Histologic and ultrastructural studies on the mineralization process in hypophosphatasia

Chondroosseous tissue from six infants with infantile hypophosphatasia and six control infants were studied by light, transmission, and scanning electron microscopy. Alkaline phosphatase histochemical reaction of the growth plate was studied in two infants and was greatly reduced when compared to two control infants. Hypertrophic chondrocytes were increased in number with persisting cartilage islets in the metaphysis. In five of the six cases studied, chondrocytes and intercartilagenous intercellular chondroid matrix appeared ultrastructurally normal. Matrix vesicle distribution was similar to that of control subjects, but they were associated with few mineral crystals. In two infants, the matrix vesicles were alkaline phosphatase nonreactive. In the calcifying zone of the growth plate and in the newly formed metaphyseal trabecular bone, cartilagenous calcospherites often were small and the orientation of crystals was nonradial when compared to that of control infants. The mineralization of diaphyseal bone appeared normal. It seems that matrix vesicles are present in hypophosphatasia and that the impaired mineralization of cartilage is due primarily to the deficiency of alkaline phosphatase. In spite of the lack of alkaline phosphatase, secondary mineralization of bone which is not mediated by matrix vesicles was normal.