Answer: The Weil stain is used for low-resolution identification of myelin in fixed tissue.
Myelin is a substance that wraps around the axons of neurons. This additional membrane creates an insulating effect that decreases the membrane resistance of the neuron, which allows the axons to send their electrical signals more quickly. Bundles of axons are called tracts, and bundles of myelinated axons are called white matter tracts. Myelinated brain tissue looks white because it is mostly made of fatty lipid membranes which reflect light. It is helpful anatomically for identifying white matter tracts since groups of neurons in certain brain areas often communicate with other neurons long distances away using white matter tracts.
In a cross section of the brain, it is possible to distinguish between two types of brain matter: The gray matter, where the neuronal cells are located, and the white matter, where the myelinated axonal tracts are located.
A Weil stain is an experimental anatomical technique that is used on thinly sliced sections of nervous tissue, including the brain or spinal cord. The Weil stain was first published in 1928, so it has been largely replaced by more advanced techniques that allow for higher resolution images to be captured. First, the tissue samples must be fixed with formalin, and then embedded in paraffin. Thin slices of tissue between 20 and 50 microns will allow the best staining.
The chemistry of the Weil stain relies on the chrome solution that the brain sections are immersed in. The reaction with the myelin sheaths creates chromium dioxide, which then reacts with a hematoxylin step. When the stain is completed, myelin will appear black which should stand out boldly against the clear or yellow background tissue.