Abstract
It is well known that the phonatory process changes during the life span. However, detailed investigations on potential factors concerned are rare. To deal with this issue, we performed extended biomechanical, macro anatomical, and histological analyses of the contributing laryngeal structures in ex vivo juvenile sheep models. Altogether twelve juvenile sheep larynges were analyzed within the phonatory experiments. Three different elongation levels and 16 different flow levels were applied to achieve a large variety of phonatory conditions. Vocal fold dynamics and acoustical and subglottal signals could be analyzed for 431 experimental runs. Subsequently, for six juvenile larynges microcomputed tomography following virtual 3D reconstruction was performed. The remaining six juvenile larynges as well as six ex vivo larynges from old sheep were histologically and immunohistologically analyzed. Results for juveniles showed more consistent dynamical behavior compared to old sheep larynges due to vocal fold tissue alterations during the life span. The phonatory process in juvenile sheep seems to be more effective going along with a greater dynamic range. These findings are supported by the histologically detected higher amounts of elastin and hyaluronic acid in the lamina propria of the juvenile sheep. The 3D reconstructions of the thyro-arytenoid muscles (TAM) showed a symmetrical shape. Intraindividual volume and surface differences of the TAM were small and comparable to those of aged sheep. However, TAM dimensions were statistically significant smaller for juvenile larynges. Finally, topographical landmarks were introduced for later comparison with other individuals and species. This work resulted in detailed functional, immunohistological, and anatomical information that was not yet reported. This data will also provide reference information for therapeutic strategies regarding aging effects, e.g. laryngeal muscle treatment by functional electrical stimulation.
Highlights
In vivo investigation of the larynx and especially the vocal folds (VF) is limited due to their small size, their inaccessibility in the in vivo state, and the sensitivity of the microarchitecture
In the tissue section of the old sheep, smaller gaps in the hyaluronic acid tissue distribution are observable in contrast to the very even distribution in juvenile sheep
More fat inclusions being rather randomly scattered was observable in the aged sheep. These results show that old sheep exhibit (1) stiffer collagen network due to thicker and shortened fibers; (2) lower tissue viscoelasticity due to reduced HAlevel, resulting in reduced water storage and in reduced ability to shock absorption due to the constant trauma caused by the vibratory actions of phonation; (3) increased inertance yielding a reduced dynamic range; (4) more random distributed fat inclusion that may to a certain point explain the reduced systematic behavior in the phonatory experiments; (5) reduced elastin concentration, yielding less changes in vocal fold elongation and less overall accurately dynamical measurable differences
Summary
In vivo investigation of the larynx and especially the vocal folds (VF) is limited due to their small size, their inaccessibility in the in vivo state, and the sensitivity of the microarchitecture. Image-based investigation is constricted by the limited accessibility [1]. Biopsies of the laryngeal structures (mucosa, muscles) carry the risk of long-lasting damage of these, going along with a hoarse and breathy voice (dysphonia). Most phonatory investigations in humans are restricted to acoustic analysis (perception and quantitative analysis), electroglottagraphy (EGG), and visualization techniques imaging the superior VF surfaces [3, 4]. The complex microarchitecture of the VF (consisting of multi-layered epithelium and the layered lamina propria) as well as the underlying muscle structures can hardly be visualized [5, 6] and not quantified. The muscular structures play a crucial role in coordinating the BioMed Research International
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