two-photon microscopy

two-photon microscopy. It is a form of fluorescence microscopy that enables high-resolution, three-dimensional imaging of living tissues and intact organisms. The technique relies on the near-simultaneous absorption of two infrared photons by a fluorophore to excite fluorescence, a process first theorized by Maria Goeppert-Mayer in her doctoral dissertation. Its development into a practical laboratory instrument is primarily credited to Winfried Denk, working at the Cornell University laboratory of Watt W. Webb in 1990.

Principle of operation

The fundamental principle relies on nonlinear optics, specifically two-photon excitation, which occurs with significant probability only at the focal point of an intense, pulsed laser. This is because the probability of a fluorophore absorbing two photons simultaneously scales with the square of the light intensity. Consequently, excitation is confined to a tiny sub-femtoliter volume, eliminating the need for a pinhole to reject out-of-focus light, as required in confocal microscopy. The emitted fluorescence photons, which are of a shorter wavelength than the excitation light, are then collected by a detector. Key enabling technologies include mode-locked titanium-sapphire lasers, which produce the necessary high-intensity, ultrashort pulses, and high-numerical aperture objective lenses to tightly focus the beam.

Advantages over confocal microscopy

This technique offers several distinct benefits for imaging biological specimens. It causes significantly less photobleaching and phototoxicity outside the focal plane, as excitation is intrinsically localized. This allows for longer-term observation of live samples, such as developing embryos or functioning neurons. The use of longer-wavelength infrared light also results in deeper penetration into scattering tissues like the brain or skin, due to reduced light scattering and absorption by hemoglobin and water. Furthermore, it enables the simultaneous excitation of multiple fluorophores with a single laser line, simplifying multicolor imaging protocols compared to some confocal setups.

Applications in biological research

It has become a cornerstone technology in neuroscience, particularly for imaging calcium dynamics in neuronal networks within living brain tissue, as pioneered by researchers like Karel Svoboda at the Janelia Research Campus. It is extensively used in developmental biology to track cell lineages and morphogenetic movements in model organisms like zebrafish and mouse embryos. In immunology, it allows visualization of immune cell interactions within lymph nodes. The method is also applied in tumor biology to study the tumor microenvironment and angiogenesis, and in dermatology for non-invasive imaging of human skin.

Technical considerations and limitations

Successful implementation requires careful attention to several factors. The high peak power of the pulsed laser, while essential, can induce optical damage in the sample if not properly managed. Imaging depth, though superior to confocal, is ultimately limited by light scattering and the working distance of the objective lens. The resolution, while high, is slightly lower than that of a confocal microscope under ideal conditions. Furthermore, the equipment, particularly the titanium-sapphire laser and associated dispersion compensation optics, represents a significant financial investment and requires specialized technical expertise for maintenance and alignment.

Historical development and variants

The theoretical foundation was laid by Maria Goeppert-Mayer in 1931 in her work on quantum mechanics, though experimental verification awaited the invention of the laser. The practical microscope was realized in 1990 by Winfried Denk, then a postdoctoral fellow with James Strickler in the lab of Watt W. Webb at Cornell University. Subsequent variants have expanded its capabilities. Multiphoton microscopy encompasses three-photon excitation for even deeper imaging. Second-harmonic generation microscopy, which images non-centrosymmetric structures like collagen, is often implemented on the same platform. Other advanced implementations include two-photon laser scanning microscopy combined with fluorescence lifetime imaging and adaptations for high-speed volumetric imaging. Category:Microscopy Category:Fluorescence techniques Category:Neuroimaging