Zenkers fixative

Zenker's fixative is a rapid-acting solution used to prepare animal and plant tissues for microscopic study. It effectively fixes nuclear chromatin, connective tissue fibers, and some cytoplasmic features but does not preserve delicate organelles like mitochondria, for which Helly's fixative is more suitable. Zenker's fixative permeabilizes the plasma membrane but not the nuclear membrane, allowing selective staining of mitotic cells using antibodies against chromatin.

The fixative contains mercuric chloride (corrosive sublimate), potassium dichromate, sodium sulfate, water, and acetic acid. These components are toxic, making disposal hazardous and costly. Mercuric chloride can be replaced with zinc chloride to reduce toxicity, though this may compromise fixation quality.

Named after German histologist Konrad Zenker, who died in 1894, the fixative is typically prepared by mixing 50g of mercuric chloride, 25g of potassium dichromate, and 10g of sodium sulfate in distilled water to make 1000 ml. Before use, 5 ml of glacial acetic acid is added to 100 ml of this stock solution. Both the stock and complete Zenker fixative are stable for many years.

Helly's fixative, a variation, replaces acetic acid with formalin (37–40% formaldehyde). It is less stable, reacting within hours to form formic acid and chromium(III) ions, which changes its color from orange to greenish.