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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...

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This week’s interesting image shows the examination of skeletal muscle in cross-section at the level of the A zone by electron microscopy in Figure 1, and corresponding illustration of the sarcomere in longitudinal section, Figure 2. Figure 1. Ultrastructure of sarcomere bundles.  The contractile elements of skeletal muscle are seen in cross-section at the level where myosin thick filaments and actin thin filaments overlap at the "anisotropic" or "A" zone of the sarcomere.  Electron Microscopy (EM): 25000x original magnification. Figure 2.  Illustration of sarcomere bundles.  The sarcomere contractile apparatus viewed in longitudinal section shows various degrees of overlap of the...

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Can you identify the structure(s) identified by arrows in this muscle biopsy?   Tangential sections through three triads (pale T-tubules and darker adjacent sarcoplasmic reticulum ….. "Oreo cookie-like") in association with sarcomeres (electron microscopy 15000x magnification). T-tubules distribute the surface action potential into the interior of the muscle fiber, where it triggers release of calcium stored in the adjacent sarcoplasmic reticulum (i.e. excitation-contraction coupling).  Release and reuptake of calcium by the sarcoplasmic reticulum is critical to muscle contraction and relaxation.  Note the location of the triads adjacent to thick filaments where calcium is utilized for contraction. 

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Contraction band artifact (Hematoxylin and Eosin stained section from formalin fixed paraffin embedded tissue 400x magnification)   This skeletal muscle biopsy shows contraction band artifact.  This artifact is caused by prolonged ischemic time prior to fixation.  To avoid this artifact, the best practice is to triage and submit the skeletal muscle tissue into it's appropriate "fixative" as soon as possible (i.e. snap freeze, formalin, or glutaraldehyde).   Contraction band artifact (electron microscopy 500x magnification)

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This skeletal muscle biopsy shows grouped atrophy, which is a finding characteristic of denervation.  This morphologic alteration indicates the presence of prior denervation and subsequent successful re-innervation.  The surrounding muscle fibers often show enlarged least mean fiber diameter due to compensatory hypertrophy.  (Image:  Hematoxylin and eosin-stained frozen section, 400x magnification)  

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What is the dark staining structure?  (Hint: note the small nerve twig associated with the structure).   The neuromuscular junction (NMJ; AKA myoneural junction or motor end-plate) is the synapse between the nerve and muscle.  It has three basic components: 1) presynaptic motor nerve terminal, 2) synaptic space (synaptic cleft), and 3) the postsynaptic surface of the skeletal muscle fiber. It is important to note that the nerve ending does not come into direct contact with the surface of the muscle fiber.  Release of the neurotransmitter acetylcholine from the presynaptic nerve terminal into the synaptic space is triggered by the...

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This week's Neuro Notes shows ultrastructural examination of a muscle biopsy can sometimes yield additional useful information, such as evaluation of these small nerve twigs within the endomysial connective tissue. Electron photomicrograph, 6000x original magnification.

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This previously healthy pediatric patient presented with peculiar facial muscle hypertrophy, and genetic testing revealed one pathologic variant and two variants of uncertain significance (VUSs).  A muscle biopsy of the thigh was performed to further evaluate this patient.  Frozen sections and ultrastructural examination images of the inflammatory infiltrate are shown below.  What is your diagnosis?     ​ ​   ​   ​ ​   ​   ​   ​   ​   ​   ​   ​   ​   ​   ​   ​   ​   ​   ​   ​   ​   ​   ​...

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Additional unintended useful information can be obtained from many histochemical stains used in the routine evaluation of skeletal muscle biopsies.  In addition to identifying amyloid deposition, Congo red stain provides some morphologic detail.  In this case, granular brown lipofuscin pigment can be seen in the atrophic muscle fibers under routine light microscopy.  The lipofuscin pigment is also autofluorescent, showing a white-yellow color under fluorescence microscopy. Congo Red stained frozen section of skeletal muscle under routine light microscopy (top) and fluorescence microscopy with FITC filter (bottom); original magnification: 400x each.  

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