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- Published: January 15, 2021
- By: Kelly Mrachek, MD
What are these dark (osmophilic) structures seen in this electron micrograph of skeletal muscle? These osmophilic structures are Nemaline rods (largest, white arrow). These structures are classically seen in "Nemaline Myopathy" (NM; aka rod body myopathy). The rods are located in the sarcoplasm, and may also be seen within nuclei. NM is most commonly caused by dominant mutations of ACTA1 (encoding skeletal a-actin) and recessive mutations of NEB (encoding nebulin). Intranuclear rods are associated with pathogenic ACTA1 mutations. There are variable phenotypes of NM ranging from the severe congenital myopathy to adult-onset disease. Limited numbers of muscle fibers containing...