Fig 4.

Standard and modified aortic ring model of angiogenesis with quiescent aortic rings. Freshly cut rings of rat aorta embedded in collagen gel generate outgrowths of branching microvessels. Aortic rings lose their spontaneous angioformative properties and become angiogenically quiescent if kept in serum-free growth medium for ∼2 weeks prior to collagen embedding. The medium is changed three times a week during this time to deplete the cultures of endogenous growth factors. Quiescent aortic rings remain viable and produce an angiogenic response comparable to that of freshly cut rings when they are re-injured or treated with exogenous angiogenic factors.