Eyal, S. | Search bioRxiv

bioRxiv2018

Bone Morphology is Regulated Modularly by Global and Regional Genetic Programs

During skeletogenesis, a variety of protrusions of different shapes and sizes develop on the surfaces of long bones. These superstructures provide stable anchoring sites for ligaments and tendons during the assembly of the musculoskeletal system. Despite their importance, the mechanism by which superstructures are patterned and ultimately give rise to the unique morphology of each long bone is far from understood. In this work, we provide further evidence that long bones form modularly from Sox9+ cells, which contribute to their substructure, and from Sox9+/Scx+ progenitors that give rise to superstructures. Moreover, we identify components of the genetic program that controls the patterning of Sox9+/Scx+ progenitors and show that this program includes both global and regional regulatory modules.\n\nUsing light sheet fluorescence microscopy combined with genetic lineage labeling, we mapped the broad contribution of the Sox9+/Scx+ progenitors to the formation of bone superstructures. Additionally, by combining literature-based evidence and comparative transcriptomic analysis of different Sox9+/Scx+ progenitor populations, we identified genes potentially involved in patterning of bone superstructures. We present evidence indicating that Gli3 is a global regulator of superstructure patterning, whereas Pbx1, Pbx2, Hoxa11 and Hoxd11 act as proximal and distal regulators, respectively. Moreover, by demonstrating a dose-dependent pattern regulation in Gli3 and Pbx1 compound mutations, we show that the global and regional regulatory modules work coordinately. Collectively, our results provide strong evidence for genetic regulation of superstructure patterning that further supports the notion that long bone development is a modular process.

developmental biology↗

bioRxiv2018

On the Development of Sesamoid Bones

Sesamoid bones are a special group of small auxiliary bones that form in proximity to joints and contribute to their stability and function. Sesamoid bones display high degree of variability in size, location, penetrance and anatomical connection to the main skeleton across vertebrate species. Therefore, providing a comprehensive developmental model or classification system for sesamoid bones is challenging. Here, we examine the developmental mechanisms of three anatomically different sesamoid bones, namely patella, lateral fabella and digit sesamoids. Through a comprehensive comparative analysis at the cellular, molecular and mechanical levels, we demonstrate that all three types of sesamoid bones originated from Sox9+/Scx+ progenitors under the regulation of TGF{beta} and independent of mechanical stimuli from muscles. We show that BMP4 was necessary specifically for differentiation of patella but not of lateral fabella or digit sesamoids, whereas BMP2 regulated the growth of all examined sesamoids. Next, we show that whereas patella and digit sesamoids initially formed in juxtaposition to long bones, the lateral fabella formed independently at a distance. Finally, we provide evidence suggesting that while patella detached from the femur by formation of a synovial joint, digit sesamoids detached from the phalanx by a fibrocartilage joint. Collectively, these findings highlight both common and divergent molecular and mechanical features of sesamoid bone development, thereby advancing our understanding of their evolutionary plasticity.

developmental biology↗