Elbow Extensor Muscles in Humans and Chimpanzees: Adaptations to Different Uses of the Upper Extremity in Hominoid Primates.

The anatomical and functional characteristics of the elbow extensor muscles (triceps brachii and anconeus) have not been widely studied in non-human hominoid primates, despite their great functional importance. In the present study, we have analyzed the muscle architecture and the expression of the myosin heavy chain (MHC) isoforms in the elbow extensors in humans and chimpanzees. Our main objective was to identify differences in these muscles that could be related to the different uses of the upper extremity in the two species. In five humans and five chimpanzees, we have analyzed muscle mass (MM), muscle fascicle length (MFL), and the physiological cross-sectional area (PCSA). In addition, we have assessed the expression of the MHC isoforms by RT-PCR. We have found high MM and PCSA values and higher expression of the MHC-IIx isoform in the triceps brachii of chimpanzees, while in humans, the triceps brachii has high MFL values and a higher expression of the MHC-I and MHC-IIa isoforms. In contrast, there were no significant differences between humans and chimpanzees in any of the values for the anconeus. These findings could be related to the participation of the triceps brachii in the locomotion of chimpanzees and to the use of the upper extremity in manipulative functions in humans. The results obtained in the anconeus support its primary function as a stabilizer of the elbow joint in the two species.

mRNA expression of myosin heavy chain isoforms in the sphenomandibularis portion of the temporalis muscle / Expresión de ARNm de las isoformas de la cadena pesada de la miosina en la porción esfenomandibular del músculo temporal

SUMMARY: The main objective of this study was to analyze by real-time quantitative polymerase chain reaction (RT-qPCR) the expression patterns of the myosin heavy chain (MHC) isoforms (MHC-I, MHC-IIa, MHC-IIx) in the sphenomandibularis portion of the temporalis muscle. We expected to find differences between the sphenomandibularis and the other portions of the temporalis that could be related to the functional characteristics of the sphenomandibularis identified by electromyography. We dissected the right temporalis muscle of ten adult human individuals (five men and five women). Samples of the anterior and posterior temporalis and of the sphenomandibularis portion were obtained from each dissected muscle. These samples were analyzed by RT-qPCR to determine the percentages of expression of the MHC-I, MHC-IIa and MHC-IIx isoforms. No significant differences were identified between the anterior and the posterior temporalis in the expression patterns of the MHC-I, MHC-IIa and MHC-IIx isoforms. However, there were significant differences between the sphenomandibularis and the anterior temporalis. Specifically, the sphenomandibularis portion had a higher percentage of expression of the MHC-I isoform (P=0.04) and a lower percentage of expression of the MHC-IIx isoform (P=0.003). The pattern of expression that we observed in the sphenomandibularis reflects a greater resistance to fatigue, a lower contraction speed, and a lower capacity of force generation in the sphenomandibularis compared to the anterior temporalis. These characteristics are consistent with electromyographic findings on the functional differences between these two portions.

RESUMEN: El principal objetivo de este estudio fue analizar mediante real-time quantitative polymerase chain reaction (RT-qPCR) los patrones de expresión de las isoformas de la cadena pesada de la miosina (MHC-I, MHC-IIa y MHC-IIx) en la porción esfenomandibular del músculo temporal. Se esperó encontrar diferencias entre el esfenomandibular y las otras porciones del músculo temporal que se pudieran relacionar con las características funcionales del esfenomandibular, identificadas mediante electromiografía. Para obtener estos resultados, se diseccionó el músculo temporal derecho en diez humanos adultos (cinco hombres y cinco mujeres) y se obtuvieron muestras de la porción anterior y posterior del músculo temporal y de su porción esfenomandibular. Estas muestras fueron analizadas mediante RT-qPCR para determinar los porcentajes de expresión de las isoformas MHC-I, MHC- IIa y MHC-IIx. No se identificaron diferencias significativas de los patrones de expresión entre la porción anterior y la porción posterior del músculo temporal, pero sí que se observaron diferencias significativas entre la porción anterior del músculo temporal y su porción esfenomandibular. Concretamente, la porción esfenomandibular presentó un mayor porcentaje de expresión de la isoforma MHC-I (P=0.04) y un menor porcentaje de expresión de la isoforma MHC-IIx (P=0.003). El patrón de expresión que hemos observado en la porción esfenomandibular del músculo temporal refleja una mayor resistencia a la fatiga, una velocidad de contracción más lenta y una menor capacidad de generar fuerza si se compara esta porción con la porción anterior del músclo temporal. Estas características son consistentes con las diferencias funcionales que presentan estas dos porciones, que han sido descritas mediante electromiografía.

3D geometric morphometric analysis of the distal radius insertion sites of the palmar radiocarpal ligaments indicates a relationship between wrist anatomy and unique locomotor behavior in hylobatids.

OBJECTIVES: The objective of this study is to explore the anatomical differences in the insertion sites of the palmar radiocarpal ligaments between hylobatids and other hominoids that may be related to their different locomotor behaviors. MATERIALS AND METHODS: The morphology of the insertion sites of the palmar radiocarpal ligaments was analyzed with three-dimensional geometric morphometrics (3D GM) in the distal radial epiphysis of 44 hylobatids, 25 Pan, 31 Gorilla and 15 Pongo. RESULTS: Relative to other hominoids, hylobatid insertion sites of the palmar radiocarpal ligaments were relatively larger and the insertion site of the short radiolunate ligament had a palmar orientation. DISCUSSION: Larger palmar radiocarpal ligaments in hylobatids can help stabilize the wrist during the radial and ulnar displacement that occurs in ricochetal brachiation, the characteristic locomotor behavior of hylobatids, and compensate for the large traction loads on the wrist during extended-elbow vertical climbing.

Expression of myosin heavy chain isoform mRNA transcripts in the masseter and medial pterygoid muscles / Expresión de transcripciones de ARNm de isoforma de cadena pesada de miosina en los músculos masetero y pterigoideo medial

SUMMARY: Both the masseter and medial pterygoid muscles elevate the mandible, raising the lower jaw by acting simultaneously on the lateral and medial surfaces of the mandibular ramus. Nevertheless, electromyographic studies indicate that these muscles, as well as the superficial and deep heads of the masseter, act in a different way during mastication. We have analyzed by real time quantitative polymerase chain reaction (RT-qPCR) the expression of myosin heavy chain (MHC) isoforms in the masseter and medial pterygoid muscles in humans in order to identify possible differences in the expression patterns that may be related to functional differences identified with electromyography. Our findings indicate that the expression pattern of MHC isoforms in the two muscles is characteristic of fast and powerful phasic muscles. We have also observed a high percentage of expression of the MHC-IIx isoform and the expression of the MHC-M isoform at the mRNA level in both muscles, an isoform that does not translate into protein in the masticatory muscles of humans. The high percentage of expression of the MHC-IIx isoform in humans can be related to a high contractile speed of the masseter and medial pterygoid in humans. On the other hand, the low percentage of expression of the MHC-M isoform at the mRNA level in both muscles can be related to the complex evolutionary process that has reduced the size and force of the masticatory muscles in humans.

RESUMEN: Los músculos masetero y pterigoideo medial elevan la mandíbula actuando de forma simultánea sobre las caras lateral y medial de su rama. Sin embargo, los estudios electromiográficos indican que estos dos músculos actúan de forma diferente durante la masticación, de la misma forma que lo hacen las porciones superficial y profunda del músculo masetero. En el presente estudio hemos analizado mediante PCR en tiempo real la expresión de las isoformas de la cadena pesada de la miosina o myosin heavy chain (MHC) en los músculos masetero y pterigoideo medial en humanos, con la finalidad de identificar diferencias en los patrones de expresión que se puedan relacionar con las diferencias funcionales identificadas con la electromiografía. Nuestros resultados indican que el patrón de expresión de las isoformas de la MHC en los dos músculos es la característica de los músculos rápidos y potentes. También hemos observado un elevado porcentaje de expresión de la isoforma MHC-IIx y la expresión a nivel de ARNm de la isoforma MHC-M en los dos músculos, una isoforma que no se detecta a nivel de proteína en los músculos masticadores humanos. El elevado porcentaje de expresión de la isoforma MHC-IIx que hemos observado se puede relacionar con una elevada velocidad de contracción de los músculos masetero y pterigoideo medial en los humanos. Por otro lado, el bajo porcentaje de expresión de la isoforma MHC-M a nivel de ARNm en ambos músculos se puede relacionar con los procesos evolutivos complejos que han reducido el tamaño y la fuerza de los músculos masticadores en los humanos.

Expression of myosin heavy chain isoforms mRNA transcripts in the temporalis muscle of common chimpanzees (Pan troglodytes).

PURPOSE: The common chimpanzee (Pan troglodytes) is the primate that is phylogenetically most closely related to humans (Homo sapiens). In order to shed light on the anatomy and function of the temporalis muscle in the chimpanzee, we have analyzed the expression patterns of the mRNA transcripts of the myosin heavy chain (MyHC) isoforms in different parts of the muscle. BASIC PROCEDURES: We dissected the superficial, deep and sphenomandibularis portions of the temporalis muscle in five adult P. troglodytes and quantified the expression of the mRNA transcripts of the MyHC isoforms in each portion using real-time quantitative polymerase chain reaction. MAIN FINDINGS: We observed significant differences in the patterns of expression of the mRNA transcripts of the MyHC-IIM isoform between the sphenomandibularis portion and the anterior superficial temporalis (33.6% vs 47.0%; P=0.032) and between the sphenomandibularis portion and the anterior deep temporalis (33.6% vs 43.0; P=0.016). We also observed non-significant differences between the patterns of expression in the anterior and posterior superficial temporalis. PRINCIPAL CONCLUSIONS: The differential expression patterns of the mRNA transcripts of the MyHC isoforms in the temporalis muscle in P. troglodytes may be related to the functional differences that have been observed in electromyographic studies in other species of primates. Our findings can be applicable to the fields of comparative anatomy, evolutionary anatomy, and anthropology.

Expression of MyHC isoforms mRNA transcripts in different regions of the masseter and medial pterygoid muscles in chimpanzees.

OBJECTIVE: The aim of this study is to examine the expression pattern of the different myosin heavy chain (MyHC) isoforms in the masseter and medial pterygoid muscles by real time quantitative polymerase chain reaction (RT-qPCR) to obtain information at molecular level which can be related to the functional characteristics of these two muscles. DESIGN: The masseter, deep and superficial portion, and medial pterygoid muscles of five adult Pan troglodytes were dissected in order to obtain samples of the anterior and posterior regions of each portion of the masseter and of the medial pterygoid. The expression of MyHC isoforms mRNA transcripts was analyzed by RT-qPCR. RESULTS: No significant differences in expression of MyHC isoforms between the masseter and the medial pterygoid were found. In contrast, when comparing the superficial and the deep portion of the masseter, we found that the MyHC-IIM isoform was expressed at a significantly higher level in the superficial portion. CONCLUSIONS: The superficial portion of the masseter and the medial pterygoid muscle have the same expression pattern regarding the different MyHC isoforms. On the other hand, the deep portion of the masseter, which is activated mainly during lateral and repositioning movements of the mandible, has a lower MyHC-IIM isoform expression than the superficial portion. Our findings provide new data on functional aspects of the masseter and medial pterygoid that can complement results obtained by other techniques.