Fig. 2. Capillary sprouts from the HITA wall. HITA rings embedded in collagen gel or in matrigel show extensive capillary sprouting into the lumen (A) and outside the rings (B). Light microscopic studies on paraffin wax sections and electron microscopic studies on fine sections of HITA after the ring assay demonstrate the formation of tubular structures (arrowheads) in the lumen and outside the rings (C). The formation of capillaries (Cap) is also confirmed by electron microscopic analysis (D). New capillaries develop in close contact with mature endothelial cells based on the internal elastic membrane (arrow).