Targeted ultramicrotomy: a valuable tool for correlated light and electron microscopy of small model organisms.

Correlative light and electron microscopy (CLEM) is used when one needs to combine both imaging modalities on the same sample. When working on living small model organisms, such as Caenorhabditis elegans, specific CLEM protocols are required to acquire high-resolution light microscopic images of a region of interest and thereafter to relocate and study the same object at the ultrastructural level using a transmission electron microscope. In this chapter, we describe how to process living specimens from the confocal microscope to the transmission electron microscopy (TEM), focusing on an improved ultramicrotomy technique that allows a precise and reliable targeting of the object of interest. This improvement significantly reduces the time consuming and frequently frustrating search for the region of interest. Our targeted ultramicrotomy protocol is versatile enough to be applied on a variety of bulk specimens, such as fly and fish embryos, or mouse tissues.