A tissue culture system supporting cartilage cell differentiation, extracellular mineralization, and subsequent bone formation, using mouse condylar progenitor cells

Undifferentiated progenitor cells of mandibular condyles of neonatal mice were kept in a tissue culture system for up to 9 days. After 2 days in culture, new chondroblasts developed within the explants, whereas the peripheries of the latter were occupied by undifferentiated cells undergoing mitosis. By 4 days in culture, many of the cartilage cells showed signs of hypertrophy, while the matrix revealed positive reactivity for type II collagen and matrix metachromasia. The process of maturation of cartilage cells appeared to have reached its final stages after 6 days in culture, at a time when the initial loci of matrix mineralization could be easily identified. Concomitantly, peripheral areas bordering the cartilaginous core, as well as portions of the cartilage, reacted positively for type I collagen and fibronectin. Light microscopy examination showed signs of new bone formation after 9 days in culture. The extracellular matrix at the upper portion of the explant, facing the medium-air interface, reacted intensely with antibodies against type I collagen and fibronectin, but not with antibodies against type II collagen. Further, the newly formed osteoid was found to have undergone mineralization, thus forming an expanded layer of new bony tissue. A close spatial association was found between mature, mineralized cartilage and new bone. Hence, we hereby introduce a new in vitro system serving as an experimental model for studies related to the differentiation of progenitor cells into chondroblasts, which in turn promote ossification.