Preparation of Drosophila specimens for examination by transmission electron microscopy.

There is no single, simple procedure for fixing and embedding all tissues for transmission electron microscopy (TEM). The chemistry of different cell types is to some extent unique, and this affects the way each cell type reacts to the wide array of fixatives, buffers, organic solvents, and resins used in TEM specimen preparation. A recurring theme in those organisms or cell types that are difficult to fix is the presence of a diffusion barrier that prevents the free diffusion of fixative and other chemicals in and out of the cell or tissue. This in turn means that fixation takes a relatively long time (measured in minutes or tens of minutes in some cases), during which the cells begin autolysis or are otherwise degraded from their original state. Drosophila requires specific preparation methods for TEM because most fly tissues are surrounded by significant diffusion barriers. In the embryo, it is the vitelline envelope, and in larvae and adults, it is the cuticle. In this article, we discuss methods that have evolved to cope with these barriers to achieve reasonable preservation of ultrastructure.