Common cellular origin and diverging developmental programs for different sesamoid bones

Abstract

Sesamoid bones are small auxiliary bones that form near joints and contribute to their stability and function. Thus far, providing a comprehensive developmental model or classification system for this highly diverse group of bones has been challenging. Here, we compare our previously reported mechanisms of patella development with those of two anatomically different sesamoids, namely lateral fabella and digit sesamoids. We show that all three types of sesamoid bones originated from Sox9+/Scx+ progenitors under the regulation of TGFβ and independently of mechanical stimuli from muscles. Whereas BMP2 regulated the growth of all examined sesamoids, the differentiation of lateral fabella or digit sesamoids was regulated redundantly by BMP4 and BMP2. Next, we show that whereas patella and digit sesamoids initially formed in juxtaposition to long bones, lateral fabella formed independently and at a distance. Finally, our evidence suggests that unlike the synovial joint separating patella from femur, digit sesamoids detached from the phalanx by formation of a fibrocartilaginous joint. These findings highlight both common and divergent molecular and mechanical features of sesamoid bone development, underscoring their evolutionary plasticity.