Immunohistochemistry: Double labelling with anti-CD31 and α-actin. Photomicrographs of specimens from perfusion culture and controls labeled with antiCD31 (Tetrazolium, blue) and anti-α-actin (DAB, brown). 2A shows a low power magnification of a cross section of an perfused specimen. Arrows demonstrate entrances from the central lumen to the capillary-like network in the subendothelial region. 2B and 2C show high power magnifications of these "entrances" or "branches". 2D represents an example of a rotated specimen, also with a luminal layer of endothelial cells, but in contrast to the perfused specimen, capillary-like network formation in the underlying region is less marked, and the "branches" represent rather empty spaces of the scaffold material lined by endothelial cells. lu = central lumen of the vessel construct.