Mechanism of WAT anastomosis. (A) Nonincorporated ECs (green) organize in the implanted gel to form an unperfused network in the vicinity of a perfused host vessel that has grown into the gel. The host vessel consists of host ECs (brown), BM (blue), and pericytes (red) and contains flowing blood. (B) Engrafted ECs associated with the nascent network wrap around the existing host vessel segment and produce MMPs (yellow), which cause reorganization of the existing BM and pericytes. (C) The engrafted ECs lie on top of the host segment to form a bilayer. At this stage, there is not yet perfusion of the nascent vascular network. Note that the wrapping coverage is generally not as complete as depicted. (D) On degradation of a portion of the underlying host endothelium, blood is allowed to flow into the nascent vascular network. (E) The engrafted ECs are fully incorporated, forming junctions with the host ECs. The segment has matured, achieving normal BM and pericyte fortification.