Histopathological examination studies tissues under the microscope. During this study, the pathologist looks for abnormal structures in the tissue. Tissues for histopathological examination are obtained by biopsy.
Biopsy is a tissue sample from a living person to identify the disease. Biopsy can be either incisional or excisional.
Once the tissue is removed from the patient, it has to be immediately fixed by putting it into adequate amount of 10% Formaldehyde (10% formalin) before sending it to the pathologist.
The purpose of fixation is:
- To prevent autolysis and bacterial decomposition and putrefaction
- To coagulate the tissue to prevent loss of easily diffusible substances
- To fortify the tissue against the deleterious effects of the various stages in the preparation of sections and tissue processing.
- To leave the tissues in a condition this facilitates differential staining with dyes and other reagents.
Once the tissue arrives at the pathology department, the pathologists will exam it macroscopically (i.e. naked-eye examination of tissues). Then the tissue is processed to make it ready for microscopic examination. The whole purpose of the tissue processing is to prepare a very thin tissue (i.e. five to seven μm or one cell thick tissues) which can be clearly seen under the microscope. The tissue is processed by putting it into different chemicals. It is then impregnated (embedded) in paraffin, sectioned (cut) into thin slices, and is finally stained. The stains can be Hematoxylin/Eosin stain or special stains such as PAS, Immuno-histochemistry, etc…
The Hematoxylin/Eosin stain is usually abbreviated as H&E stain. The H and E stain is routinely used. It gives the nucleus a blue color & the cytoplasm and the extra cellular-matrix a pinkish color. Then the pathologist will look for abnormal structures in the tissue.
And based on this abnormal morphology he/she will make the diagnosis. Histopathology is usually the gold standard for pathologic diagnosis