Further characterisation of the extracellular matrix in the mandibular condyle in neonatal mice.

This study provides newer information concerning the extracellular matrix of neonatal condylar cartilage--a genuine representative of a secondary type of cartilage. In addition, the data presented hereby indicate that the condylar cartilage contains a population of progenitor cells that synthesise Type I collagen rather than Type II. Under normal conditions in vivo local biomechanical factors influence the progenitor cells to differentiate into cartilage cells and thereby to shift their synthetic pathway from Type I collagen to Type II collagen--the typical collagen of cartilage extracellular matrix. In the absence of such biomechanical effects the condylar progenitor cells seem to proceed with their inherent differentiation pathway and express an osteogenic phenotype (Fig. 21).

Chondroid bone arises from mesenchymal stem cells in organ culture of mandibular condyles.

Mandibular condyles of fetal mice 19 to 20 days in utero comprising clean cartilage and its perichondrium were cultured for up to 14 days, and their capacity to develop osteoid and to mineralize in vitro was examined. After 3 days in culture the cartilage of the mandibular condyle appeared to have lost its inherent structural characteristics, including its various cell layers: chondroprogenitor, chondroblastic, and hypertrophic cells. At that time interval no chondroblasts could be seen; instead, most of the cartilage consisted of hypertrophic chondrocytes. By that time, the surrounding perichondrium, which contains pluripotential mesenchymal stem cells, revealed the first signs of extracellular matrix enclosing type I collagen, bone alkaline phosphatase, osteonection, fibronectin, and bone sialoprotein as demonstrated by immunofluorescent techniques. Electron microscopic examinations of the newly formed matrix revealed foci of mineralization within and along collagen fibers as is normally observed during bone development. The composition of the latter mineral deposits resembled calcium pyrophosphate crystals. Following 14 days in culture larger portions of the condyle revealed signs of osseous matrix, yet the tissue reacted positively for type II collagen. Hence, the condylar cartilage, a genuine representative of secondary-type cartilage, elaborated in vitro a unique type of bone that would be most appropriately defined as chondroid bone. Biochemical assays indicated that the de novo formation of chondroid bone was correlated with changes in alkaline phosphatase activity and 45Ca incorporation. The findings of the present study imply that mesenchymal stem cells that ordinarily differentiate into cartilage possess the capacity to differentiate into osteogenic cells and form chondroid bone.

The in vitro behavior of fetal condylar cartilage in serum-free hormone-supplemented medium.

Mandibular condyles of fetal mice 19-20 days in utero were kept in a serum-free organ culture system for up to 14 days and were investigated for their capacity to develop osteoid and to mineralize in vitro. After 3 days in culture, the cartilage of the mandibular condyle appeared to have maintained all its inherent structural characteristics, including its various cell layers: chondroprogenitor, chondroblastic, and hypertrophic. After 7-9 days in culture, no chondroblasts could be seen; instead, most of the cartilage consisted of hypertrophic chondrocytes. In addition, various areas throughout the explant revealed the appearance of osteoidlike material. The process of matrix mineralization progressed with time, and by the 14th day new bonelike material was found to occupy a larger portion of the explant. The newly formed matrix reacted positively with antibodies against type I and type III collagens, as well as against bone alkaline phosphatase. Electron microscopic examination showed that the mineralization appeared to be associated with collagen fibers as well as the matrix vesicles. In composition, the in vitro-formed mineral deposits resembled hydroxyapatite crystals. Biochemical assays indicated that the newly formed tissue reacted strongly for alkaline phosphatase and incorporated 45Ca. The findings of the present study imply that fetal condylar cartilage possesses the potential to develop in vitro osseouslike tissue even in a system that is serum-free. Due to the fact that the newly formed extracellular matrix mineralized and reacted positively to bone markers as well as to cartilage macromolecules, it would seem justifiable to define the new tissue as chondroid bone.