The nature of these sites is altered as methyl groups are added to tissue molecules. Cationic tissue groups are softened and anionic tissue groups are hardened. Tissues immersed in formalin for 4 hr were irradiated for 1.5 min. Consistent microscopic results were evident throughout the block, even in the central portion. Reducing atmospheric pressure to increase the rate of fixation with formaldehyde contradicts the generally accepted rule that reactions increase with increases in pressure.
Free water is removed in a simple exchange process by diffusion.
Bound water, which is attached to tissue molecules by hydrogen bonds, may be removed during excessive dehydration. Such excessive dehydration causes tissue to become hard, brittle, and difficult to section. In addition, some lipids are also dissolved. The efficiency of the dehydrating agent depends on its hydrogen bonding strength and molecular weight. A powerful hydrogen bonding dehydrant that competes with tissue molecules, will more readily remove bound water, and a smaller molecular size penetrates the tissue more readily.