Resting zone of the growth plate houses a unique class of skeletal stem cells

Abstract

Summary Skeletal stem cells regulate bone growth and homeostasis by generating diverse cell types including chondrocytes, osteoblasts and marrow stromal cells. The emerging model postulates a distinct type of skeletal stem cells closely associated with the growth plate 1 - 4 , a special cartilaginous tissue playing critical roles in bone elongation 5 . The resting zone maintains the growth plate by expressing parathyroid hormone-related protein (PTHrP) that interacts with Indian hedgehog (Ihh) released from the hypertrophic zone 6 - 10 , while providing a source of other chondrocytes 11 . However, the identity of skeletal stem cells and how they are maintained in the growth plate are unknown. Here we show that skeletal stem cells are formed among PTHrP + chondrocytes within the resting zone of the postnatal growth plate. PTHrP + chondrocytes expressed a panel of markers for skeletal stem/progenitor cells and uniquely possessed the properties as skeletal stem cells in cultured conditions. Cell lineage analysis revealed that PTHrP + resting chondrocytes continued to form columnar chondrocytes long term, which underwent hypertrophy and became osteoblasts and marrow stromal cells beneath the growth plate. Transit-amplifying chondrocytes in the proliferating zone, which was concertedly maintained by a forward signal from undifferentiated cells (PTHrP) and a reverse signal from hypertrophic cells (Ihh), provided instructive cues to maintain cell fates of PTHrP + resting chondrocytes. Our findings unravel a unique somatic stem cell type that is initially unipotent and acquires multipotency at the post-mitotic stage, underscoring the malleable nature of the skeletal cell lineage. This system provides a model in which functionally dedicated stem cells and their niche are specified postnatally and maintained throughout tissue growth by a tight feedback regulation system.

Growth Plate Borderline Chondrocytes Behave as Transient Mesenchymal Precursor Cells

“…L ); these cells go through a unique state expressing Col10a1 without hypertrophy on their way to becoming mesenchymal cells in the marrow space. In combination with our previous finding that hypertrophic chondrocytes in the central growth plate can produce marrow mesenchymal cells predominantly in the adult stage, we speculate that different routes of chondrocytes are utilized to generate osteoblasts and marrow stromal cells in a stage‐specific manner. Additionally, our data indicate that Pthrp‐creER + borderline chondrocytes represent a non‐self‐renewing subset of Col2a1‐creER + chondrocytes; we observed that borderline chondrocyte‐derived marrow mesenchymal cells could only transiently persist within the marrow space.…”

“…PTHrP is initially expressed in the periarticular region of the fetal and neonatal growth plate and later in the resting zone of the postnatal growth plate …”

“…The growth plate can provide a substantial source of mesenchymal cells in endochondral ossification, which has been supported by a series of murine lineage‐tracing studies using cre/creER transgenes active in chondrocytes and their close relatives, such as Sox9‐creER , type II collagen (Col2)‐creER , aggrecan (Acan)‐creER , and type X collagen (Col10 )‐ cre/creER. Our lineage‐tracing study using a Pthrp‐creER line demonstrated that PTHrP + resting chondrocytes can establish columnar chondrocytes, some of which can further differentiate into osteoblasts and marrow stromal cells through chondrocyte‐to‐osteoblast transformation . However, these columnar chondrocytes of the central growth plate can contribute only to a minor fraction of marrow mesenchymal cells in adulthood; therefore, chondrocytes in other locations may contribute more significantly in earlier stages.…”

“…Pthrp‐creER , Cxcl12 GFP/+ mice have been described elsewhere. Col1a1(2.3 kb)‐ GFP (JAX013134), Col2a1‐creER (JAX006774), Rosa26‐CAG‐loxP‐stop‐loxP‐ tdTomato (Ai14: R26R ‐tdTomato, JAX007914) mice were acquired from the Jackson Laboratory (Bar Harbor, ME, USA).…”

Possible Contribution of Wnt-Responsive Chondroprogenitors to the Postnatal Murine Growth Plate

“…Previous studies have demonstrated that decreases in number and proliferation activity of the resting chondrocytes are closely linked to reduction in bone growth with age and pathological growth arrest, indicating that the resting zone contains chondroprogenitors . More recently, lineage tracing analysis directly demonstrated that a subset of cells labeled with the PTHrP reporter system form the growth plate . The ZsG+ Wnt‐responsive cells were also found in the resting zone and proliferating zone of P20 mice.…”