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WIREs Dev Biol
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Tendon development and diseases

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Tendon is a uniaxial connective tissue component of the musculoskeletal system. Tendon is involved in force transmission between muscle and bone. Tendon injury is very common and debilitating but tendon repair remains a clinical challenge for orthopedic medicine. In vertebrates, tendon is mainly composed of type I collagen fibrils, displaying a parallel organization along the tendon axis. The tendon‐specific spatial organization of type I collagen provides the mechanical properties for tendon function. In contrast to other components of the musculoskeletal system, tendon biology is poorly understood. An important goal in tendon biology is to understand the mechanisms involved in the production and assembly of type I collagen fibrils during development, postnatal formation, and healing processes in order to design new therapies for tendon repair. In this review we highlight the current understanding of the molecular and mechanical signals known to be involved in tenogenesis during development, and how development provides insights into tendon healing processes. WIREs Dev Biol 2016, 5:5–23. doi: 10.1002/wdev.201 This article is categorized under: Vertebrate Organogenesis > Musculoskeletal and Vascular
Tendon organization. (a) Tendon links muscle to bone and is attached at one end to muscle by the myotendinous junction and at the other end to bone by the enthesis. Tendon is mainly composed of type collagen and of very few cells. Type I collagen displays a specific spatial organization parallel to the tendon axis. Tendon is formed of collagen fascicles, which are composed of collagen fibers, which are formed of collagen fibrils. The endotenon separates collagen fascicles. Tendon is surrounded by the tendon sheaths named the peritenon, which comprises paratenon and epitenon. (b and c) Collagen fibres and fibrils can be visualized at different scales with electron microscopy. Electron microscopy of transverse sections of a mouse Achilles tendon showing collagen fibrils (b,c).
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Schematic representation of tendon pathologies. (a) Normal tendons. (b) Tendons in genetic diseases affecting collagen fibrillogenesis. (c) Chronic tendon injury or tendinopathy. (d) Acute tendon injury.
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Muscle‐dependency for head, limb, and axial tendon development. Muscle and tendon are schematized in red and green, respectively. In the head (a) and limbs (b), tendons initiate their development independently of muscle, but further tendon development requires the presence of muscle. In contrast, the initiation of axial tendon development requires the presence of muscle (c).
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Distinct embryological origins of vertebrate tendons. Tendons can be divided into head, axial, and limb tendons. Head tendons originate from neural crest cells (orange). Axial tendons originate from somites (purple). Limb tendons originate from limb lateral plate (green).
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Expression of Scx and Tnmd in chick limbs and schematic representation of Scx expression in developmental, postnatal, and adult tendons. (a–d) In situ hybridization to adjacent and transverse forelimb sections of Embryonic Day 9 (E9) chick embryos with Scx (a and c) and Tnmd (b and d) probes. Scx and Tnmd are expressed in tendons. (e) Scx‐positive cells are schematized in green. During development, Scx expression is expressed in all tendon cells. During tendon maturation at postnatal stages, Scx is expressed in the tendon proper, endotendon, and external sheaths including epitenon and paratenon, but is restricted to the epitenon by the fourth postnatal month.
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Expression of Col1a1 and Scx in chick limbs. (a–d) Adjacent and transverse forelimb sections of Embryonic Day 10 (E10) chick embryos were hybridized with Col1a1 (a and b) and Scx (c and d) probes (blue) and then immunostained with the MF20 antibody, which recognizes myosins in skeletal muscles (brown). Col1a1 is expressed in tendons but also around cartilage elements, in feather buds and connective tissues (a). Scx is expressed in tendons (c). (b and d) are higher magnifications of two dorsal muscles of forelimbs. Col1a1 is expressed in tendons and muscle connective tissue (b), while Scx is expressed only in tendons (d). u, ulna; r, radius.
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