The surgical pathologist does not often need to look at skeletal muscle biopsies on a daily basis. An exception to this is the neuropathologist, a type of pathologist that diagnoses disorders of the central and peripheral nervous system, and also including myopathies (diseases of muscle). Soft tissue and dermatopathologists also see muscle excisions. The rest of us often see muscle tissue as part of an excision or biopsy where we are looking for something else. Most of the time, the skeletal muscle is normal and we can glean over it at lower power magnification and not give it a second thought. But the normal muscle cell is one of my favorites to look at, and it can actually help you understand those diagrams that you see in anatomy and physiology textbooks. Look at this high power magnification of a skeletal muscle cell:
In the center of the photo you can see a pink lattice-like structure, and that is the skeletal muscle cell! It is seen in a longitudinal section. The nuclei are the purple elongated structures at the top and bottom of that cell. At the bottom of the photo is vein (notice its very thin wall) filled with red blood cells. But why does it have this lattice-like appearance? If you’ve had a basic anatomy/physiology class you may have seen a figure like this:
Those proteins called actin and myosin are what give the muscle cell its striated appearance. The crosslinking and release of these proteins occur during contraction and relaxation of the muscle. In the microscopic image above, there are no abnormalities.
Because most of the skeletal muscle we (working in general surgical pathology) see is normal, its beauty and organization is sometimes neglected. Just as a note for any future pathologists out there, the difference between the skeletal muscle and smooth muscle, is that the smooth muscle will not have any striations. Smooth muscle is everywhere in the body, from inside blood vessels, to our skin, bladder, gastrointestinal mucosa. So it is very important to be able to differentiate these types of muscle when grading things like a malignant tumor invasion.
—DLW