Matrix components in the developing bones and cartilage of embryonic chick skulls: an in situ hybridization study

Matrix components involved in the development of various types of bone and cartilage in chick skulls were analyzed from the gene expression viewpoint. The quadrate cartilage (primary cartilage), quadrate bone (endochondral bone), pterygoid bone (membrane bone), and secondary cartilages, such as squamosal secondary cartilage and palatoquadrate cartilage, were investigated by in situ hybridization using digoxigenin-labeled RNA probes for VCAN, ACAN, COL1A2, COL2A1, COL10A1, integrin-binding bone sialoprotein (IBSP), secreted phosphoprotein 1 (SPP1), and dentin matrix protein 1 (DMP1). VCAN mRNA was expressed in the anlagen of quadrate cartilage and pterygoid bone, and reduced as chondrogenesis or osteogenesis progressed. Conversely, VCAN was neither expressed in the anlagen nor formed new secondary cartilages, indicating VCAN may not be highly significant in secondary chondrogenesis. Newly formed chondrocytes in the squamosal secondary cartilage simultaneously expressed COL1A2, COL2A1, and COL10A1 mRNAs, indicating its fibrocartilaginous nature and the rapid differentiation of progenitor cells into hypertrophic chondrocytes. Compared with SPP1 mRNA, IBSP was earlier expressed during pterygoid bone formation. SPP1 expression was more extensive in the central position than that of IBSP in the quadrate cartilage, whereas IBSP expression was more extensive in newly formed squamosal secondary cartilage, reflecting the difference in the differentiation process of primary and secondary chondrogenesis. Alkaline phosphatase activity was detected in the anlagen and newly formed chondrocytes in the squamosal secondary cartilage, indicating it originated from periosteum-like tissues. However, the palatoquadrate cartilage showed no alkaline phosphatase activity. In conclusion, gene expression of the extracellular matrix during chondrogenesis and osteogenesis showed tissue-specific expression patterns.