Tendon-bone attachment unit is formed modularly by a distinct pool of Scx- and Sox9-positive progenitors

Abstract

SUMMARYThe 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, Scxpositive, 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 eminenceforming 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.

Musculoskeletal integration at the wrist underlies modular development of limb tendons

“…Cre deleter, which labels all cartilage cells as well as a subset of tendon cells (Blitz et al, 2013;Soeda et al, 2010;Sugimoto et al, 2013), we indeed identified a cell lineage boundary at the MP level of the FDS Individual tendons in I can be identified using the schematic and numerical assignments in Fig. 1. tendons.…”

“…A few studies have suggested that the tendon and cartilage cell fates represent alternative lineages derived from a common tendo-chondro progenitor population and that inhibition of chondrogenesis may drive these progenitors toward a tendon cell fate (Brent et al, 2003(Brent et al, , 2005Lorda-Diez et al, 2009). This assertion was also supported by recent data showing that the tendon and cartilage cells comprising the entheses, which are the skeletal insertions of tendons, are derived from a pool of Sox9 and Scx co-expressing progenitors that then differentiate toward their respective lineages (Blitz et al, 2013;Sugimoto et al, 2013). There is, however, also some evidence that cartilage may play a direct role in tendon induction.…”

“…Interestingly, it was recently demonstrated that induction of a progenitor population for the cartilage and tendon components of the entheses depends on TGFβ signaling (Blitz et al, 2013). The wrist tendon anlagen might therefore represent a unique version of such a progenitor population that, instead of mediating connection of muscle to cartilage, mediates attachment of the muscles to the autopod tendon segments.…”

“…In recent years, a number of studies have posited the existence of a chondro-tendo progenitor population (co-expressing Scx and Sox9) that gives rise to either chondrocytes or tenocytes during early development (Blitz et al, 2013;Sugimoto et al, 2013). The failure of cartilage differentiation in Sox5 −/− ;Sox6 −/− double mutants has also been associated with an expansion of axial tendon progenitors, suggesting a conversion of chondroprogenitors to a tenogenic cell fate in the absence of these cartilage-specific transcription factors (Brent et al, 2005).…”

Multiphase Biodegradable Scaffolds for Tissue Engineering a Tendon-Bone Junction

“…Ablation of the TGFBR2 gene induces loss of tendon/ligament formation [9,10]. Further research has shown that TGF-β-type-2(TGFBR2) singling plays an essential role for tendon morphogenesis via regulating scleraxis (SCX), which is expressed in all the cells of tendon tissues as a key transcription factor for tenogenic differentiation [11]. TGFBR2 expressing progenitor cells are maintained in postnatal tendonbone junctions [8].…”