# Neurodegeneration of the mastoid segment of the human facial nerve associated with senescence

## TL;DR

This study shows that the facial nerve in older adults undergoes structural changes, including reduced myelination and smaller axon sizes, suggesting age-related neurodegeneration.

## Contribution

The study provides new morphometric evidence of age-related neurodegeneration in the mastoid segment of the human facial nerve.

## Key findings

- Myelination of the facial nerve decreases progressively with aging.
- Older age groups show a reduction in axon cross-sectional area and large myelinated fibers.
- The number and density of nerve fibers remain unchanged despite other age-related changes.

## Abstract

•There was a progressive reduction in myelination of the facial nerve with aging.•Reduction in the cross-sectional area of facial nerve axons occurs at older ages.•Increase in the quantity of small myelin fibers at older ages.•Decrease in the quantity of large myelin fibers at older ages.•The number and density of nerve fibers were not altered during aging.

There was a progressive reduction in myelination of the facial nerve with aging.

Reduction in the cross-sectional area of facial nerve axons occurs at older ages.

Increase in the quantity of small myelin fibers at older ages.

Decrease in the quantity of large myelin fibers at older ages.

The number and density of nerve fibers were not altered during aging.

To describe and analyze the microscopic morphometry of the mastoid segment of the human facial nerve in the region of the stylomastoid foramen, in different age groups of elderly individuals.

Eighteen blocks of temporal bones from cadavers aged between 63 and 94 years old were used and divided into 3 age groups: 60–70, 71–80 and over 80-years-old. The nerve was collected, fixed in a glutaraldehyde and paraformaldehyde solution and then subjected to dehydration and resin embedding processes. Semi-thin cross-sectional sections (1 μm) were obtained using an ultramicrotome and stained with toluidine blue. Histological images were captured and analyzed using Zen 2.6 software. The variables were described by mean and standard deviation and compared between age groups (SPSS; p < 0.05).

The Cross-Sectional Area (CSA) of the nerve, diameter of the myelinated fiber, CSA of the myelinated fiber, thickness of the myelin sheath and percentage of total area occupied by myelinated fibers were lower in the groups with older ages, with significant differences among all age groups (p < 0.001). The total number of myelinated fibers and density remained constant and did not present significant differences between age groups. CSA and axon diameter decreased at older ages, with significant differences only between the second and third groups (p < 0.001). The degree of myelination (g ratio) and the percentage of area occupied by the endoneurium, unmyelinated fibers and degenerating tissue were higher in the older age groups (p < 0.001). In the older age groups, there was an increase in the percentage of myelinated fibers with lower CSA and a reduction in the percentage of myelinated fibers with higher CSA.

The results indicate that the facial nerve presents morphometric variations suggestive of neurodegeneration, associated with aging.

Level V.