Dermal bone grows by cellular invasion and intercalary biomineralization
Dermal bone architecture bears great evolutionary significance for jawed vertebrates. Its thickness growth is believed to rely on matrix apposition by superficial osteoblasts. To test this hypothesis we employ genetic lineage mapping, conditional gene ablation, intravital matrix labelling and molecular 3D analysis at single cell resolution in vivo. We uncover an invasive mechanism incompatible with apposition. The pervasive arrangement of two layers sandwiching a third spongy layer develops from a molecularly defined bi-layer via a previously unknown developmental module: Outer layer osteoblasts form rosette-like assemblies around single invading cells. The latter bear unique molecular signatures and form de novo sheets inside the spongy layer, communicating with the layer below. The transcription factor Hand2 organizes this module by orchestrating rosette formation, their molecular heterogeneity, cellular invasion and spongy layer thickness growth. Surprisingly, invading osteoblasts secrete new biomineral matrix inside the older matrix, which keeps expanding. Such intercalary biomineralization provides new perspectives for bone biology and the evolution of endochondral ossification.