All-optical histology

With femtosecond lasers. To study the microscopic anatomy of tissue, histologists typically stain it, freeze it, slice it thinly, and sequentially look at the individual slices to get an overall picture. Now, a multidisciplinary and multi-institutional collaboration, led by neurophysicist David Kleinfeld (University of California, San Diego), has developed an automated, all-optical technique for cutting and imaging brain tissue. As described by Jeffrey Squier (Colorado School of Mines) at the June CLEO/QELS meeting in Baltimore, Maryland, the researchers first stained or otherwise labeled a tissue specimen and then imaged the desired structures in 1-µm steps to a depth of about 150 µm, using µJ pulses of their laser. Next, with µJ pulses, they ablated the previously imaged tissue layer. The newly exposed layer was then stained (if necessary), the laser intensity was reduced to take another set of images, and the process continued until no tissue remained. Stacking up the successive images results in a diffraction-limited three-dimensional picture, such as that of the vasculature of mouse brain tissue shown here. The laser ablation left a surface that was smooth to within 1 µm and preserved protein viability. Because the femtosecond technique completely destroys its tissue samples, it may be inappropriate for certain clinical applications like tumor biopsies, in which physicians wish to preserve the tissue for future reference. However, the automated technique may be well suited for many other applications in the burgeoning field of molecular medicine. (P. S. Tsai et al., Neuron 39 , 27, 2003 http://dx.doi.org/10.1016/S0896-6273(03)00370-2 .)