Brain tissue is commonly embedded in paraffin for microscopic examination.
Microscopic examination is a means of seeing things not visible to the naked eye, such as microorganisms and cells. In order for specimens to be viewed using a microscope, they must first be mounted on a glass slide. Types of mounts include dry mounts, wet mounts and prepared mounts. Paraffin embedding is a technique used for making prepared mounts.
Purpose
Viewing a specimen using a microscope requires that the specimen be thin enough to allow light to pass through it. Many tissues, such as those from the brain, liver and kidneys, are thick and must be cut into thin slices prior to being examined. Paraffin embedding is used to prepare the the tissue specimen so it does not shred upon slicing. The tissue specimen is infused with paraffin (wax) and placed within a mold to which more melted paraffin is added. The result is a small cube of wax with the paraffin-filled tissue specimen within it. Tissue embedding is routinely used to prepare tissue biopsies for examination. The tissue embedding process involves several steps including fixation, dehydration, embedding and sectioning.
Fixation
When a piece of tissue is removed from the body, it immediately begins to die. As a result, the cells of which it is made begin to deteriorate, making accurate microscopic evaluation impossible. Fixation is a method of preserving the cells and is accomplished by placing the specimen in a liquid fixative, usually formaldehyde. The fixative works by destroying the chemicals responsible for cell breakdown. The tissue sample remains in the fixative for at least 4 hours.
Dehydration
Following fixation, the piece of tissue is rinsed with water to remove any remaining fixative. Water and paraffin do not mix. Therefore, all traces of water need to be removed from the rinsed tissue, a process called dehydration. Dehydration is performed by placing the tissue specimen in a series of alcohol baths of increasing strengths. Dehydration is not the same as drying. It is replacing the water with another liquid that, unlike water, will mix with paraffin. The dehydration process takes approximately 15 hours.
Embedding
Once the tissue is dehydrated, it is placed into pure melted paraffin for 3 to 4 hours, resulting in a specimen that has been completely infiltrated by the wax. In other words, the paraffin fills all the nooks and crannies within the tissue that used to be filled with water. The infiltrated tissue is placed into a block-shaped mold. Melted paraffin is added to the mold and it is allowed to cool. The process results in a block of wax with a piece of paraffin infiltrated tissue embedded in it. The specimen is now ready to be sliced.
Sectioning
Sectioning is the term used for tissue slicing and is performed using an instrument called a microtome. The microtome is similar to a meat slicer except that rather than using a back and forth motion, a circular wheel is turned that advances the paraffin block as it is being sliced. The slices are placed on glass microscope slides for staining and examination. Sectioning requires great skill. The specimen slices are so thin that it is difficult to transfer them to the glass slide without them tearing, creasing or folding. An accurate examination is possible only if the specimen has been properly prepared, embedded and sectioned.
Tags: tissue specimen, glass slide, infiltrated tissue, melted paraffin, paraffin added, Paraffin embedding
