Heterotopic ossification (HO) is a pathological process resulting in aberrant bone formation and often involves synovial lined tissues. During this process, mesenchymal progenitor cells undergo endochondral ossification. Nonetheless, the specific cell phenotypes and mechanisms driving this process are not well understood, in part due to the high degree of heterogeneity of the progenitor cells involved. Here, using a combination of lineage tracing and single-cell RNA sequencing (scRNA-seq), we investigated the extent to which synovial/tendon sheath progenitor cells contribute to heterotopic bone formation. For this purpose, Tppp3 (tubulin polymerization-promoting protein family member 3)-inducible reporter mice were used in combination with either Scx (Scleraxis) or Pdgfra (platelet derived growth factor receptor alpha) reporter mice. Both tendon injury- and arthroplasty-induced mouse experimental HO models were utilized. ScRNA-seq of tendon-associated traumatic HO suggested that Tppp3 is an early progenitor cell marker for either tendon or osteochondral cells. Upon HO induction, Tppp3 reporter⁺ cells expanded in number and partially contributed to cartilage and bone formation in either tendon- or joint-associated HO. In double reporter animals, both Pdgfra⁺Tppp3⁺ and Pdgfra⁺Tppp3^- progenitor cells gave rise to HO-associated cartilage. Finally, analysis of human samples showed a substantial population of TPPP3-expressing cells overlapping with osteogenic markers in areas of heterotopic bone. Overall, these data demonstrate that synovial/tendon sheath progenitor cells undergo aberrant osteochondral differentiation and contribute to HO after trauma.

异位骨化（HO）是一种导致异常骨形成的病理过程，通常涉及滑膜衬里组织。在此过程中，间充质祖细胞经历软骨内骨化。尽管如此，驱动这一过程的特定细胞表型和机制尚不清楚，部分原因是所涉及的祖细胞具有高度异质性。在这里，我们结合谱系追踪和单细胞 RNA 测序 (scRNA-seq)，研究了滑膜/腱鞘祖细胞对异位骨形成的贡献程度。为此，将Tppp3 （微管蛋白聚合促进蛋白家族成员3）诱导型报告小鼠与Scx（Scleraxis）或Pdgfra （血小板衍生生长因子受体α）报告小鼠组合使用。使用肌腱损伤和关节成形术诱导的小鼠实验 HO 模型。肌腱相关创伤性 HO 的 ScRNA-seq 表明Tppp3是肌腱或骨软骨细胞的早期祖细胞标记。在 H2O 诱导后，Tppp3报告基因⁺细胞数量增加，并部分促进肌腱或关节相关 H2O 中的软骨和骨形成。在双报告动物中，Pdgfra ⁺ Tppp3 ⁺和Pdgfra ⁺ Tppp3 ^-祖细胞均产生 HO 相关软骨。最后，对人类样本的分析显示，大量表达 TPPP3 的细胞与异位骨区域中的成骨标记物重叠。总的来说，这些数据表明滑膜/腱鞘祖细胞经历异常的骨软骨分化，并有助于创伤后的 HO。