Male β3-null coronary capillaries appear immature and angiogenic.Transmission electron micrographs show that coronary capillaries from adult wild-type mice (A-B) are composed of endothelial cells with a smooth luminal surface and uniform thickness around the vessel perimeter. In contrast, coronary capillaries from adult male β3-null mice (C-E) often have irregular endothelial-cell thickness (arrows) along with filopodia extending into the vessel lumen (arrows). The luminal filopodia can originate from cell-cell junctions as well as from the central region of a single endothelial cell (E). All filopodia appear to extend toward the vessel lumen rather than into the extravascular space. The frequent presence of organelles such as mitochondria and ribosomes in regions of thickened endothelium is consistent with activated endothelial cells. Luminal filopodia in male β3-null coronary capillaries appear to extend toward (F) or interact with (G-H) adjacent filopodia. The presence of electron-dense staining at points of contact suggests that interactions between filopodia may represent adhesive events. Connections between filopodia appear to form pockets within a single vessel lumen, consistent with angiogenesis by division of existing vessels. Bar = 500 nm.