Type Ⅱ collagen-positive progenitors are important stem cells in controlling skeletal development and vascular formation

Type Ⅱ collagen-positive (Col2⁺) cells have been reported as skeletal stem cells (SSCs), but the contribution of Col2⁺ progenitors to skeletal development both prenatally and postnatally during aging remains unclear. To address this question, we generated new mouse models with ablation of Col2⁺ cells at either the embryonic or postnatal stages. The embryonic ablation of Col2⁺ progenitors resulted in the death of newborn mice due to a decrease in skeletal blood vessels, loss of all vertebral bones and absence of most other bones except part of the craniofacial bone, the clavicle bone and a small piece of the long bone and ribs, which suggested that intramembranous ossification is involved in long bone development but does not participate in spine development. The postnatal ablation of Col2⁺ cells resulted in mouse growth retardation and a collagenopathy phenotype. Lineage tracing experiments with embryonic or postnatal mice revealed that Col2⁺ progenitors occurred predominantly in the growth plate (GP) and articular cartilage, but a limited number of Col2⁺ cells were detected in the bone marrow. Moreover, the number and differentiation ability of Col2⁺ progenitors in the long bone and knee joints decreased with increasing age. The fate-mapping study further revealed Col2⁺ lineage cells contributed to, in addition to osteoblasts and chondrocytes, CD31⁺ blood vessels in both the calvarial bone and long bone. Specifically, almost all blood vessels in calvarial bone and 25.4% of blood vessels in long bone were Col2⁺ lineage cells. However, during fracture healing, 95.5% of CD31⁺ blood vessels in long bone were Col2⁺ lineage cells. In vitro studies further confirmed that Col2⁺ progenitors from calvarial bone and GP could form CD31⁺ vascular lumens. Thus, this study provides the first demonstration that intramembranous ossification is involved in long bone and rib development but not spine development. Col2⁺ progenitors contribute to CD31⁺ skeletal blood vessel formation, but the percentage differs between long bone and skull bone. The number and differentiation ability of Col2⁺ progenitors decreases with increasing age.