7. Conclusion

Biological tissues are mainly composed of water, and are transparent. The development of contrast methods based on index gradients (phase contrast, Nomarski contrast, Hoffmann contrast) has enabled progress to be made. Cytological analysis only developed with chemical and then immunochemical dyes. Fluorescence microscopy brought about a revolution by enabling targeted structures to be observed against a dark background. Different-colored probes reveal their quantities, positions and interactions. New approaches make it possible to image right down to the single molecule.

Fluorescence microscopy is currently being combined with other techniques:

• with electron microscopy for resolution down to the nanometric level;

• with "dual" probes for NMR or MRI, which enable us to scale up to the...