What is often used to fix tissues before applying hematoxylin?

Formalin is the most commonly used fixative in histopathology for preserving tissues before applying stains such as hematoxylin. It is a solution of formaldehyde gas in water, often diluted to about 10% for use in tissue fixation. Formalin penetrates tissues well, cross-linking proteins and stabilizing cellular structures, which is essential for preserving the shape and integrity of the tissue during processing.

The fixation process is crucial for proper staining, as it helps maintain the morphology of the cells and tissue architecture. Hematoxylin, a basic dye, requires well-preserved tissues to bind effectively to the nucleic acids within the cell nuclei, allowing for accurate visualization of cellular features under a microscope.

While other options, such as alcohol and acetone, are used in certain protocols, they are not the primary choice for tissue fixation prior to hematoxylin staining. Heat can be used in tissue processing but does not serve as a fixative. Therefore, formalin is clearly the preferred option in this scenario.