Golgi Tendon Organ

In reviewing this skeletal muscle biopsy, what is this structure and what is its function? (Hematoxylin and Eosin stained FFPE tissue section, original magnification:  400x)

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Answer: This is the Golgi Tendon Organ (GTO), a normal finding in skeletal muscle, and it serves as a proprioceptive receptor that senses tension within tendons.  Note the small nerve twig entering the structure from the left (9 o’clock).