Characterization of Angiogenesis and Microcirculation of High–Grade Glioma: An Intravital Multifluorescence Microscopic Approach in the Athymic Nude Mouse

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Abstract
The current study follows angiogenesis and microcirculatory changes associated with malignant glioma growth by means of an intravital fluorescence microscopic approach, which allows for the direct and continuous visualization of the glioma microvasculature and its quantitative analysis. Fluorescently labeled C6 rat glioma cells (5 × 105) were implanted into dorsal skinfold chamber preparations of athymic nude mice. Glioma growth, vascularization, microhemodynamics, vascular permeability, and leukocyte–endothelial cell interactions were simultaneously followed over a 22-day observation period using intravital epiillumination microscopy and a multifluorescent labeling technique. Analysis of the process of glioma vascularization revealed three stages with distinct microvascular characteristics: avascular stage (days 0 to 6), lag of glioma growth but initial glioma-induced angiogenesis within the host tissue in peritumoral areas; early vascular stage (days 6 to 14), glioma cell proliferation associated with a spatially homogeneous development of a glioma microvasculature; and late vascular stage (days 14 to 22), exponential tumor growth and expansion (> 400 mm3) with high vascular densities in the peritumoral region and reduced vascularization (microvascular perfusion) in the glioma center. Within the center, the functional vessel length per area correlated inversely with glioma size ( P<0.01). In the peritumoral region, functional vessel length per area was independent of glioma size, indicating persistent, high angiogenic activity throughout the observation period. Thus, the microvasculature of mature gliomas revealed a microvascular zonal division with a progressive reduction of the functional vessel length per area within the tumor center. The perfusion failure of individual microvessels within the glioma center was partly compensated by an increase of diameters ( P<0.05), and thus by an increase of blood flow in these functional microvessels ( P<0.05) over time. Histologic analysis demonstrated both expanding and infiltrating growth patterns, as well as focal necroses on day 22. These are the first data from repeated in vivo analysis of glioma growth, vascularization, and microcirculation.