Arterial-Venous Plasticity in Adult Figure 2. Microvessel segments of arterial identity form a competent, hierarchical microcirculation when implanted. A–D, Confocal microscopy images (Z-projections) of microvascular networks in MVCs derived from total isolates (tMVC) or arterial-enriched isolates (aMVC) and implanted for 6 weeks. In all cases, microvessel segments were isolated from brain tissue of GFP-expressing transgenic rats. The scale bar in (A) = 100 mm and applies to all panels. (A, B). Host mice were injected intravenously with dextran-TRITC (red) to identify implant microvessels (green) that were perfusion-competent or (C, D) MVC implants were immunostained for alpha smooth muscle actin (red) to identify perivascular cell morphology. Arrows indicate arterioles, arrowheads indicate venules and capillaries. (E) Analysis of perfusion-competence of microvessels in total- (tMVC) and arterial-derived (aMVC) implanted MVCs 6 weeks post-implantation. Vessel perfusion was measured as the % fraction of green vessels exhibiting red dye in their lumens. (F) The distribution of vessel diameters, as an index of network architecture, in 4 week total (tMVC) and arterial (aMVC) MVC implants plotted as the frequency of diameters placed into bins of 5 mm increments. Vessel diameter distribution from tMVC is Reprinted from [16] with permission from Elsevier. doi:10.1371/journal.pone.0027332.g002 formed and fully perfused microvasculatures with heterogeneous perivascular cell coverage and microvessel morphologies consistent with mature arteriole, capillary and venule structures (Fig. 2A–E). Importantly, all perfused mature vessels were isolate-derived as these microvessels were GFP-positive (isolates were prepared from rats that ubiquitously express GFP) (Fig. 2A, B). Assessment of the microvessel caliber within the aMVC implants demonstrated a broad distribution of diameters consistent with a typical hierarchical vascular tree and similar to neovasculatures formed from total microvessel isolates (Fig. 2F). Repeating the experi- ments using the Efnb2-reporter mouse as the source of microvessel segments (to identify mature arterial identity) demonstrated that ,60% of the mature microvessels within the microvasculatures of the implanted aMVCs were positive for the arterial marker (Fig. 3A–D), implying that the remaining nearly 40% of the mature microvessels were either venous or capillary in identity. PLoS ONE | www.plosone.org 3 November 2011 | Volume 6 | Issue 11 | e27332