The decision to use a preservative or fixative on your tissue will very much depend on the methods of sectioning and processing you will use, the data you ultimately wish to collect, desired length of storage, and on the quality, age and inherent characteristics of your tissue. As with many experimental techniques, it is necessary to allow time to optimize your protocol to your tissue.
Ethanol is a very good fixative, when used in various proportions with water. Proportions that include less than 50/50 ethanol/water introduce the possibility of pathogen growth in your tissue. This is a possibility even with short-term tissue storage, if the percentage of ethanol is low. However, such preparations are acceptable if tissue is to be sectioning within 7 days; in such a case, refrigeration of the tissue in the fixative is recommended. Proportions of more than 75% ethanol introduce the danger of making the tissue hard or rubbery, which can make it very difficult to get thin sections by hand. This proportion is acceptable if tissue is to be sectioned within 2-4 weeks.
As an alcohol, ethanol tends to dehydrate and contract tissue to a certain degree. This can be avoided by adding a small amount of glacial acetic acid (~ 5% of the total volume). The classic plant fixative FAA (Formaldehyde Alcohol Acetic Acid, 10%:50%:5% + 35% water) can be used successfully. Advantages include rapid and thorough penetration of tissue by formaldehyde, long-term storage possibilities, and balancing of tissue contraction/expansion between the alcohol and acid. Further, formaldehyde is a non-coagulating fixative that chemically cross-links cellular constituents, allowing for preservation of structure. Disadvantages include the toxicity of formaldehyde.
For further information, researchers are referred to the excellent text by Steven E. Ruzin. (Plant Microtechnique and Microscopy, ISBN: 13 978-0-19-508956-1)
