Summary Osteoclasts are large multinucleated bone-resorbing cells formed by the fusion of monocyte/macrophage-derived precursors that are thought to undergo apoptosis once resorption is complete. Here, by intravital imaging, we reveal that RANKL-stimulated osteoclasts have an alternative cell fate in which they fission into daughter cells called osteomorphs. Inhibiting RANKL blocked this cellular recycling and resulted in osteomorph accumulation. Single-cell RNA sequencing showed that osteomorphs are transcriptionally distinct from osteoclasts and macrophages and express a number of non-canonical osteoclast genes that are associated with structural and functional bone phenotypes when deleted in mice. Furthermore, genetic variation in human orthologs of osteomorph genes causes monogenic skeletal disorders and associates with bone mineral density, a polygenetic skeletal trait. Thus, osteoclasts recycle via osteomorphs, a cell type involved in the regulation of bone resorption that may be targeted for the treatment of skeletal diseases.

Osteocytes are master regulators of the skeleton. We mapped the transcriptome of osteocytes from different skeletal sites, across age and sexes in mice to reveal genes and molecular programs that control this complex cellular-network. We define an osteocyte transcriptome signature of 1239 genes that distinguishes osteocytes from other cells. 77% have no previously known role in the skeleton and are enriched for genes regulating neuronal network formation, suggesting this programme is important in osteocyte communication. We evaluated 19 skeletal parameters in 733 knockout mouse lines and reveal 26 osteocyte transcriptome signature genes that control bone structure and function. We showed osteocyte transcriptome signature genes are enriched for human orthologs that cause monogenic skeletal disorders (P = 2.4 × 10−22) and are associated with the polygenic diseases osteoporosis (P = 1.8 × 10−13) and osteoarthritis (P = 1.6 × 10−7). Thus, we reveal the molecular landscape that regulates osteocyte network formation and function and establish the importance of osteocytes in human skeletal disease.

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Highlights d Osteoclasts fission into daughter cells called osteomorphs d Osteomorphs fuse and recycle back into osteoclasts d Osteomorph upregulated genes control bone structure and function in mice d Osteomorph upregulated genes are implicated in rare and common bone diseases in humans

Osteocytes are master regulators of the skeleton. We map the transcriptome of osteocytes at different skeletal sites, across age and sexes in mice to reveal genes and molecular programs that control this complex cell-network. We define an osteocyte transcriptome signature, 1239 genes that distinguishes osteocytes from other cells. 77% have no known role in the skeleton.We show they are enriched for genes controlling neuronal network formation, suggesting this program is important in the osteocyte network. We evaluated 19 skeletal parameters in 733 mouse lines with functional-gene-deletions and reveal 26 osteocyte transcriptome signature genes that control bone structure and function. We showed osteocyte transcriptome signature genes are enriched for human homologues that cause monogenic skeletal dysplasias (P=6x10 -17 ), and associated with polygenic diseases, osteoporosis (P=1.8×10 -13 ), and osteoarthritis (P=2.6×10 -6 ). This reveals the molecular landscape that regulates osteocyte network formation and function, and establishes the importance of osteocytes in human skeletal disease. IntroductionThe skeleton is a highly dynamic structure that changes in shape and composition throughout life. Osteocytes are the most abundant cell type in bone and have emerged as master regulators of the skeleton. These enigmatic cells are connected via ramifying dendritic processes that form a complex multicellular network distributed throughout mineralized bone 1,2 .The scale and complexity of the osteocyte network is comparable to neurons in the brain, with 42 billion osteocytes present in the human skeleton forming 23 trillion connections 2,3 . This network enables osteocytes to detect and respond to mechanical strain, hormones and local growth factors and cytokines 1 . The network responds by regulating the formation and activity of osteoclasts and osteoblasts, instructing these cells to repair damaged bone, controlling bone mass and composition, and ensuring the optimal distribution of bone tissue in response to mechanical stress. Osteocytes also remove and replace bone surrounding the osteocyte network by the process of perilacunar remodeling, liberating calcium and phosphate in response to endocrine demands 4 . These features allow the osteocyte network to maintain both the structural integrity of the skeleton and mineral homeostasis. Osteocytes also have regulatory functions beyond the skeleton, including in skeletal muscle, adipose tissue, the central nervous system and in the control of phosphate homeostasis and energy expenditure, indicating the network acts as an important and endocrine organ 5-8 .Although osteocytes are pivotal in controlling the skeleton, the molecular programs that regulate their formation and function are poorly defined. Osteocytes are entombed in bone making them challenging to isolate and study. As a result osteocytes have been omitted from large-scale efforts to map tissue-specific transcriptomes 9-12 and studies of their transcriptome are limited [13][14][15][16] . Consequently, the influenc...