part of muscle system tonofibrilla Attachment fibers that are embedded in the cuticle, and attach the tendon cells to it. Tonofibrils are closely associated with hemidesmosomes found in the external, apical face of the tendon cell that abuts the cuticle. Tonofibrils are made up of dense extracellular material. fly_anatomy.ontology FBbt:00005092 acellular anatomical structure