Chondrogenic cell differentiation from membrane bone periostea

Most craniofacial membrane bones are derived from neural crest (NC) cells. Interaction between NC cells and epithelium, and cellular condensation, are two major events that lead NC cells to become osteoblasts that deposit membrane bone. Unlike endochondral bone, membrane bone formation is not preceded by cartilage formation in normal development. However, chondrogenic potential in membrane bone is evidenced by several cartilage-associated phenomena in vivo. Furthermore, in vitro, periosteal cells of some membrane bones express cartilage phenotype gene products and even differentiate into chondrocytes. Hence, membrane bone periosteal cells can undergo chondrogenic differentiation. The precursor of chondrogenic cells in membrane bone is not clear: chondrocytes were proposed to arise from unipotential chondroprogenitor cells, bi- or multipotential progenitor cells, or differentiated osteogenic cells. There is experimental support for each, but studies on clonal and cell cultures provided more support for a common precursor of both chondro- and osteogenic cells. Moreover, in periostea, chondrogenesis probably arises from a differentiated cell type. Membrane bone formation in periostea may include a transient cell stage that is able to undergo both osteo- and chondrogenesis. Osteogenesis would be the normal pathway, but chondrogenesis can be evoked in certain microenvironments. It is not known whether microenvironmental factors trigger chondrogenesis through a universal molecular mechanism, nor is the molecule that triggers chondrogenesis known. Expression of neural cell adhesion molecule (NCAM) is down-regulated during commitment of periostal cells for secondary chondrogenesis, suggesting a possible regulatory role for NCAM in the alternative differentiation pathways of periosteal cells.