Editing Histology (section)

===Embedding===
Tissues are embedded in a harder medium both as a support and to allow the cutting of thin tissue slices.<ref name="Ross and Pawlina, 2016" /><ref name="Leeson and Leeson, 1981" /> In general, water must first be removed from tissues (dehydration) and replaced with a medium that either solidifies directly, or with an intermediary fluid (clearing) that is miscible with the embedding media.<ref name="Bancroft and Stevens, 1982" />

==== Paraffin wax ====
[[File:Tissue processing - Embedding station.jpg|thumb|300px|left|Histologic sample being embedded in paraffin wax (Tissue is held at the bottom of a metal mold, and more molten paraffin is poured over it to fill it.)]]

For light microscopy, [[paraffin wax]] is the most frequently used embedding material.<ref name="Bancroft and Stevens, 1982" /><ref name="Wick, 2019" /> Paraffin is immiscible with water, the main constituent of biological tissue, so it must first be removed in a series of dehydration steps.<ref name="Bancroft and Stevens, 1982" /> Samples are transferred through a series of progressively more concentrated [[ethanol]] baths, up to 100% ethanol to remove remaining traces of water.<ref name="Ross and Pawlina, 2016" /><ref name="Bancroft and Stevens, 1982" /> Dehydration is followed by a ''clearing agent'' (typically [[xylene]]<ref name="Wick, 2019" /> although other environmental safe substitutes are in use<ref name="Wick, 2019" />) which removes the alcohol and is [[miscible]] with the wax, finally melted paraffin wax is added to replace the xylene and infiltrate the tissue.<ref name="Ross and Pawlina, 2016" /> In most histology, or histopathology laboratories the dehydration, clearing, and wax infiltration are carried out in ''tissue processors'' which automate this process.<ref name="Wick, 2019" /> Once infiltrated in paraffin, tissues are oriented in molds which are filled with wax; once positioned, the wax is cooled, solidifying the block and tissue.<ref name="Wick, 2019" /><ref name="Bancroft and Stevens, 1982" />

==== Other materials ====

Paraffin wax does not always provide a sufficiently hard matrix for cutting very thin sections (which are especially important for electron microscopy).<ref name="Bancroft and Stevens, 1982" /> Paraffin wax may also be too soft in relation to the tissue, the heat of the melted wax may alter the tissue in undesirable ways, or the dehydrating or clearing chemicals may harm the tissue.<ref name="Bancroft and Stevens, 1982" /> Alternatives to paraffin wax include, [[epoxy]], [[Poly(methyl methacrylate)|acrylic]], [[agar]], [[gelatin]], [[Micro technique#Celloidin Method|celloidin]], and other types of waxes.<ref name="Bancroft and Stevens, 1982" /><ref name="Drury and Wallington, 1980" />

In electron microscopy epoxy resins are the most commonly employed embedding media,<ref name="Ross and Pawlina, 2016" /> but acrylic resins are also used, particularly where [[immunohistochemistry]] is required.

For tissues to be cut in a frozen state, tissues are placed in a water-based embedding medium. Pre-frozen tissues are placed into molds with the liquid embedding material, usually a water-based glycol, [[Optimal cutting temperature compound|OCT]], [[Tris-buffered saline|TBS]], Cryogen, or resin, which is then frozen to form hardened blocks.