Table 1.
Specimen details. vocal fold length measured between insertion at thyroid cartilage and ventral tip of processus vocalis of arytenoid cartilage.
Figure 1.
Panthera vocal folds possess a large lamina propria.
A: Schematic frontal section through a larynx. The square indicates where the histological sections (Haematoxylin-eosin stain) in B, C and D were taken from. The mid-membraneous sections of the vocal fold are from Bengal tiger (B), Siberian tiger (C), and Lion (D). The terms ‘lateral’ and ‘medial’ are used in reference to the body axis and are indicated in D to explain how they apply to the vocal fold. In all three species, the vocal fold consists of epithelium, lamina propria and the thyroarytenoid muscle. LP: lamina propria; E: epiglottis; T: thyroid cartilage; TA: thyroarytenoid muscle; CT: cricothyroid muscle; C: cricoid cartilage; B: tracheal ring. Arrows in A indicate the airflow during expiration. Horizontal arrows in LP of B indicate medio-lateral depth. Vertical arrows in LP of B indicate cranio-caudal thickness.
Figure 2.
Fat cells are only found in a deep portion of the lamina propria.
Haematoxylin-eosin stain of a Bengal tiger vocal fold. A: Overview of a midmembraneous section indicating a densely packed lamina propria without fat cells medially (LP2) and with fat cells laterally (LP1). The approximate border between both regions is indicated by a dotted line. Bar indicates a 5 mm distance. Larger magnification of an area in LP1 region (B) and in LP2 region (C) are shown. Bars in B and C indicate a 100 µm distance. TA: thyroarytenoid muscle; LP: lamina propria; BV: blood vessel; FC: fat cells.
Figure 3.
Elastic fibers are densely packed in the medial Lion vocal fold.
A: Overview of a midmembraneous section (Elastica-van-Gieson stain). Bar indicates a 5 mm distance. Larger magnifications indicated by four squares in the overview are shown in B–E. The black dots in B are cross sections of elastic fibers. They indicate that most of the elastic fibers are oriented dorso-ventrally. More laterally (C, D and E), fibers are cut along as well as perpendicular to their longitudinal axis suggesting that fibers are more variably oriented. Bars in B and C indicate a 100 µm distance. TA: thyroarytenoid muscle; BV: blood vessel; FC: fat cells; EP: epithelium.
Figure 4.
Proteins and fat cells are distributed differently in the medial and lateral lamina propria.
Elastin, collagen and fat distribution along a medio-lateral transect for Siberian Tiger (A) Lion (B) and Bengal Tiger (C). The area is the relative number of pixels stained positively (elastin, collagen) or representing empty fat cells. See Figure 1 for an explanation of ‘lateral’ and ‘medial’.
Figure 5.
Collagen fibers are densely packed in the medial layer and provide a bed for fat cells in the lateral portion.
A: Overview of a midmembraneous section (Trichrome stain) of a lion vocal fold. Bar indicates a 5 mm distance. Larger magnifications indicated by four squares in the overview are shown in B–E. Bars in B to E indicate a 100 µm distance. TA: thyroarytenoid muscle; BV: blood vessel; FC: fat cells; EP: epithelium.
Figure 6.
Hyaluronan is found throughout the lamina propria.
Overview of an alcian blue stain (A) and an alcian blue stain after hyaluronidase treatment (B) of two consecutive midmembraneous sections of a Siberian tiger vocal fold. Sections in (C) to (F) are larger magnifications of two different locations of the lamina propria. The locations are indicated by little squares with the respective letter in (A) and (B). (C) and (D): locations in the densely packed lamina propria without fat cells medially (LP2 in Figure 2) and (E) and (F) are lamina propria locations with fat cells laterally (LP1 in Figure 2). Scale bar in A and B is 5 mm, scale bar in C–F is 100 µm. TA: thyroarytenoid muscle; LP: lamina propria; FC: fat cells; GL: glands; EP: epithelium.
Figure 7.
A linear followed by a nonlinear stress response at about 15% strain occurs when Lion 2 vocal fold lamina propria is subjected to a 1 Hz sinusoidal elongation.
A: loading and unloading phase. The upper part of the “banana-shaped” curve is the loading phase (stretching). The lower part is the unloading phase (relaxation). The difference between both curves is due to hysteresis of the tissue, i.e., lower stress in the tissue during the unloading phase. The low strain region of the loading phase was fitted with a linear regression line (B), while the high-strain region was modeled with an exponential function (C). The limit of the linear region (‘Linear strain limit’) was determined by maximizing the sum of the two regression coefficients (‘sum of r2’). D: Tangents modulus versus strain for the high-strain (>Linear strain limit) region.
Table 2.
Parameters of the linear model (Eq. 6) for curve-fitting the empirical stress-strain response of the lamina propria; linear strain limit (ε1) in %; and parameters of the exponential model (Eq. 7) for curve-fitting the empirical stress-strain response of the lamina propria.
Figure 8.
Elastic and viscous moduli of lion and tiger vocal fold tissues increase with frequency, where elastic modulus dominates material response to oscillation (i.e., loss tangent values are 0.1–0.4).
Shear elastic modulus G′ (A), viscous modulus G″ (B) and loss tangent = G″/G′ (C) lion, Siberian tiger, and Sumatran tiger vocal fold tissue as a function of frequency.
Figure 9.
Viscous modulus and loss tangent diverge at frequencies larger 10 Hz indicating different behavior between the fat-free and fat-containing layer.
Mean elastic modulus G′ (A), viscous modulus G″ (B), and loss tangent = G″/G′ (C) of the lion LP2 and LP1 as a function of frequency. Error bars correspond to ±1 standard error().
Figure 10.
Phonation threshold is surprisingly low for Panthera vocal folds.
The values were calculated from biomechanical properties of lion and Sumatran tiger across frequency with varying glottal half-widths.
Table 3.
Mean tangent Young's moduli of lamina propria tissue of vocal folds from males of different mammal species.