Figure 8. Model Summarizing Our Findings.

Schematic outline of the events taking place during the initial invasion of the cartilaginous bone model and its transformation into the primary ossification center (POC). Col2-expressing chondro-perichondrial progenitors (green) give rise to cells in the central growth cartilage and to transversally oriented cells at the periphery with presumed perichondrial/osteoblastic fates. The first committed osteoblast lineage cells appear in the perichondrium surrounding the middiaphyseal hypertrophic cartilage. The cells display differential destinies in the developing bone depending on their stage of maturation. Early cells of the lineage, represented by the Osx-expressing osteoblast precursors (Osx/LacZ+, yellow), move into the developing POC and populate it as stromal cells or differentiate further to become bone-forming trabecular osteoblasts. The entrance of the osteoblast precursors into the POC coincides with the initial invasion by blood vessels (red) and osteoclasts (blue), and is associated with a pericytic localization of a subset of the precursors onto the endothelium. In contrast, cells that differentiated to the stage of Col1 (3.2 kb) expression within the perichondrium/periosteum (Col1/LacZ+ mature osteoblasts, orange) are not found in vessel-covering positions and do not have the capacity to translocate into the POC. These osteoblasts are retained on and within the cortical bone surfaces.