Histology slides are made by:

1. fixing (preserving) tissue
2. embedding it in a medium that will withstand thin slicing
3. laying the slices on a glass slide
4. staining the thin slice
5. covering the stained slice with mounting medium and a coverslip, then letting the new slide dry and harden.

There are many fixatives, but the most common is 10% buffered neutral formalin, the smelly stuff you associate with dissections. Fixation requires 1-2 days in solution, preferably with an abundance of fixative, and with the samples cut small enough to permit thorough penetration of the aldehyde molecules. The usual choice for embedding is paraffin, a close relative of plain old candle wax. In order to take tissues soluble in molten wax, they are dehydrated in alcohol, then xylene, then a solution of wax dissolved in xylene.

The device that cuts thin sections of tissue mounted in paraffin is called a microtome. Most of our class set has been cut at a thickness of 5 micrometers.

After sectioning, the thin samples are laid on a water bath, then picked up onto glass slides.

The wax is removed from the sample by a brief bath in xylene, then the tissue is stained, to permit the human eye to resolve features of value. The usual stain is a combination of hematoxylin and eosin. These will be discussed in more detail below.

After staining, the specimens are dehydrated again, returned to xylene, then coated with a permanent mounting medium which is optically clear, to bond the glass, tissue and coverslip together.

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