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Infrared (IR) and Raman spectroscopy are emerging biophotonic tools to recognize various diseases. The current review gives an overview of the experimental techniques, data-classification algorithms and applications to assess soft tissues, hard tissues and body fluids. The methodology section presents the principles to combine vibrational spectroscopy with microscopy, lateral information and fiber-optic probes. A crucial step is the classification of spectral data by a variety of algorithms. We discuss unsupervised algorithms such as cluster analysis or principal component analysis and supervised algorithms such as linear discriminant analysis, soft independent modeling of class analogies, artificial neural networks support vector machines, Bayesian classification, partial least-squares regression and ensemble methods. The selected topics include tumors of epithelial tissue, brain tumors, prion diseases, bone diseases, atherosclerosis, kidney stones and gallstones, skin tumors, diabetes and osteoarthritis.

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Raman microspectroscopic mapping enables one to study the chemical composition and molecular structure of subcellular components in individual cells without the need for labeling. Lung fibroblast cells were prepared under normal conditions and under stress, which was induced by 24 h of exposure to glyoxal. Raman microspectroscopic maps were recorded from fixed cells with 785-nm excitation and with 1-microm step width. Cluster analysis was applied to generate pseudocolor images of the cell morphology. Raman maps revealed that the cell nucleus shrinks in stressed cells, called pyknosis, which refers to an early stage of apoptosis. The intensity of nucleic acid bands decreased in cluster-averaged Raman spectra of the nucleus and cytoplasm, which is consistent with degradation and conformational changes of DNA and RNA. During a later stage of apoptosis, Raman maps indicate a rounding of cells, a further intensity decrease of nucleic acids bands, fragmentation of the nucleus, disappearance of lipid bodies, and formation of blisters at the cell surface. Whereas the peripheral membrane of the undisturbed cell is composed of lipids and cholesterol, the blisters have a higher protein content with nucleic acids incorporated. The results demonstrate that Raman spectroscopic mapping might become a powerful tool in cell biology for single cell analysis.