Histopathology of Skeletal Muscle Degeneration: A Review, Study notes of Pathology

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Figure Legend: Figure 1 Skeletal muscle - Degeneration in a male Harlan Sprague-Dawley rat from a subchronic study. A central myofiber is swollen and hypereosinophilic (arrows), and a fragmented segment of another fiber (arrowhead) demonstrates segmental degeneration. Figure 2 Skeletal muscle

• Degeneration in a male Harlan Sprague-Dawley rat from a subchronic study. An enlarged, hypereosinophilic muscle fiber with subtle vacuolation is present in an otherwise normal muscle bundle. Figure 3 Skeletal muscle - Degeneration in a male Harlan Sprague-Dawley rat from a subchronic study. A rounded fiber contains multiple peripheral nuclei, a single internal nucleus, and abundant cytoplasmic vacuoles. Figure 4 Skeletal muscle - Degeneration in a male Harlan Sprague-Dawley rat from a subchronic study. Multiple macrophages and interstitial cells have phagocytized degenerative portions of an enlarged muscle fiber. Figure 5 Skeletal muscle - Degeneration in a male F344/N rat from a chronic study. In this cross section of muscle, several adjacent muscle fibers exhibit multiple features of degeneration; increased sarcolemmal nuclei are indicative of an early regenerative response. Figure 6 Skeletal muscle - Degeneration in a male Harlan Sprague-Dawley rat from a subchronic study. Degeneration is represented by a swollen, hyalinized, and partly fragmented muscle fiber. Figure 7 Skeletal muscle - Degeneration in a male Harlan Sprague-Dawley rat from a subchronic study. A fragmented and partly hyalinized muscle fiber has lost its striations and is accompanied by early infiltration of macrophages.

Comment: Degenerated muscle can grossly appear either pale or dark. Histologically, degenerating myofibers can exhibit a variety of microscopic changes, including cell swelling, hypereosinophilia,

inflammation and hemorrhage, should not be diagnosed separately unless warranted by severity but should be described in the narrative.

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Author:

Torrie A. Crabbs, DVM, DACVP Pathologist Experimental Pathology Laboratories, Inc. Research Triangle Park, NC