineage infiltrate the collagen CAM onplants. Collagen on-plants supplemented with buffer alone (A,C,E) or bFGF/VEGF (B,D,F) were harvested with the underlying CAM at 24 hours (A-B), 48 hours (C-D), and 66 hours (E-F), embedded in OCT compound, and frozen. Cryosections were immunostained with a chicken monocyte/macrophage-specific monoclonal antibody (red). The sections were counterstained with YO-PRO-1 iodide, resulting in green-stained cell nuclei. (A-B) At 24 hours, positively stained cells (arrowheads) were present only in the underlying CAM. (C-F) By 48 to 66 hours, positively stained monocytes/macrophages appeared also in the upper areas of onplants as isolated cells or clusters of cells. Bar represents 200 μm. At a higher magnification (A inset; bar represents 20 μm), the stained cells display macrophage-like morphologic characteristics (rounded or bean-shaped nuclei and a relatively high cytoplasm-to-nucleus ratio). lu denotes the lumens of large vessels. Large, unstained, circular areas in the upper portions of the sections represent the sites where the grids of nylon meshes were displaced during the processing of sections.