The use of a laser for correlating light and electron microscopic images in thick tissue specimens

Correlating images between light and electron microscopy is difficult especially in tissue specimens with a substantial z-dimension. To facilitate correlated light and electron microscopy (CLEM) in thick tissue specimens, we describe a basic method of using a femto-pulsed near-infrared laser to burn precise three-dimensional fiducial markers that circumscribe cells or regions of interest for easy identification between imaging methods. This rapid and reliable approach permits traditional fixation and avoids the use of electron-dense labeling methods that can obscure ultrastructural details. The versatility of the technique permits CLEM in a variety of tissue specimens to allow interpretation of highly resolved ultrastructural data in the more macroscopic and potentially dynamic context of light microscopy.