Are CD117, MMP-9 and Type II Collagen Positive Undifferentiated Cells involved in Cartilage Growth? A Description of Unusual Interstitial Type of Cartilage Growth.

Cartilage growth requires undifferentiated perichondrial cells in appositional growth and chondrocytes in interstitial growth. This study was performed to identify a third type of cartilage growth in different skeletal elements of quail embryos. Quail embryos were collected, fixed, dehydrated, and embedded in paraffin. Paraffin sections of the whole embryos were stained using H&E and Safranin O stains. Immunohistochemical staining for CD117, matrix metalloproteinase-9 (MMP-9), and type II collagen was performed on embryonic cartilage. Other samples were processed for scanning electron microscopy. During embryonic development, cells extended from mesenchymal tissue or perichondrium penetrated the cartilage templates of different skeletal elements. The morphological and immunological characteristics of these cells were identical to mesenchymal cells characterized by their small size having multiple cell processes and staining positive for CD117. MMP-9 positive cells were detected in the cartilage matrix. The mesenchymal cells further differentiated into chondrocytes which were positive for type II collagen immunostaining and secreted Safranin O positive cartilage matrix. In conclusion, our results suggest that CD117 positive mesenchymal cells secrete MMP-9 to penetrate growing cartilage, produce type II collagen and proteoglycan, and have a role in the interstitial growth of embryonic cartilage templates in quail.