Golgi stain

The Golgi stain is a laboratory recipe that turns a tiny fraction of brain cells solid black, so that single neurons stand out sharply against a clear, pale background. Brain tissue is soaked first in a chromium salt and then in silver nitrate; inside a few scattered cells, these two chemicals react to form dark crystals of silver chromate that fill the cell completely. The strange and lucky part is that this only happens to maybe one cell in a hundred, seemingly at random. If every neuron were blackened, the slice would look like a solid ink blot and you could see nothing; because only a sprinkling light up, each one can finally be traced on its own.

Why this matters: a neuron is not a simple blob but a branching tree, with a cell body, a bushy crown of fine receiving branches called dendrites, and a long output cable. The Golgi stain fills all of these with black, revealing the full dendritic arbor — the spreading, antler-like shape of a single cell — in a way no earlier method could. Invented by the Italian scientist Camillo Golgi in 1873, it became the favorite tool of Santiago Ramón y Cajal, who used it to draw thousands of neuron shapes and to argue that the brain is built from separate cells touching one another, not one continuous web. The two men shared the 1906 Nobel Prize, and this single black-and-clear trick is a big reason we know what neurons look like at all.