Single cell transcriptomes reveal a mesenchymal state during chondrocyte to osteoblast transition
Kwok Yeung Tsang1, Xiangyu Li2, Ron Wu1, Jin Gu2, Danny Chan1, Michael Q. Zhang2,3, Kathryn SE Cheah1
1School of Biomedical Sciences, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong SAR, China; 2Bioinformatics Division, Center for Synthetic and Systems Biology, TNLIST, Tsinghua University, Beijing, China; 3Department of Molecular and Cell Biology, Center for Systems Biology, The University of Texas at Dallas, Richardson, TX, 75080, USA
During mammalian endochondral bone formation, growth plate cartilage laid down by chondrocytes is replaced with bone produced by osteoblasts. Although these two cell types were thought to be distinct lineages that are descendants of a common mesenchymal progenitor, we have shown a chondrocyte-osteoblast continuum during endochondral bone formation. In this process hypertrophic chondrocytes survive during the cartilage-to-bone transition and contribute to all osteoblast lineages. However, molecular control of the lineage progression is unclear. We have applied single cell transcriptomics to identify lineage intermediates during the chondrocyte to osteoblast transition. Genetically tagged fluorescent single cells of this lineage were manually isolated from the proximal tibias of neonatal mice, from which 320 single cell transcriptomic data were generated. Pseudotemporal ordering revealed three distinct populations; two of which are characteristic of hypertrophic chondrocyte and osteoblast, and an additional one expressing cell cycle markers and/or mesenchymal/stromal cell markers, including Cxcl12 that is associated with maintenance of hematopoietic niche. Single cell transcriptome analysis serves as a powerful tool to elucidate lineage hierarchy and provides important insights into the transition from chondrocyte to osteoblastic lineage in bone development.