Growth Plate Borderline Chondrocytes Behave as Transient Mesenchymal Precursor Cells

Abstract

The growth plate provides a substantial source of mesenchymal cells in the endosteal marrow space during endochondral ossification. The current model postulates that a group of chondrocytes in the hypertrophic zone can escape from apoptosis and transform into cells that eventually become osteoblasts in an area beneath the growth plate. The growth plate is composed of cells with various morphologies; particularly at the periphery of the growth plate immediately adjacent to the perichondrium are “borderline” chondrocytes, which align perpendicularly to other chondrocytes. However, in vivo cell fates of these special chondrocytes have not been revealed. Here we show that borderline chondrocytes in growth plates behave as transient mesenchymal precursor cells for osteoblasts and marrow stromal cells. A single‐cell RNA‐seq analysis revealed subpopulations of Col2a1‐creER‐marked neonatal chondrocytes and their cell type–specific markers. A tamoxifen pulse to Pthrp‐creER mice in the neonatal stage (before the resting zone was formed) preferentially marked borderline chondrocytes. Following the chase, these cells marched into the nascent marrow space, expanded in the metaphyseal marrow, and became Col(2.3 kb)‐GFP+ osteoblasts and Cxcl12‐GFPhigh reticular stromal “CAR” cells. Interestingly, these borderline chondrocyte‐derived marrow cells were short‐lived, as they were significantly reduced during adulthood. These findings demonstrate based on in vivo lineage‐tracing experiments that borderline chondrocytes in the peripheral growth plate are a particularly important route for producing osteoblasts and marrow stromal cells in growing murine endochondral bones. A special microenvironment neighboring the osteogenic perichondrium might endow these chondrocytes with an enhanced potential to differentiate into marrow mesenchymal cells. © 2019 American Society for Bone and Mineral Research.

Insights into Gene Regulatory Networks in Chondrocytes

“…In addition, detailed transcriptional profiling in distinct cell types can be linked to the gene regulatory network during cartilage formation and pathogenesis. Single-cell RNA-seq (scRNA-seq) analyses in growth plate chondrocytes [57,58], skeletal development [59], and cartilage pathogenesis [60] may provide insights into the dynamic molecular link at the single-cell level.…”

Skeletal stem cells: insights into maintaining and regenerating the skeleton

“…At postnatal stages, mTORC1 signaling was found to fine-tune chondrocyte cell divisions to achieve the correct balance of asymmetric divisions that optimally maintain chondrocyte stem cells in the resting zone (Newton et al, 2019). A separate study used PTHrP-CreER to label a subset of resting zone chondrocytes and found that they are a major source of chondrocytes for several months after labeling (Mizuhashi et al, 2019). In agreement with this observation, diphtheria-toxin-mediated ablation of PTHrP + cells disrupts bone elongation.…”

Systematic Study of Single-Cell Isolation from Musculoskeletal Tissues for Single-Cell Sequencing

“…Also utilizing the single-cell platform, there were two nice reports that systematically uncovered the cell atlas and transcriptional regulation mechanisms of OA [30,31]. As for other related studies, several reports showed the single-cell pro ling results of developing limbs, including growth plate cartilage as well as articular cartilage [32][33][34][35]. The single-cell isolation methods used in the above studies were summarized in Table 1.…”