In biological imaging of fluorescent molecules, multiphoton laser scanning microscopy (MPLSM) has become the favorite method of fluorescence microscopy in tissue explants and living animals. The great power of MPLSM with pulsed lasers in the infrared wavelength lies in its relatively deep optical penetration and reduced ability to cause potential nonspecific phototoxicity. These properties are of crucial importance for long time-lapse imaging. Since the excited area is intrinsically confined to the high-intensity focal volume of the illuminating beam, MPLSM can also be applied as a tool for selectively manipulating fluorophores in a known, three-dimensionally defined volume within the tissue. Here we introduce localized multiphoton photoactivation (MP-PA) as a technique suitable for analyzing the dynamics of photoactivated molecules with three-dimensional spatial resolution of a few micrometers. Short, intense laser light pulses uncage photoactivatable molecules via multiphoton excitation in a defined volume. MP-PA is demonstrated on photoactivatable paGFP in Drosophila wing imaginal discs. This technique is especially useful for extracting quantitative information about the properties of photoactivatable fusion proteins in different cellular locations in living tissue as well as to label single or small patches of cells in tissue to track their subsequent lineage.
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