Mechanical fixation techniques for processing and orienting delicate samples, such as the root of Arabidopsis thaliana, for light or electron microscopy

Despite improvements in live imaging, fixation followed by embedding and sectioning for light or electron microscopy remains an indispensible approach in biology. During processing, small or delicate samples can be lost, damaged or poorly oriented. Here we present a protocol for overcoming these issues when, along with chemical fixation, the sample is fixed mechanically. The protocol features two alternatives for mechanical fixation: the sample is encased either in a rectangular block of agarose or between Formvar films suspended on a wire loop. We also provide methods for key steps all the way through to sectioning. We illustrate the method on the root of Arabidopsis thaliana, an object that is similar to 0.15 mm in diameter and difficult to process conventionally. With this protocol, well-oriented sections can be obtained with excellent ultrastructural preservation. The protocol takes about 1 week.