Two-photon excited autofluorescence (AF) is based on the fluorescence of the endogenous intracellular fluorophores NADH and NADPH, flavins, porphyrins and keratin. Furthermore, the pigment melanin is fluorescent. Also the extracelluar matrix proteins elastin and collagen are fluorescent. However, the fluorescence of collagen is weak compared to the SHG signal.
Photo: Autofluorescence of human cells recorded by the multiphoton tomograph MPTflex