Search

Digastric muscle (DGM) is a powerful jaw-opening muscle that participates in chewing, swallowing, breathing, and speech. For better understanding of its contractile properties, five pairs of adult human DGMs were obtained from autopsies and processed with immunocytochemistry and/or immunoblotting. Monoclonal antibodies against α-cardiac, slow tonic, neonatal, and embryonic myosin heavy chain (MHC) isoforms were employed to determine whether the DGM fibers contain these MHC isoforms, which have previously been demonstrated in restricted specialized craniocervical skeletal muscles but have not been reported in normal adult human trunk and limb muscles. The results showed expression of all these MHC isoforms in adult human DGMs. About half of the fibers reacted positively to the antibody specific for the α-cardiac MHC isoform in DGMs, and the number of these fibers decreased with age. Slow tonic MHC isoform containing fibers accounted for 19% of the total fiber population. Both the α-cardiac and slow tonic MHC isoforms were found to coexist mainly with the slow twitch MHC isoform in a fiber. A few DGM fibers expressed the embryonic or neonatal MHC isoform. The findings suggest that human DGM fibers may be specialized to facilitate performance of complex motor behaviors in the upper airway and digestive tract. (J Histochem Cytochem 52:851–859, 2004)

Some adult cranial muscles have been reported to contain unusual myosin heavy-chain (MHC) isoforms (i.e., slow-tonic, α-cardiac, embryonic, and neonatal), which exhibit distinct contractile properties. In this study, adult human mylohyoid (MH) muscles obtained from autopsies were investigated to detect the unusual MHC isoforms. For comparison, the biceps brachii and masseter muscles of the same subjects were also examined. Serial cross-sections from the muscles studied were incubated with a panel of isoform-specific anti-MHC monoclonal antibodies that distinguish major and unusual MHC isoforms. On average, the slow type I and fast type II MHC-containing fibers in the MH muscle accounted for 54% and 46% of the fibers, respectively. In contrast to limb and trunk muscles, the adult human MH muscle was characterized by a large proportion of hybrid fibers (85%) and a small percentage of pure fibers (15%; P < 0.01). Of the fast fiber types, the proportion of the type IIa MHC-containing fibers (92%) was much greater than that of the type IIx MHC-containing fibers (8%; P < 0.01). Our data demonstrated that the adult human MH fibers expressed the unusual MHC isoforms that were also identified in the masseter, but not in the biceps brachii. These isoforms were demonstrated by immunocytochemistry and confirmed by electrophoretic immunoblotting. Fiber-to-fiber comparisons showed that the unusual MHC isoforms were coexpressed with the major MHC isoforms (i.e., MHCI, IIa, and IIx), thus forming various major/unusual (or m/u) MHC hybrid fiber types. Interestingly, the unusual MHC isoforms were expressed in a fiber type-specific manner. The slow-tonic and α-cardiac MHC isoforms were coexpressed predominantly with slow type I MHC isoform, whereas the developmental MHC isoforms (i.e., embryonic and neonatal) coexisted primarily with fast type IIa MHC isoform. There were no MH fibers that expressed exclusively unusual MHC isoforms. Approximately 81% of the slow type I MHC-containing fibers expressed slow-tonic and α-cardiac MHC isoforms, whereas 80% of the fast type IIa MHC-containing fibers expressed neonatal MHC isoform. The m/u hybrid fibers (82% of the total fiber population) were found to constitute the predominant fiber types in the adult human MH muscle. At least seven m/u MHC hybrid fiber types were identified in the adult human MH muscle. The most common m/u hybrid fiber types were found to be the MHCI/slow-tonic/α-cardiac and MHCIIa/neonatal, which accounted for 39% and 33% of the total fiber population, respectively. The multiplicity of MHC isoforms in the adult MH fibers is believed to be related to embryonic origin, innervation pattern, and unique functional requirements. © 2004 Wiley-Liss, Inc.

Most of the sounds of human speech are produced by vibration of the vocal folds, yet the biomechanics and control of these vibrations are poorly understood. In this study the muscle within the vocal fold, the thyroarytenoid muscle (TA), was examined for the presence and distribution of slow tonic muscle fibers (STF), a rare muscle fiber type with unique contraction properties. Nine human TAs were frozen and serially sectioned in the frontal plane. The presence and distribution pattern of STF in each TA were examined by immunofluorescence microscopy using the monoclonal antibodies (mAb) ALD-19 and ALD-58 which react with the slow tonic myosin heavy chain (MyHC) isoform. In addition, TA muscle samples from adjacent frozen sections were also examined for slow tonic MyHC isoform by electrophoretic immunoblotting. STF were detected in all nine TAs and the presence of slow tonic MyHC isoform was confirmed in the immunoblots. The STF were distributed predominantly in the medial aspect of the TA, a distinct muscle compartment called the vocalis which is the vibrating part of the vocal fold. STF do not contract with a twitch like most muscle fibers, instead, their contractions are prolonged, stable, precisely controlled, and fatigue resistant. The human voice is characterized by a stable sound with a wide frequency spectrum that can be precisely modulated and the STF may contribute to this ability. At present, the evidence suggests that STF are not presented in the vocal folds of other mammals (including other primates), therefore STF may be a unique human specialization for speech.

It is not understood how different parts of the thyroarytenoid muscle contribute to vocal fold vibration. This study investigated the medial part of the thyroarytenoid muscle, the vocalis compartment, for anatomic differences that might suggest functionally distinct areas. Twenty human vocal folds were frontally sectioned and stained with hematoxylin and eosin. A single section from the middle of each vocal fold was magnified, and the muscle fascicles of the most superficial 25% of the vocalis compartment were then examined. In all 20 specimens the vocalis compartment could be separated into 2 plainly distinct subcompartments: the inferior vocalis compartment was composed of a single large muscle fascicle that contained densely packed muscle fibers of similar size; the superior vocalis compartment was composed of multiple small fascicles in which the muscle fibers were loosely arranged and varied greatly in size. On average, the inferior vocalis subcompartment composed 60% of the medial surface of the thyroarytenoid muscle. The superior subcompartment composed the remaining 40% of the medial surface, but also continued past the vocal ligament to make up the superior surface of the thyroarytenoid muscle. It is concluded that 2 distinct entities make up the vocalis compartment of the thyroarytenoid muscle. Their anatomy is so markedly different it suggests that they may function independently. One possibility is that they reflect the 2 masses observed in the superior and inferior aspects of the vocal fold during vibration.

It is hypothesized that different parts of the thyroarytenoid muscle (TA) are functionally specialized. Specifically, the TA is divided into a lateral muscularis compartment and a medial vocalis compartment. This study examined the distribution of muscle spindles throughout the human TA as an indicator of these functional differences. Histological cross-sections from the anterior, middle, and posterior regions of five human membranous vocal folds were examined for the number and location of muscle spindles. There was an average of 6.1 muscle spindles in sections from each region with no significant variation between the different regions (p.05). However, in sections from all three regions, the muscle spindles were always found to be concentrated in the superior medial quadrant of the TA (mean 85.9%, p.01). The inferior medial, superior lateral, and inferior lateral quadrants of the TA contained 11.96%, 2.17%, and 0%, respectively, of the total muscle spindles. Within the superior medial quadrant, most of the muscle spindles were localized in the most superficial part of the muscle. The results of this study demonstrate that the majority of TA muscle spindles are concentrated in its superior medial quadrant, an area we have termed the superior vocalis subcompartment (SC). This finding suggests that the superior vocalis SC is functionally distinct from the remainder of the TA. It is hypothesized that tension in the superior vocalis SC can be controlled independently from the remainder of the TA, and this capability is used to effect the biomechanics of vocal fold vibration during phonation.

The goal of this research is to identify and characterize the protein machinery that functions in the intracellular translocation and assembly of peroxisomal proteins in Saccharomyces cerevisiae. Several genes encoding proteins that are essential for this process have been identified previously by Kunau and collaborators, but the mutant collection was incomplete. We have devised a positive selection procedure that identifies new mutants lacking peroxisomes or peroxisomal function. Immunofluorescence procedures for yeast were simplified so that these mutants could be rapidly and efficiently screened for those in which peroxisome biogenesis is impaired. With these tools, we have identified four complementation groups of peroxisome biogenesis mutants, and one group that appears to express reduced amounts of peroxisomal proteins. Two of our mutants lack recognizable peroxisomes, although they might contain peroxisomal membrane ghosts like those found in Zellweger syndrome. Two are selectively defective in packaging peroxisomal proteins and moreover show striking intracellular clustering of the peroxisomes. The distribution of mutants among complementation groups implies that the collection of peroxisome biogenesis mutants is still incomplete. With the procedures described, it should prove straightforward to isolate mutants from additional complementation groups.

Problem: Fiber type and myosin-heavy chain (MHC) composition in the human pharyngeal constrictor (PC) and upper esophageal sphincter (UES) muscles were analyzed to better understand the muscle functions. Methods: Twelve adult human PC and UES muscles obtained from autopsies were studied using immunohistochemical and immunoblotting techniques. For comparison, masseter and biceps brachii muscles of the same subjects served as controls. Serial cross-sections were incubated with a panel of isoform-specific anti-MHC monoclonal antibodies. Results: The human PC and UES muscles were composed of 2 histochemically defined fiber layers, a slow inner layer (SIL) (78% type I) and a fast outer layer (FOL) (59% type II). The adult PC and UES fibers expressed unusual MHC isoforms (ie, slow-tonic, alpha-cardiac, embryonic, and neonatal) that were also identified in the masseter, but not in the biceps brachii. These unusual MHC isoforms demonstrated by immunocytochemistry were confirmed by electrophoretic immunoblotting. The slow-tonic and alpha-cardiac fibers were concentrated mainly in the SIL, whereas the developmental MHC-containing fibers were located primarily in the FOL. The unusual MHC isoforms were coexpressed with the major MHC isoforms (ie, slow type I and fast type II), thus forming various major/unusual (m/u) MHC hybrid fiber types. The slow-tonic and alpha-cardiac MHC isoforms coexisted mainly with type I MHC, whereas the embryonic and neonatal MHC isoforms were coexpressed primarily with type IIa MHC. At least 8 m/u MHC hybrid fiber types were identified in these muscles. Conclusion: The multiplicity of MHC isoforms in the PC and UES fibers is believed to be related to embryonic origin, innervation, and unique functional requirements. Significance: Age-related changes in the fiber type and MHC composition in the human PCs and UES may be associated with pathogeneses of dysphagia and obstructive sleep apnea that are commonly seen in the elderly. Support: None reported.