A tiny microscope developed by research teams at Bell Labs and the Max-Planck Institute for Medical Research has enabled the imaging of the brain of a live, moving rat. The device only weighs about 25 grams and employs multiphoton fluorescence. Because it is so small, the microscope can be attached to the head of a rat so that images can be obtained through a window in its cranium while it is free to roam about its environment. Fluorescent agents introduced into the bloodstream of the animal facilitate the targeting of specific components of the brain.

Immunofluorescence was utilized to label neurofilaments and astrocytes in a horizontal section of rat brain tissue. First, the specimen was fixed, permeabilized, blocked with 10-percent normal goat serum, and treated with a cocktail of mouse anti-NF-H Non PO₄ and rabbit anti-GFAP primary antibodies. Then, to visualize the primary targets, the tissue section was treated with goat anti-mouse and anti-rabbit secondary antibodies (IgG) conjugated to Alexa Fluor 488 and Alexa Fluor 568, respectively. Finally, Hoechst 33342 was employed to counterstain cell nuclei. Nuclear DNA was counterstained with Hoechst 33342. Images were recorded in grayscale with a 12-bit digital camera coupled to a Nikon Eclipse 80i microscope equipped with bandpass emission fluorescence filter optical blocks. During the processing stage, individual image channels were pseudocolored with RGB values corresponding to each of the fluorophore emission spectral profiles.