Novel Events in Cartilage Remodeling During Endochondral Bone Formation

Cartilage remodeling is one of the main events occurred to permit invasion of osteogenic cells and deposition of endochondral bone. Many types of cells secrete varieties of proteolytic enzymes such as matrix metalloproteinases (MMP) family and ADAMS (Disintegrin and metalloprotease with thrombospondin type 1 motifs) which exert degradation of cartilage matrix. The current study aimed to specify types of cells involved in cartilage remodeling and distinguish new dramatic events accompanied cartilage remodeling. Bone samples were taken from tibia of quail (Coturnix coturnix) at different ages and processed for light and electron microscopic examination. Two types of chondrocytes; light and dark chondrocytes involved in degradation of the pericellular matrix. Cartilage eroding cells originated form the periosteum. In the eroding surface, populations of mononucleated and multinucleated cells can be distinguished connected by cytoplasmic processes. Eroding cells were categorized according to heterogeneity. Chondroclasts were characterized by high electron cytoplasmic density, rich in secretory vesicles and covered by microvilli. Chondroclasts acquired polarized profile in lower metaphysis. Two types of heterogenic different osteoblasts were observed; osteoblasts of moderate electron density presented in the tranphyseal cartilage canals and were specialized to deposit the osteoid tissue on the surface of cartilage matrix. This type was not involved in remodeling process. The other type was Intralacunar osteoblasts (light osteoblasts) which possessed the low electron cytoplasmic density, vesicular nucleus and rough endoplasmic reticulum. Light osteoblasts removed the septal matrix of the cartilage in order to get access into the empty lacunae and deposit bone tissue inside the lacunae.