The use of percutaneous muscle biopsy represents one of the few ways in which the effects of aging on human muscles can be studied at the biochemical, molecular, or cellular level. A single biopsy can yield 25 to 75 milligrams of tissue, which can then be analyzed in multiple ways including via microscopy or biochemical assays (Coggan, 1995).
Muscle biopsy involves the use of a biopsy needle, such as the one developed by Bergstrom, which consists of a closed hollow cylinder with a pointed tip (Coggan, 1995). A cylindrical cutting blade fits inside the first cylinder and is responsible for obtaining the biopsy. An opening a few centimeters from the tip in the outer cylinder serves as the location where the actual biopsy is taken since tissue can bulge into the center of the cylinder through the opening when the blade is removed. Reinsertion of the blade then cuts the protruding tissue and obtains the biopsy.
Technically, biopsies are performed on patients by first injecting local anesthetic into the skin and fascia over the muscle of interest. The vastus lateralis is the most commonly used muscle, but the biceps, tibialis anterior, and gastrocnemius have also been used (Coggan, 1995). Following anesthesia, a scalpel is used to make a small incision in the skin and the needle is then inserted into the muscle belly. During the insertion of the needle, the cutting blade is placed inside to occlude the opening in the needle. Once placed, the blade is removed and then reinserted to obtain the biopsy. After removal of the needle, a solid rod is inserted into the center of the blade cylinder to eject the obtained tissue. Post biopsy, pressure is held at the side for 30 minutes to minimize bleeding, and the incision is closed with a suture or tape dressing like a Steri-strip. The biopsy samples are either fixed and embedded if the ultimate goal is either light or electron microscopy, or frozen if the ultimate goal is biochemical or functional analysis.
The risks of biopsy include infection and intramuscular hematoma, which are both low when the biopsy is performed in sterile fashion and pressure is held for adequate time (Coggan, 1995). For most patients the biopsy produces only mild discomfort, consisting of a pressure or cramp-like sensation when the needle is inserted into the muscle and a few days of muscle soreness after the biopsy.
The biopsy samples have a wide variety of uses, which include analysis of morphology, assessment of metabolism, analysis of gene or protein expression, culture of satellite cells, and evaluation of contractile properties of muscle fibers (Coggan, 1995; Peterson, 1995). Morphologic analysis can include examination of the numbers and size of type I and type II fibers or the examination of specific cellular organelles like the contractile apparatus by either light or electron microscopy. Metabolic assessment can involve the measurement of specific enzyme activities, the measurement of glucose, triglyceride, and protein levels, the measurement of intermediate metabolite levels, or the measurement of cellular energy stores like ATP or phosphocreatine. Gene or protein expression can be analyzed at the single gene or protein levels through use of Northern or Western blotting. Biopsy samples would also be amenable to modern genomic or proteome techniques like microarray or 2D gel electrophoresis to identify differentially expressed genes (Roth et al., 2002; van den Heuvel et al., 2003). Biopsy samples also can serve as a source for human satellite cell myoblasts for use in cell culture experiments, as discussed in a later section (Peterson, 1995). Finally, the contractile properties of single fibers from biopsy samples can be functionally assessed by the measurement of maximal force or shortening velocity (Coggan, 1995).
An important limitation of muscle biopsy is the observed sample-to-sample variation seen even in biopsies taken from the same muscle in the same patient (Coggan, 1995). The variation is probably due to differences between the areas of the muscle sampled by individual biopsies. For example, 10 to 20% variation has been observed in measures of fiber type or muscle fiber area. Ways to overcome this limitation are either to take two or more biopsies at the same time to obtain a more accurate measurement for an individual, or, if the goal of a study is to compare two or more groups at a population level, then increasing the number of subjects would also be an effective way to cope with the variability of individual biopsies.
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