Membrana limitans shape variation and its role in vestibular form

Abstract

The membrana limitans (ML) is a membranous vestibular structure found within the bony labyrinth, housed in the petrous temporal bone. It is located inferior to the semicircular canals (which detect changes in angular acceleration of the head) and adjacent to both otolith organs, the utricle and saccule (responsible for detecting linear acceleration of the head and gravity). The ML forms the keystone to vestibular geometry, acting as a support for the majority of the utricular macula and respective neural substrate, and provides a structural boundary between the superior (pars superior) and inferior (pars inferior) portions of the vestibular labyrinth. Despite its importance, knowledge of ML 3D structure and morphological variation is limited. Here we test the hypothesis that there is consistency among ML bony attachment sites in humans. We use a sample comprised of micro-CT scans of nine human temporal bones soaked in 2 per cent osmium tetroxide after fixation in Karnovsky's fixative in order to better visualize the membranous labyrinth. Each ML was 3D reconstructed and the extent of bony attachment site variation was assessed using landmark-based Generalized Procrustes and Principal Component Analyses. Results indicate a consistent, yet locally variable configuration with distinct anterolateral, medial and posterior attachments of the ML to the bony wall of the labyrinth. The anterolateral attachment originates just superior to the posteriormost edge of the stapes footplate and stretches anteriorly. This is often a discontinuous attachment (observed in seven of nine ears) which occasionally attaches onto the posterior one-third of the footplate itself (observed in two of nine ears). The length of the medial attachment is the most variable, yet in all ears it attaches along the superior border of the spherical recess which houses the saccule. The posterior attachment is the least variable and its medial boundary forms a consistent attachment to the bony lip of the vestibular aqueduct (observed in all ears). These results also support previous findings suggesting the ML forms a discontinuous support system within the vestibule among humans. This exploratory analysis sets the stage for further understanding into utricular biomechanics and vestibular geometry, especially related to details of otolith organ structure and their relationships to bony anatomy.