Tissue-engineered breast tumors require Axl expression. (A) MDA-MB-231/GFP-Luc tumor growth and colonization within polylactic acid tissue engineering scaffolds in NOD-SCID mice (n = 6 per group) is Axl dependent. (B) Axl is required for the growth of tricellular tissue implants comprising primary human microvascular cells (ECs), vascular smooth muscle cells (SMCs), and MDA-MB-231/GFP-Luc in NOD-SCID mice (n = 7 per group). Temporal in vivo bioluminescence image analysis was used to measure tumor cell number (Upper, total photon) and the extent of radial infiltration (Lower, signal diameter) in controls (solid line) and shAxl2 implants (dashed line). (C) Intrascaffold human vessel diameter (Upper) is unaffected, whereas microvascular density (Lower) is slightly enhanced in tissue-engineered (tricellular) tumors inhibited by shAxl2 expression. *P < .05; **P < .005; ***P < .0005 (paired t test) versus control.