Immature coronary capillaries in the male β3-null mouse heart. Using TEM, we evaluated whether coronary capillary maturation is affected by null expression of β3 integrin. Whereas the frequency of immature vessels decreases with age in male and female wild-type mice as well as female β3-null mice, β3-null mice continue to display these immature characteristics, even in senescent (27-week-old) mice.