The myotendinous junction of the smooth feather muscles (mm. pennati)

Summary Smooth feather muscles (mm. pennati) consist of bundles of smooth muscle cells which are attached to the feather follicles by short elastic tendons. In addition, some muscle bundles are interrupted by elastic tendons. The elastic tendon is composed of longitudinally arranged elastic fibers which branch and wavy bundles of collagen fibrils. Smooth muscle cells of the muscle bundles are attached to each other by desmosome-like junctions and by fusion of the basal laminae. The cytoplasm of the muscle cells is characterized by conspicuous thick filaments and abundant thin and intermediate filaments. These are attached to band-like dense patches (dense bands) at the plasma membrane which are particularly broad at the tapering end of the muscle cell. The contact surface between smooth muscle cells and their elastic tendon is considerably increased (i) by deep finger-like invaginations and indentations located at the tapering muscle end, and (ii) by branching of the coarse elastic fibers into slender processes, which are attached to the richly folded surface of the muscle cell endings by peripheral microfibrils. This intimate interlocking closely resembles the myotendinous junctions in skeletal muscle. In addition to fibroblasts and fibrocytes, the myotendinous junction of the young growing chicks contains numerous so-called myofibroblasts, which are suggested to represent smooth muscle cells differentiating into fibroblasts of the developing tendon.Dedicated to Professor Dr. Helmut Leonhardt on the occasion of his 60th birthdaySupported by a grant from the Deutsche Forschungsgemeinschaft (Dr. 91/1)     

Rest length and compliance of non-immobilised and immobilised rabbit soleus muscle and tendon

The first aim of this study was to measure the contributions of muscle and tendon to the total compliance of resting muscle-tendon units. A second aim was to determine whether the decrease in muscle-tendon unit rest length produced by prolonged immobilisation in a shortened position is mediated primarily by adaptations of the muscle or tendon. One ankle joint from each of five rabbits was immobilised in a plantarflexed position for 14 days. The passive length-tension properties of soleus muscle fascicles and tendons from both hindlimbs were measured using a video-based tensile-testing system. In non-immobilised muscles, muscle fascicle strains exceeded tendon strains by up to four times. However, because the rest length of tendon was much greater than that of muscle fascicles, changes in tendon length accounted for nearly half of the total change in muscle-tendon unit length. The rest length of immobilised muscle-tendon units was less than that of non-immobilised muscle-tendon units from contralateral limbs. Most of this difference was attributable to a change in the rest length of the tendon; there was little change in the rest length of muscle fascicles. It is concluded that the tendon is responsible for a large part of the compliance of rabbit soleus muscle-tendon units at physiological resting tensions, and that adaptation of tendon rest length is the primary mechanism by which the rabbit soleus shortens in response to immobilisation at short lengths. Accepted: 7 May 1997     

Ethnic differences in triceps surae muscle-tendon complex and walking economy

The purposes of this study were to (a) determine whether structural differences in triceps surae muscle-tendon complex and walking economy exist between 14 African American and 19 Caucasian sedentary women and (b) determine whether muscle-tendon parameters are associated with walking economy. African American and Caucasian subjects were matched on body weight, height, and body composition. Muscle-tendon parameters were determined by magnetic resonance imaging and walking economy was evaluated at 4.8 km.h(-1). Medial gastrocnemius and total triceps surae muscle shape were different across ethnicity despite no ethnic differences in plantar flexion strength or in maximal cross-sectional area for any triceps surae muscles. African American women had shorter gastrocnemius muscles and longer tendons and performed walking more economically. Tendon length was the only variable related to walking economy. No ethnic differences were observed in walking economy after adjusting for tendon length. Data show gastrocnemius tendon length is related to level walking and longer gastrocnemius tendons may partly explain more economical walking in African American women. These preliminary findings indicate the structure of the muscle-tendon complex could be a factor partially accounting for reported ethnic differences in certain types of athletic-related performance.     

“Importin” signaling roles for import proteins

The formation of a mature myotendinous junction (MTJ) between a muscle and its site of attachment is a highly regulated process that involves myofiber migration, cell-cell signaling, and culminates with the stable adhesion between the adjacent muscle-tendon cells. Improper establishment or maintenance of muscle-tendon attachment sites results in a decrease in force generation during muscle contraction and progressive muscular dystrophies in vertebrate models. Many studies have demonstrated the important role of the integrins and integrin-associated proteins in the formation and maintenance of the MTJ. We recently demonstrated that moleskin (msk), the gene that encodes for Drosophila importin-7 (DIM-7), is required for the proper formation of muscle-tendon adhesion sites in the developing embryo. Further studies demonstrated an enrichment of DIM-7 to the ends of muscles where the muscles attach to their target tendon cells. Genetic analysis supports a model whereby msk is required in the muscle and signals via the secreted epidermal growth factor receptor (Egfr) ligand Vein to regulate tendon cell maturation. These data demonstrate a novel role for the canonical nuclear import protein DIM-7 in establishment of the MTJ.     

Muscle Function in vivo: A Comparison of Muscles used for Elastic Energy Savings versus Muscles Used to Generate Mechanical Power1

SYNOPSIS. The function of muscles used to generate force economicallyand facilitate elastic energy savings in their tendons is comparedwith muscles that function to produce mechanical power. Theunderlying architectural design of the muscle and its tendon(if present) dictate much of their functional capacity and rolein animal locomotion. Using methods that allow direct recordingsof muscle force and fiber length change, the functional designof muscle-tendon systems can now be investigated in vivo. Thesestudies reveal that, in the case of wallaby hindleg muscles,the fibers can maintain sufficient stiffness during tendon stretchand recoil to ensure useful elastic energy recovery and savingsof metabolic energy. In the case of the pectoralis muscle ofpigeons, although isometric or active lengthening of the muscle'sfibers may occur late in the upstroke of the wing beat cycleto enhance force development, the fibers shorten extensivelyduring the downstroke (up to 35% of their resting length) toproduce mechanical power for aerodynamic lift and thrust. Oscillatorylength change, with force enhancement during active lengtheningmay be a general feature of muscles that power aerial and aquaticlocomotion. Similarly, force enhancement by active lengtheningis likely to be important to the design and function of musclesthat primarily generate force to minimize energy expenditure/unitforce generated, as well as for elastic energy savings withina long tendon. Architectural features of muscle-tendon unitsfor effective elastic energy savings, however, are likely toconstrain locomotor performance when mechanical work is required,as when an animal accelerates, either limiting performance orrequiring the recruitment of functional agonists with greatermechanical power generating capability (i.e., longer fibers)     

Interrelationships of crural muscles and tendons in a range of birds

The crural musculature in 19 species of birds was examined. New data on the mechanical properties of avian tendons showed a mean linear elastic modulus of about 1.3 GPa and failure stress of 74MPa. Physiological cross-sectional areas were calculated for 12 muscles in each species, together with the cross-sectional areas of their attached tendons. Investigation into potential various sources of error demonstrate this is a robust method for studying muscle-tendon units. The mode for calculated peak tendon stresses was 17.5 MPa. The results were compared to those obtained from mammal limbs by Ker, Alexander & Bennett (1988). It appears that the muscle-tendon units in birds are not as strongly optimized for the minimization of mass as those in the limbs of mammals. This may reflect a compromise between optimization for minimum mass and other functional requirements.     

Cleaved Slit directs embryonic muscles

The formation of functional musculoskeletal system relies on proper connectivity between muscles and their corresponding tendon cells. In Drosophila, larval muscles are born during early embryonic stages, and elongate toward tendons that are embedded within the ectoderm in later. The Slit/Robo signaling pathway had been implicated in the process of muscle elongation toward tendons. Here we discuss our recent findings regarding the critical contribution of Slit cleavage for immobilization and stabilization of the Slit signal on the tendon cells. Slit cleavage produces 2 polypeptides, the N-terminal Slit-N, which is extremely stable, undergoes oligomerization, and associates with the tendon cell surfaces, and the C-terminal Slit-C, which rapidly degrades. Slit cleavage leads to immobilization of Slit signaling on tendons, leading to a short-range repulsion, which eventually arrest further muscle elongation. Robo2, which is co-expressed with Slit by the tendon cells facilitates Slit cleavage. This activity does not require the cytoplasmic signaling domain of Robo2. We suggest that Robo2-dependent Slit cleavage, and the formation of Slit-N oligomers on the tendon cell surfaces direct muscle elongation, and provide a stop signal for the approaching muscle, through binding to Robo and Robo3 receptors expressed by the muscles.     

Structure-function considerations of muscle-tendon junctions

Skeletal muscle cells transmit force across the cell membrane to the extracellular matrix and ultimately to tendons. Force transmission may occur both along the lateral surfaces of muscle fibers and at their ends. Forces within muscles may follow the path of greatest resistance. Sites of force transmission are morphologically and compositionally specialized for this function. They are also specialized to provide stress-information that feeds into the synthetic programs of the muscle cell. A detailed analysis of the structures and functions of muscle-tendon junctions is essential to a comprehensive understanding of the way in which muscles and their connective tissues are controlled to move joints and to respond to mechanical stresses.     

“Importin” signaling roles for import proteins: The function of Drosophila importin-7 (DIM-7) in muscle-tendon signaling

The formation of a mature myotendinous junction (MTJ) between a muscle and its site of attachment is a highly regulated process that involves myofiber migration, cell-cell signaling, and culminates with the stable adhesion between the adjacent muscle-tendon cells. Improper establishment or maintenance of muscle-tendon attachment sites results in a decrease in force generation during muscle contraction and progressive muscular dystrophies in vertebrate models. Many studies have demonstrated the important role of the integrins and integrin-associated proteins in the formation and maintenance of the MTJ. We recently demonstrated that moleskin (msk), the gene that encodes for Drosophila importin-7 (DIM-7), is required for the proper formation of muscle-tendon adhesion sites in the developing embryo. Further studies demonstrated an enrichment of DIM-7 to the ends of muscles where the muscles attach to their target tendon cells. Genetic analysis supports a model whereby msk is required in the muscle and signals via the secreted epidermal growth factor receptor (Egfr) ligand Vein to regulate tendon cell maturation. These data demonstrate a novel role for the canonical nuclear import protein DIM-7 in establishment of the MTJ.     

The integrated function of muscles and tendons during locomotion

The mechanical roles of tendon and muscle contractile elements during locomotion are often considered independently, but functionally they are tightly integrated. Tendons can enhance muscle performance for a wide range of locomotor activities because muscle-tendon units shorten and lengthen at velocities that would be mechanically unfavorable for muscle fibers functioning alone. During activities that require little net mechanical power output, such as steady-speed running, tendons reduce muscular work by storing and recovering cyclic changes in the mechanical energy of the body. Tendon stretch and recoil not only reduces muscular work, but also allows muscle fibers to operate nearly isometrically, where, due to the force-velocity relation, skeletal muscle fibers develop high forces. Elastic energy storage and recovery in tendons may also provide a key mechanism to enable individual muscles to alter their mechanical function, from isometric force-producers during steady speed running to actively shortening power-producers during high-power activities like acceleration or uphill running. Evidence from studies of muscle contraction and limb dynamics in turkeys suggests that during running accelerations work is transferred directly from muscle to tendon as tendon stretch early in the step is powered by muscle shortening. The energy stored in the tendon is later released to help power the increase in energy of the body. These tendon length changes redistribute muscle power, enabling contractile elements to shorten at relatively constant velocities and power outputs, independent of the pattern of flexion/extension at a joint. Tendon elastic energy storage and recovery extends the functional range of muscles by uncoupling the pattern of muscle fiber shortening from the pattern of movement of the body.     

The fine structure and development of calcified skeletal elements in the body wall of holothurian echinoderms

The calcareous ossicles and iron-rich calcified granules in the integument of sea cucumbers (Echinodermata: Holothuroida) have been examined by light and electron microscopy. Most ossicles are perforated, platelike structures that measure 100–500 μm long and consist of magnesium-rich calcite. The formation of ossicles occurs within multinucleated syncytia of sclerocytes that are situated in the dermal layer of the body wall. Sclerocytes possess fairly large numbers of mitochondria and are characteristically enveloped by an external lamina. Each ossicle arises as a minute rodlike spicule that branches and develops into a fenestrated skeletal element. When viewed by SEM, fully developed ossicles appear non-crystalline. Following decoration with synthetically grown calcite seeds, however, each ossicle reveals an ordered array of crystallites that seem to be aligned perpendicular to the ossicle's original plane of growth. Examinations of sectioned ossicles indicate that the mineral phase in each skeletal element lacks a well developed matrix of EDTA-insoluble organic substances. Ossicles first arise in the ciliated, lecithotrophic larvae of Eupentacta quinquesimita at about 1 week postfertilization. Two-week-old specimens adopt an epibenthic existence and subsequently become enveloped by imbricated ossicles that are secreted by sclerocytes resembling those found in adults. In molpadiid holothurians, the adult body wall contains numerous reddish-brown granules that are chiefly composed of iron-rich subunits. The dermal granules differ from calcitic ossicles in developing extracellularly in association with finely filamentous material.     

Structural features associated with movement and ‘catch’ of sea-urchin spines

Spine catch ligaments of a sea urchin Arbacia punctulata were extended under constant load. Ligaments from an undisturbed animal may show any extension rate from zero (catch state) to rapid extension to failure. Replacing the preparation bath with Ca²⁺- and Mg²⁺-free sea water reversibly abolishes the catch state. The fine structure of the outer muscle layer and inner ligament cone associated with the spine base is described. The unstriated paramyosin muscles bear thin flanges and form compact interlocking rows. Subsurface cisternae are associated with the plasma membrane. The muscles are innervated by glia-free axons ending in bulbous terminals containing lucent synaptic vesicles. The ligament comprises cylindrical bundles of collagen fibrils: one or more minute muscle fibers (paramyosin) lie parallel with and closely adjoining each bundle. The mean diameter of these muscles is 0.3 μg and they occupy 2–3 % of the ligament's cross-sectional area. Axons containing electronopaque secretory droplets accompany the muscles between the collagen bundles: the cell bodies of these neurones generally lie on the outer surface of the ligament. When an urchin points a spine, the ligament on the side of the contracting spine muscle shortens but does not buckle. A function of the intraligamental muscles is to effect this non-buckled shortening. The catch mechanism (which resides entirely within the ligament) may be due either to the intraligamental muscles and/or to a locked polymer mechanism in which matrix molecules between collagen fibrils are reversibly crosslinked by divalent cations.     

A dynamic model of the forearm including fatigue

A biomechanical model of the forearm, consisting of 61 muscle-tendon systems or tendons and 8 sections, is presented. The model can be used to calculate the muscle forces when resultant of the external forces and the motion is known. Calculations are based on constraints of muscle forces, joint forces, contact forces, and tendon junctions, and a load sharing principle telling which of the feasible solutions are likely and which are not. Fatigue is accounted for by updating the upper limits of the muscle forces according to the loading history. As an example, the model is used to predict the load sharing between the fingers when they are pressed against a table with a given total force.     

Moleskin is essential for the formation of the myotendinous junction in Drosophila

It is the precise connectivity between skeletal muscles and their corresponding tendon cells to form a functional myotendinous junction (MTJ) that allows for the force generation required for muscle contraction and organismal movement. The Drosophila MTJ is composed of secreted extracellular matrix (ECM) proteins deposited between integrin-mediated hemi-adherens junctions on the surface of muscle and tendon cells. In this paper, we have identified a novel, cytoplasmic role for the canonical nuclear import protein Moleskin (Msk) in Drosophila embryonic somatic muscle attachment. Msk protein is enriched at muscle attachment sites in late embryogenesis and msk mutant embryos exhibit a failure in muscle–tendon cell attachment. Although the muscle–tendon attachment sites are reduced in size, components of the integrin complexes and ECM proteins are properly localized in msk mutant embryos. However, msk mutants fail to localize phosphorylated focal adhesion kinase (pFAK) to the sites of muscle–tendon cell junctions. In addition, the tendon cell specific proteins Stripe (Sr) and activated mitogen-activated protein kinase (MAPK) are reduced in msk mutant embryos. Our rescue experiments demonstrate that Msk is required in the muscle cell, but not in the tendon cells. Moreover, muscle attachment defects due to loss of Msk are rescued by an activated form of MAPK or the secreted epidermal growth factor receptor (Egfr) ligand Vein. Taken together, these findings provide strong evidence that Msk signals non-autonomously through the Vein-Egfr signaling pathway for late tendon cell late differentiation and/or maintenance.     

Cleaved Slit directs embryonic muscles

The formation of functional musculoskeletal system relies on proper connectivity between muscles and their corresponding tendon cells. In Drosophila, larval muscles are born during early embryonic stages, and elongate toward tendons that are embedded within the ectoderm in later. The Slit/Robo signaling pathway had been implicated in the process of muscle elongation toward tendons. Here we discuss our recent findings regarding the critical contribution of Slit cleavage for immobilization and stabilization of the Slit signal on the tendon cells. Slit cleavage produces 2 polypeptides, the N-terminal Slit-N, which is extremely stable, undergoes oligomerization, and associates with the tendon cell surfaces, and the C-terminal Slit-C, which rapidly degrades. Slit cleavage leads to immobilization of Slit signaling on tendons, leading to a short-range repulsion, which eventually arrest further muscle elongation. Robo2, which is co-expressed with Slit by the tendon cells facilitates Slit cleavage. This activity does not require the cytoplasmic signaling domain of Robo2. We suggest that Robo2-dependent Slit cleavage, and the formation of Slit-N oligomers on the tendon cell surfaces direct muscle elongation, and provide a stop signal for the approaching muscle, through binding to Robo and Robo3 receptors expressed by the muscles.     

The association of microtubules with the plasmalemma in epidermal tendon cells of the river crab

The mode of association of microtubules (MTs) with the plasmalemma in epidermal tendon cells of the river crab, Potamon dehaani was studied by thin-section electron microscopy. In the leg muscle, the tendon cells connect striated muscle cells with the cuticle, forming specialized junctions at both ends. At the muscle-tendon cell junction, the apposed plasmalemmas are interdigitated in a zig-zag pattern separated by a uniform space of about 50 nm, where the basal lamina is shared by two cells. At the tendon cell-cuticle junction, the plasmalemma of the tendon cell forms many conical invaginations, into which dense fibrous material extends from the cuticle. Inside the tendon cell, numerous microtubules run parallel to the direction of tension transmission and are arranged into parallel bundles of various sizes. Within such bundles, fine filamentous structures cross-link adjacent MTs. MTs span the entire length of the cell and attach at their both ends to the junctional domains of the plasmalemma. The junctional plasmalemma is characterized by formation of an electron-dense undercoat, through which MTs are connected with the plasmalemma proper. The ultrastructural features of MT association with the plasmalemma are basically the same at both junctions. At the junctions, MTs usually terminate with free ends and are linked laterally to the plasmalemmal undercoat with fine filamentous structures. These observations emphasize the role of the plasmalemmal undercoat as a device of the attachment of MTs to the plasmalemma.     

Tendon crimps and peritendinous tissues responding to tensional forces

Tendons transmit forces generated from muscle to bone making joint movements possible. Tendon collagen has a complex supramolecular structure forming many hierarchical levels of association; its main functional unit is the collagen fibril forming fibers and fascicles. Since tendons are enclosed by loose connective sheaths in continuity with muscle sheaths, it is likely that tendon sheaths could play a role in absorbing/transmitting the forces created by muscle contraction. In this study rat Achilles tendons were passively stretched in vivo to be observed at polarized light microscope (PLM), scanning electron microscope (SEM) and transmission electron microscope (TEM). At PLM tendon collagen fibers in relaxed rat Achilles tendons ran straight and parallel, showing a periodic crimp pattern. Similarly tendon sheaths showed apparent crimps. At higher magnification SEM and TEM revealed that in each tendon crimp large and heterogeneous collagen fibrils running straight and parallel suddenly changed their direction undergoing localized and variable modifications. These fibril modifications were named fibrillar crimps. Tendon sheaths displayed small and uniform fibrils running parallel with a wavy course without any ultrastructural aspects of crimp. Since in passively stretched Achilles tendons fibrillar crimps were still observed, it is likely that during the tendon stretching, and presumably during the tendon elongation in muscle contraction, the fibrillar crimp may be the real structural component of the tendon crimp acting as shock absorber. The peritendinous sheath can be stretched as tendon, but is not actively involved in the mechanism of shock absorber as the fibrillar crimp. The different functional behaviour of tendons and sheaths may be due to the different structural and molecular arrangement of their fibrils.     

Comparative morphology of echinoderm calcified tissues: Histology and ultrastructure of ophiuroid scales (Echinodermata,Ophiuroida)

Summary The calcified body wall of an ophiuroid was investigated by a new method and compared with that of other echinoderms. The previous opinion that the epidermis of ophiuroid arm shields consists of a reduced syncytium continuous with the underlying dermis is incorrect. The epidermis is distinctly separated from the dermis by a basal layer and consists of (1) supporting cells which bear the cuticle, (2) ciliated cells (hitherto unknown and probably sensory), (3) gland cells, and (4) nerve cells with the basal nerve plexus. The overall structure of the epidermis is a three-dimensional tube system (marked by the basal lamina) which penetrates the dermal tissue of the scale's pore space and continues with nerve cords situated below the scale. This arrangement is unique in echinoderms.The dermal sclerocytes largely conform with those of the echinoid Eucidaris. The mineral skeleton is produced intracellularly or intrasyncytially. Moreover, dermal sclerocytes were found to release extracellular microfibrils which have nothing to do with calcite deposition. The attachment of the cuticle to the dermis is achieved by means of epidermal coupling areas. Collagen fibers fasten the scale to the underlying connective tissue sheath. The supposed fibrocytes within this sheath resemble sclerocytes. Each collagen bundle is provided with a strand of nerve fibers which, in contrast to the basal nerve plexus, are naked. They are said to infuence the mechanical properties of the connective tissue.Structures associated with cilia occur in cell types which normally lack a cilium. This finding suggests that most echinoderm cells are potentially monociliate.Abbreviations A apical shield - asp secretory products - B bacteria - bb basal body - bl basal lamina - C ciliated cell - ca coupling area - ci cilium, - cf collagen fibrils - cs cell surface - CTS connective tissue sheath - cu _i inner cuticular layer - cu _m middle cuticular layer - dp distal processes (Sc) - EC epineural canal - G Golgi complex - gv granular vesicle - H haemal vessel - hb homogeneous body - hl horizontal lamina (Su) - j cell junction - L lateral shield - le boundary layer (Sc) - lo distal lobe (Su) - M intervertebral muscle or its attachment - m mitochondrium - mf microfibrils - mu mucus - mv microvilli - mvb multivesicular body - N nerve cell - n nucleus - nf neurofibrils - ng neurogranules - nn naked neurofibrils - O oral shield - P tube foot - Pc phagocyte - pg pigment granules - rl rootlet - RN radial nerve - RV radial vessel - Sc sclerocyte - sh cytoplasmic sheath (Sc) - sj septate junction - Su supporting cell - sv secretory vesicle - T calcite trabeculum - V vertebral ossicle - v vesicle (Su)     

Mechanical response of individual collagen fibrils in loaded tendon as measured by atomic force microscopy

A precise analysis of the mechanical response of collagen fibrils in tendon tissue is critical to understanding the ultrastructural mechanisms that underlie collagen fibril interactions (load transfer), and ultimately tendon structure–function. This study reports a novel experimental approach combining macroscopic mechanical loading of tendon with a morphometric ultrascale assessment of longitudinal and cross-sectional collagen fibril deformations. An atomic force microscope was used to characterize diameters and periodic banding (D-period) of individual type-I collagen fibrils within murine Achilles tendons that were loaded to 0%, 5%, or 10% macroscopic nominal strain, respectively. D-period banding of the collagen fibrils increased with increasing tendon strain (2.1% increase at 10% applied tendon strain, p < 0.05), while fibril diameter decreased (8% reduction, p < 0.05). No statistically significant differences between 0% and 5% applied strain were observed, indicating that the onset of fibril (D-period) straining lagged macroscopically applied tendon strains by at least 5%. This confirms previous reports of delayed onset of collagen fibril stretching and the role of collagen fibril kinematics in supporting physiological tendon loads. Fibril strains within the tissue were relatively tightly distributed in unloaded and highly strained tendons, but were more broadly distributed at 5% applied strain, indicating progressive recruitment of collagen fibrils. Using these techniques we also confirmed that collagen fibrils thin appreciably at higher levels of macroscopic tendon strain. Finally, in contrast to prevalent tendon structure–function concepts data revealed that loading of the collagen network is fairly homogenous, with no apparent predisposition for loading of collagen fibrils according to their diameter.     

Relationship between Neural Crest Cells and Cranial Mesoderm during Head Muscle Development

Background

In vertebrates, the skeletal elements of the jaw, together with the connective tissues and tendons, originate from neural crest cells, while the associated muscles derive mainly from cranial mesoderm. Previous studies have shown that neural crest cells migrate in close association with cranial mesoderm and then circumscribe but do not penetrate the core of muscle precursor cells of the branchial arches at early stages of development, thus defining a sharp boundary between neural crest cells and mesodermal muscle progenitor cells. Tendons constitute one of the neural crest derivatives likely to interact with muscle formation. However, head tendon formation has not been studied, nor have tendon and muscle interactions in the head.

Methodology/Principal Findings

Reinvestigation of the relationship between cranial neural crest cells and muscle precursor cells during development of the first branchial arch, using quail/chick chimeras and molecular markers revealed several novel features concerning the interface between neural crest cells and mesoderm. We observed that neural crest cells migrate into the cephalic mesoderm containing myogenic precursor cells, leading to the presence of neural crest cells inside the mesodermal core of the first branchial arch. We have also established that all the forming tendons associated with branchiomeric and eye muscles are of neural crest origin and express the Scleraxis marker in chick and mouse embryos. Moreover, analysis of Scleraxis expression in the absence of branchiomeric muscles in Tbx1^−/− mutant mice, showed that muscles are not necessary for the initiation of tendon formation but are required for further tendon development.

Conclusions/Significance

This results show that neural crest cells and muscle progenitor cells are more extensively mixed than previously believed during arch development. In addition, our results show that interactions between muscles and tendons during craniofacial development are similar to those observed in the limb, despite the distinct embryological origin of these cell types in the head.