Retinal pigment epithelium (RPE), a monolayer of epithelial cells located between the neural retina and the choroid, plays a significant role in the maintenance of retinal function. Its in vivo imaging is still technically challenging in human eye. With the mouse eye, there is a possibility to look into the RPE through the sclera using two-photon microscopy (TPM). TPM is a two photon-excited nonlinear fluorescence microscopy that enables the observation of deep tissues up to several hundred micrometers. Since the simultaneous absorption of two photons occurs only at the focal plane, spatial resolution of the TPM is quite high, such that pinhole as used in a confocal microscope is not necessary. TPM enables observation of autofluorescence at the cellular level, and thus may provide new insights into the fluorescent molecules in/around RPE cells.
The combination of TPM with fluorescence lifetime imaging microscopy (FLIM) may expand the breadth of information about cells and tissues. Fluorescence lifetime is a fluorophore-specific property, which is independent of fluorescence intensity and changes with the alteration of molecular environment. FLIM may have therefore the potentials to distinguish different fluorophores and to indicate the change in the environment of a fluorophore. Some energy metabolisms-related intracellular fluorophores, such as NADH (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide), show characteristic fluorescence lifetimes that shift under different molecular environments, and thus their fluorescence lifetime have been used to indicate cell energy metabolic states. These nonlabeling imaging methods offer us the opportunity to engage in the study of the RPE in vivo as well as in vitro both in morphological as well as metabolic aspects.
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