Skeletal development and maintenance is a complex process. There are two essential ways to form bone: intramembranous ossification and endochondral ossification. Intramembranous ossification, which directly converts mesenchymal tissue into bone, usually occurs during the initial formation of the flat bones of skull. Mesenchymal cells aggregate and differentiate into osteoblast to produce and mineralize bone matrix, which forms the ossification center (compact nodules). With the calcification process, bony spicules are formed and surrounded by periosteum that is formed by compact mesenchymal cells. Periosteum continually provides mesenchymal cells for differentiation to osteoblasts, which deposit bone matrix to the existing spicules.
Unlike intramembranous ossification, endochondral ossification is a more complex process that forms a cartilage intermediate, which later replaced by bone. This occurs in the majority of the bones, including long bones, short bones, flat bones, and irregular bones. In endochondral ossification, mesenchymal cells form cartilaginous template by condensation. Chondrocytes deposit collagen and proteoglycans and then undergo hypertrophic growth in the primary ossification center (in the shaft of the bone). The matrix begin mineralize and later used by entered osteoprogenitor cells to form bone. Ossification in the shaft begins in the middle and then spreads towards the epiphyses (the ends of the bone). Later, secondary ossification centers appear in the epiphyses and only left a small region of cartilage (epiphyseal growth plate) in between of two ossification centers. In growth plate, chondrocytes undergo proliferation, hypertrophy and apoptosis. The cartilage extracellular matrix made by chondrocytes is converted to trabecular bone by osteoblasts. The transitions of mesenchymal to chondrocytes and osteoblasts are tightly regulated by complicated signaling pathways.
Osteoblasts derive from pluripotent mesenchymal cells and their differentiation can be divided into four major stages: cell proliferation, matrix maturation and mineralization, and apoptosis (or embedded in bone as osteocytes). Runx2 is the master gene that regulates the commitment of mesenchymal cells. Other genes like Osterix (OSX), ALP, type I collagen, osteopontin and osteocalcin are also important for osteoblast differentiation and usually used as molecular markers of osteoblast differentiation. However, the mechanisms that control osteoblast commitment and differentiation are poorly understood. A group of Cre transgenic lines have been generated using promoters of these markers genes, which greatly facilitates the study of osteoblast differentiation and bone development. In this study, we used OSX-Cre that inactivate gene in the osteoblast progenitor cells.
Chondrogenesis, the formation of cartilage, is a multi-step process that commences when pluripotent mesenchymal precursors commit to the chondrogenic cell lineage. Mesenchymal derived chondrocytes originate from several sources. Craniofacial cartilage is derived from cranial neural crest cells, the sclerotome of the somites forms the axial skeleton, and the limb skeleton is derived from lateral plate mesoderm. These mesenchymal precursor cells, under strict molecular regulation, proliferate and differentiate to form cartilage primordia that prefigure future skeletal elements. There are two main types of cartilage in the developing body. Articular cartilage located at the end of bones and in the cranium is permanent connective tissue that functions to bear weight, allow for bone movement, and is highly flexible.
Endochondral ossification, the process of appendicular skeleton bone formation, is dependent on chondrogenic precursor cells as an intermediate to bone development. Early condensed mesenchymal cells that are fated to become chondrocytes are arranged in a growth plate, also known as an epiphyseal plate, and transition through a sequential series of chondrogenic zones. Chondrocytes in the resting/reserve zone are relatively quiescent and small and rounded in appearance with abundant ECM. This reserve population of chondrogenic cells transitions to the proliferative zone where they are flattened and align in parallel, longitudinal columns of dividing cells. These cells transition from proliferative chondrocytes to prehypertrophic chondrocytes, in which chondrocytes stop dividing and start to undergo hypertrophy. Finally, the hypertrophic chondrocytes undergo apoptosis, providing the mineralized matrix as a scaffold for future bone development, osteogenesis. The cartilaginous scaffold is invaded by blood vessels whereby osteoblasts are transported to the cartilage scaffold to form a calcified bone matrix. Bone growth is an ongoing process that is driven primarily by the rate of hypertrophic chondrocyte production from the proliferating chondrocytes and ends with the cessation of puberty in humans.