Preparation of Tissues for Study

     The most common procedure used in the study of tissues is the preparation of histological sections or tissue slices that can be studied with the aid of the light microscope. Under the light microscope, tissues are examined via a light beam that is transmitted through the tissue. Because tissues and organs are usually too thick for light to pass through them, they must be sectioned to obtain thin, translucent sections and then attached to glass slides

     Most tissues studied histologically are prepared as shown. (a): Small pieces of fresh tissue are placed in fixative solutions which generally cross-link proteins, inactivating degradative enzymes and preserving cell structures. The fixed pieces then undergo "dehydration" by being transferred through a series of increasingly more concentrated alcohol solutions, ending in 100% which effectively removes all water from the tissue. The alcohol is then removed in a clearing solution miscible in both alcohol and melted paraffin. When the tissue is then placed in melted paraffin at 58°C it becomes completely infiltrated with this substance. All steps to this point are commonly done today by robotic devices in active histology or pathology laboratories. After infiltration the tissue is placed in a small mold containing melted paraffin, which is then allowed to harden. The resulting paraffin block is trimmed to expose the tissue for sectioning (slicing). Similar steps are used in preparing tissue for transmission electron microscopy, except that smaller tissue samples are fixed in special fixatives and dehydrating solutions are used that are appropriate for embedding in epoxy resins which become much harder than paraffin to allow very thin sectioning.