Postnatal angiogenesis and vascular remodeling in the heart. The cardiac coronary capillary bed undergoes significant vascular remodeling during a postnatal period of myocyte hypertrophy. (A) Immunostaining for PECAM-1/CD31 shows capillaries in cross section (top) or transverse section (bottom) for 1- and 6-week-old wild-type mice. The schematic illustrates how myocytes (blue) and capillaries (brown) are organized in the adult heart for homogeneous tissue oxygenation. Using TEM, a blinded observer examined several hundred coronary capillaries per mouse and recorded the frequency of luminal filopodia, expanded vacuoles, and/or intussusception. These characteristics of immature, angiogenic blood vessels were observed more frequently in neonatal mice (1 to 3 weeks old) of either sex (B). β3 integrin mRNA expression, considered a marker of angiogenic endothelium, decreases rapidly with age following birth on a similar time scale as capillary maturation (C). (A) Scale bar = 5 μm. (B-C) Data represent mean ± SE.