Fixation and Sectioning of Tissues:    Fixation makes cells permeable to staining and cross links their macromolecules so that they are stabilized. The most common fixative is formaldehyde which cross links proteins.

After fixation, tissue samples that are too thick for their individual cells to be examined are cut into very thin slices or sections. Vibratomes are useuly to cut weakly fixed tissue and microtomes are useful to cut frozen tissues.

Staining: Since most of the weight of cells is water, there is little to impede light rays of a microscope. Thus most cells in their natural state are almost invisible to an ordinary light micrscope. One way to make these cells visible is to stain them. Nucleic acids which are negatively charged can easily be stained with cationic dyes such as hematoxylin or nissl.