The stages below are only described in outline. Laboratories which do histology work from schedules which are much more detailed.
Chemical fixatives are used to preserve tissue from decay. This preserves the structure of the cell and of sub-cellular components such as cell organelles (e.g., nucleus, endoplasmic reticulum, mitochondria). The most common fixative for light microscopy is
formalin (4% formaldehyde in saline).
After fixing, the block of tissue is embedded in
paraffin wax. This holds and preserves the tissue as a block.
The section is cut into a series of wafer-thin slices, each of which is put on a glass
microscope slide. The machine which cuts the block is a mechanical guillotine which can be set to cut at a suitable depth for the tissue in question.
Stains are dyes, chemicals used to make cells and tissues easy to see under a microscope. There are many tissue stains, and each of them has advantages and disadvantages.
Haematoxylin and eosin (H&E)
This is the most widely used stain in biology and medicine.
Haematoxylin colours cell nuclei and
eosin colours cell cytoplasm.
Camillo Golgi developed a silver nitrate stain for nerve cells. His idea was used by Santiago Ramón y Cajal in his famous work on the structure of brain tissue.