Purpose: This article reviews and discusses different options for visualizing the microarchitecture of vessels ex vivo and in vivo with respect to reliability, practicability and availability.
Results and discussion: The investigation of angiogenesis by standard histological methods, like microvessel density counts, is limited since the three-dimensional (3-D) architecture and the functionality of vessels cannot be considered properly. Coregistration of immunostained images of vessels may be performed but is time consuming and often not sufficiently accurate. Confocal fluorescence microscopy is an alternative, but only enables 3-D stacks of less than 500 nm in thickness. Multiphoton microscopy and other advanced technologies, such as optical coherence tomography and optical frequency domain imaging, provide a deeper view into tissues and allow for in vivo imaging of microvessels, which is a precondition for longitudinal studies. Besides these microscopic techniques, the vascularization in larger tissue samples can be investigated using corrosion casts in combination with scanning electron microscopy, or microcomputed tomography (microCT). Furthermore, recent improvements in microCT technology open up new perspectives for in vivo scans with high resolution and tolerable X-ray doses. Also 3-D contrast-enhanced high-frequency ultrasound has been shown to be sensitive for angiogenic vessels and even distinguishing between mature and immature vessels appears feasible. Microvessel architecture can also be visualized by MRI. Here, T1-weighted angiography techniques after injection of blood pool contrast agents appear preferable. Optoacoustic tomographic imaging has more recently shown promise for high-resolution in vivo mapping of the microvasculature in rodents using intrinsic haemoglobin-based contrast and exogenous contrast agents.