Dissecting tumour pathophysiology using intravital microscopy

Key Points A solid tumour is an organ-like structure that consists of cancer cells and host stromal cells embedded in an extracellular matrix and nourished by a vascular network. Intravital microscopy (IVM) has provided unprecedented molecular, anatomical and functional insights into the inner workings of this organ and provided a means for testing the efficacy of various therapies. IVM is a powerful optical imaging technique that allows continuous non-invasive monitoring of molecular and cellular processes in intact living tissue with 1–10 μm resolution. Such resolution is currently not possible with non-optical techniques. IVM requires an appropriate animal model, a molecular probe (usually fluorescent), a microscope equipped with a digital camera detection system, an image acquisition system, and a computer to process and analyse the data to extract parameters of interest. Molecular imaging has revealed heterogeneity in the tumour microenvironment (for example, pO 2 and pH) and the delivery of therapeutics (micro-pharmacokinetics). The most celebrated applications of molecular imaging include measurement of promoter activity, enzyme activity and gene expression in vivo . Cellular imaging has uncovered key steps in the spread of cancer from one site to the next (invasion and metastasis), and barriers to various cell-based therapies (for example, immunotherapy and gene therapy). Cellular imaging has also allowed measurement of interactions among various subpopulations of cells in a tumour. Anatomical imaging has allowed quantification of morphological abnormalities in tumour vessels, as well as the size of pores in their walls. Functional imaging has revealed that tumour blood flow, vascular permeability, interstitial diffusion, convection and binding are spatially and temporally heterogeneous in tumours, depend on the host–tumour interactions, and change during the course of treatment. Functional lymphatics are present only in the tumour margin and the peri-tumour tissue. IVM has revealed that certain direct and indirect anti-angiogenic treatments can 'normalize' the abnormal tumour vessels so that they become more efficient. This finding underscores the importance of optimal dose and scheduling in combination therapy. With the availability of hand-held microscopes and safe (Food and Drug Administration approved) molecular probes, IVM has the potential to become a useful clinical tool to monitor integrative pathophysiology and the response of optically a