Fine Structure of the Tendon of the Human Stapedius Muscle: Comparison of Normals and Clinically Otosclerotic Patients

Abstract

The tendon of the human stapedius muscle was studied in normal post mortem material and in clinically otosclerotic patients, using light and electron microscopy. Cross section profiles of collagen fibrils were measured in various regions of the tendon and the amount of elastin was estimated. The normal stapedius tendon consisted of three concentrically arranged portions: A cylindrical central part, a tube-like mid-portion, and a cortical layer. The central part was made up of collagen fibrils with only a few elastic fibers, the mid-portion contained collagen fibrils together with significantly more elastic material, while the cortical layer, again, showed a smaller amount of elastic fibers. Mean diameters of collagen fibrils in the central part of the tendon were 65.12 ± 11.89 nm, in the intermediate layer 41.00 ± 9,63 nm, and in the cortical layer 70.28 ± 19.58 nm. Stapedius tendons from clinically otosclerotic patients, though showing the same construction, were characterized by significantly altered collagen fibrillar diameters (Mann-Whitney U-test). In the central part, mean diameters were reduced to 61.05 ± 14.70 nm, in the mid-portion increased to 50.90 ± 10.08 nm, and in the cortical layer reduced to 61.09 ± 8.49 nm. The changes of collagen cross section profiles estimated for the entire tendon were significant as well: 59.68 ± 18.74 nm in controls versus 57.82 ± 12.53 nm in otosclerotic patients. Elastin content in the mid-portion of control stapedius tendons increased with age (13% at 35 years of age to 35% at 70 years of age). In contrast, the amount of elastin constantly figured for about 10% in clinically otosclerotic patients in all age groups studied. Strain changes in otosclerotic patients were assumed to cause the remodelling of the stapedius tendon. Decrease of elastin in tendons of otosclerotic individuals is discussed with respect to the decrease of elastic material in the stapedovestibular joint.

Key Words:

• transmission electron microscopy
• collagen
• elastin
• morphometry