The chondrocytes and osteoblasts are mesenchymal in origin while osteoclasts derive from the hematopoietic system. The differentiation and function of each bone cell type appear controlled by several local and systemic factors which lead to tightly regulated patterns of specific gene expression. The essential difference between them is the presence or absence of a cartilaginous phase. Intramembranous ossification occurs when mesenchymal precursor cells proliferate and subsequently differentiate directly into osteoblasts which mineralize an immature bone tissue called woven bone, characterized by irregular bundles of randomly oriented collagen fibers and an abundance of partially calcified immature new bone, called osteoid. At later stages, this woven bone is progressively remodeled to mature, lamellar bone.
Endochondral ossification entails the conversion of a cartilaginous template into bone.
Mesenchymal cells condense and differentiate into chondrocytes which secrete the cartilaginous matrix. This embryonic cartilage is avascular, and during its early development, a ring of woven bone is formed by intramembranous ossification in the future midshaft area. This calcified woven bone is then invaded by vascular tissue, and osteoclasts and osteoblasts are recruited to replace the cartilage scaffold with bone matrix and excavate the hematopoietic bone marrow cavity. Growth plate chondrocytes appear in regular columns that initially proliferate and progressively hypertrophy. The cartilaginous matrix becomes mineralized just below the hypertrophic zone of the growth plate and the chondrocytes then die.