RESUMO
Extracellular membranous matrix vesicles were localized and described using electronmicroscopy during chondrogenesis, osteogenesis, and dentinogenesis. Evidence indicates that matrix vesicles in each of these specific tissue types function to concentrate and transport ions and enzymes which serve as nucleation sites for the mineralization of hydroxylapatite. We have examined different developmental stages of Meckel's cartilage, alveolar bone and epithelial-mesenchymal interactions associated with tooth formation in newborn mice. These ultrastructural studies indicate matrix vesicle heterogeneity. Whereas most matrix vesicles contain alkaline phosphatase activity during cartilage, bone and dentine mineralization, in earlier developmental stages matrix vesicles contain acid phosphatase activities and little, if any, alkaline phosphatase. Tissue type, specific developmental stage, and ultrastructural criteria indicate various "classes" of matrix vesicles. During epithelial-mesenchymal interactions in tooth development, mesenchymal cells (preodontoblasts) appear to be the source of matrix vesicles as indicated by the complementarity between H-2 histocompatibility alloantigen specificity on the cell surface and that of the matrix vesicle outer surface; matrix vesicles are limited by a trilaminar membrane derived from the mesenchymal cells. Some of the vesicles located adjacent to dividing inner enamel epithelial cells contain RNA's as determined by electron microscopic autoradiography in situ, as well as by direct biochemical assays. We postulate that matrix vesicles have many different and important biological functions, one of which may be to mediate developmental information from mesenchyme to epithelia during "instructive" stages of tooth development.
