Paraffin embedding and sectioning
Biological samples often need to be solidified to allow fine sectioning. Thin slices improve the access of dyes, probes, and antibodies and reduce the overlay of different cells layers in the z direction. For light microscopy, paraffin wax is the most frequently used hard matrix for cutting.
Since paraffin is immiscible with water, the main constituent of tissue, samples need to be dehydrated by progressively more concentrated ethanol baths. This is followed by a clearing agent, usually xylene, to remove the ethanol. Finally, molten paraffin wax infiltrates the sample and replaces the xylene.
Paraffin sections for light microscopy are typically 5 μm thick. Paraffin wax does not provide a sufficiently hard matrix for cutting thinner slices needed for electron microscopy, typically 80-100 nm thick. Here, epoxy or acrylic resins especially for immunohistochemistry are used.