Histological characterization of cellularity and vessel density of CT26 tumours. Histological images obtained after immunohistochemistry of PECAM 1 (positive staining in brown) and haematoxylin counterstain for ectopic (a) and orthotopic (b) 15 days after tumour implantation (original magnification 400x, scale bar 50 μm). Microvascular density (MVD) quantification (c) and cellularity (d) are presented as a function of time (mean ± SEM). The 3D vascular organization was investigated, 15 days after tumour implantation, by fluorescence using Alexa fluor 594-labeled albumin injected i.v. (e) Ectopic, (f) orthotopic, scale bar, 100 μm.