Endochondral Bone Formation

Endochondral ossification is proceeded by initial development of cartilage model. Cartilage grows to gain the size and the shape before ossification process. Two prevalent types of cells contribute to cartilage growth; chondrocytes and perichondrial cells. Recently, mesenchymal cells have been further mentioned during cartilage growth. The current study investigated contribution of mesenchymal cells in growth of cartilage model at the onset of ossification. Samples of quail embryos of 8, 9 and 10 post-incbation days were processed for the light microscopic examination. Cartilage templates of the developing femur and tibia were formed and were entirely covered by a perichondrium. Differential organization of the growth cartilage was distinguished in both cartilage templates.  At the onset of ossification (9 day), aggregations of  subperiosteal cells was observed having typical profile of undifferentiated mesenchymal cells. The subperiosteal Mesenchymal cells niche occupied a long distance in the central hypertrophic zone and some mesenchymal cell penetrated the interior of the cartilage templete. Differentiating chondrocytes secrete proteoglycan-rich cartilage matrix which appeared basophilic by H&E, and was staniws positve with both PAS-positive, and safranin O. Differentiating chondrocytes were observed adjacent to the vacated lacunae. At day 10, subperiosteal mesenchymal cells niche was reduced to solitary mesenchymal-like cells which retained the chondrogenic potential. In conclusion, The subperiosteal mesenchymal cells niche provided the cartilage templates by chondrogenic cells for cartilage growth and regeneration.  Understanding mechanisms of mesenchymal cells invasion and chondrogenic differentiation is a promise in regenerative medicine.