FIGURE 4-21 Two-photon excitation microscopy allows deep penetration for intravital imaging. (a) In conventional point-scanning confocal microscopy, absorption of a single photon results in an electron jumping to the excited state. In two-photon excitation, two lower-energy photons arrive almost instantaneously and induce the electron to jump to the excited state. (b) Two-photon microscopy can be used to observe cells up to 1 mm deep within a living animal immobilized on the microscope stage. (c) Neurons in a lobster were imaged using two-photon excitation microscopy.

[Part (c) unpublished data from Peter Kloppenburg and Warren R. Zipfel.]