A biopsy describes the procedure that is used to obtain a very small piece of the target tissue. For some tissues, like the lining of the cheek, cells can be obtained just by scrapping the tissue surface. Other samples are collected using forceps that are positioned at the end of an optical device called an endoscope. The physician can view the tissue surface (such as the wall of the large intestine) through the endoscope and use the forceps to pluck tissue from the desired region of the surface. In other cases, the tissue sample needs to be collected as a "plug," using a large hypodermic needle. Examples of the latter include liver or kidney biopsy samples. Samples of muscles and nerves can also be obtained by cutting out a small piece of the target once an incision has been made.
When a biopsy is obtained using a needle, the retrieval of a sample relies on the design of the needle and the energy of its insertion into the tissue. The needle used is a hollow tube with a sharp point capable of puncturing tissue. As the needle is driven deeper into a tissue following puncture, tissue will accumulate in the hollow tube. When the needle is withdrawn from the tissue, the plug of tissue remains in the needle tube and can be retrieved for analysis.
Many biopsy samples are examined using a light microscope to look for abnormalities in the tissues cells. This examination can involve the staining of the sample to specifically detect target molecules. As well, samples can be used for various biochemical tests, and even to test for the presence and activity of particular genes.
A biopsy can remove the entire target region (excisional biopsy) or can remove just a small portion of the target region (incisional biopsy). The latter can be done in three different ways, depending on the sample. A shave biopsy slices off surface tissue. Samples collected by piercing the tissue with a needle represent a punch biopsy. Finally, in fine needle aspiration, a needle is inserted and tissue is subsequently withdrawn into the needle using a syringe.
Brian Douglas Hoyle PhD, The Gale Group Inc., Gale, Detroit,