title: Tendon-bone attachment unit is formed modularly by a distinct pool of Scx- and Sox9-positive progenitors.
abstract: The assembly of the musculoskeletal system requires the formation of an attachment unit between a bone and a tendon. Tendons are often inserted into bone eminences, superstructures that improve the mechanical resilience of the attachment of muscles to the skeleton and facilitate movement. Despite their functional importance, little is known about the development of bone eminences and attachment units. Here, we show that bone eminence cells are descendants of a unique set of progenitors and that superstructures are added onto the developing long bone in a modular fashion. First, we show that bone eminences emerge only after the primary cartilage rudiments have formed. Cell lineage analyses revealed that eminence cells are not descendants of chondrocytes. Moreover, eminence progenitors were specified separately and after chondroprogenitors of the primary cartilage. Fields of Sox9-positive, Scx-positive, Col2a1-negative cells identified at presumable eminence sites confirm the identity and specificity of these progenitors. The loss of eminences in limbs in which Sox9 expression was blocked in Scx-positive cells supports the hypothesis that a distinct pool of Sox9- and Scx-positive progenitors forms these superstructures. We demonstrate that TGFβ signaling is necessary for the specification of bone eminence progenitors, whereas the SCX/BMP4 pathway is required for the differentiation of these progenitors to eminence-forming cells. Our findings suggest a modular model for bone development, involving a distinct pool of Sox9- and Scx-positive progenitor cells that form bone eminences under regulation of TGFβ and BMP4 signaling. This model offers a new perspective on bone morphogenesis and on attachment unit development during musculoskeletal assembly.
authors.affiliation: Department of Molecular Genetics, Weizmann Institute of Science, Rehovot 76100, Israel.
authors.forename: Einat
authors.initials: E
authors.lastname: Blitz
authors.forename: Amnon
authors.initials: A
authors.lastname: Sharir
authors.forename: Haruhiko
authors.initials: H
authors.lastname: Akiyama
authors.forename: Elazar
authors.initials: E
authors.lastname: Zelzer
mesh_terms: D000818:Animals; D051792:Basic Helix-Loop-Helix Proteins / Q000235:genetics / Q000378:metabolism*; D055415:Bone Morphogenetic Protein 4 / Q000235:genetics / Q000378:metabolism; D001842:Bone and Bones / Q000166:cytology* / Q000378:metabolism; D002356:Cartilage / Q000166:cytology; D002454:Cell Differentiation / Q000187:drug effects / Q000502:physiology; D002478:Cells, Cultured; D019902:Chondrocytes / Q000166:cytology; D005260:Female; D018507:Gene Expression Regulation, Developmental; D017403:In Situ Hybridization; D051379:Mice; D018345:Mice, Knockout; D008856:Microscopy, Fluorescence; D055758:SOX9 Transcription Factor / Q000235:genetics / Q000378:metabolism*; D013234:Stem Cells / Q000166:cytology* / Q000378:metabolism; D013629:Tamoxifen / Q000494:pharmacology; D013710:Tendons / Q000166:cytology* / Q000378:metabolism; D016212:Transforming Growth Factor beta / Q000378:metabolism; D055114:X-Ray Microtomography
chemical_list: D051792:Basic Helix-Loop-Helix Proteins; D055415:Bone Morphogenetic Protein 4; D055758:SOX9 Transcription Factor; D013629:Tamoxifen; D016212:Transforming Growth Factor beta; C503815:Scx protein, mouse; C527933:Sox9 protein, mouse
keywords: Attachment unit; BMP4; Cartilage; Modularity; Mouse; Musculoskeletal system; Patterning; Progenitors; SCX; SOX9; Skeleton; Specification; TGFβ; Tendons