Integrating levels of bone growth control: From stem cells to body proportions.

The study of the mechanisms controlling organ size during development and regeneration is critical to understanding how complex life arises from cooperating single cells. Long bones are powerful models in this regard, as their size depends on a scaffold made from another tissue (cartilage, composed of chondrocytes), and both tissues interact during the growth period. Investigating long bone growth offers a valuable window into the processes that integrate internal and external cues to yield finely controlled size of organs. Within the cellular and molecular pathways that control bone growth, the regulation of stem-cell renewal, along with amplification and differentiation of their progeny, are key to understanding normal and perturbed long-bone development. The phenomenon of "catch-up" growth-where cellular hyperproliferation occurs following injury to restore a normal growth trajectory-reveals key aspects of this regulation, such as the fact that bone growth is target-seeking. The control mechanisms that lead to this behavior are either bottom-up or top-down, and the interaction between these modes is likely critical to achieve a highly nuanced, yet flexible, degree of control. The role of cartilage-intrinsic mechanisms has been well studied, establishing a very solid groundwork for this field. However, addressing the unanswered questions of bone growth arguably requires new hypotheses and approaches. Future research could for example address to what extent extrinsic signals and cells, as well as communication with other tissues, modulate intra-limb and inter-organ growth coordination. This article is categorized under: Adult Stem Cells, Tissue Renewal, and Regeneration > Tissue Stem Cells and Niches Establishment of Spatial and Temporal Patterns > Regulation of Size, Proportion, and Timing Vertebrate Organogenesis > Musculoskeletal and Vascular.