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Synthetic hydroxyapatite crystals (Bioapatite) were implanted in a vestibular longitudinal groove made in the alveolar bone, the periodontal membrane and the superficial root dentine of 2 human canines in the absence of periodontal disease. The healing process was followed in scanning as well as in transmission electron microscopy. At 6 months, the aggregate of synthetic hydroxyapatite crystals was surrounded by a fibrous connective tissue, devoid of inflammatory cells. Small apatite crystals, similar to the neighbouring bone crystals, were deposited between the Bioapatite crystals in the central area of the aggregates. These small crystals filled the intercrystalline spaces from the center to the periphery. The mineralization extended then directly in the collagen matrix (osteoid tissue) surrounding the synthetic apatite aggregates. Bone tissue was thus formed around the apatite implants. At 1 year, the thickness of the bone tissue formed around the synthetic hydroxyapatite aggregates was increased. It consisted of bone with a well-oriented collagen matrix, with osteocyte lacunae, osteocytes and interosteocytic canaliculi, with normal ultrastructure.

The ultrastructural study of the alveolar bone surface of adult rat molars has shown the presence of resting, developing and resorbing areas. Intermediary, tight and gap junctions were observed between the cell processes of osteoblasts and osteocytes. In developing zones, the progressive differentiation of bone lacunae was followed. In resting zones, fibroblast-like cells with large intercellular spaces were observed. The ultrastructure of the osteocytes and the organic periosteocytic space was variable. The osteoclastic resorption showed an initial mineral attack followed by a collagen breakdown. Some observations suggested a relationship between the osteocytes and the origin of the osteoclast.

Among a series of 42 ossifying fibromas and 32 fibrous dysplasias, 5 cases were studied by histoenzymological and electron microscopic methods. The histological study in 2/3 of cases is sufficient for the diagnosis between the two diseases: trabecular bone in a connective tissue with regular collagen fibres in the ossifying fibroma; nodular indented bony areas with irregular collagen frame in fibrous dysplasia. By histoenzymology, the diagnosis between the two affections is not easy: same activity of ATPases and alkaline phosphatases in fibroblasts and osteoblasts. By electron microscopy, the morphology of the two lesions is different: in ossifying fibroma, numerous well-differentiated osteoblasts and large areas of ossification are seen; in fibrous dysplasia, undifferentiated cells are numerous and the collagen frame is irregularly mineralized. This method is also useful for the histogenetic understanding of these two osteopathies.

The light microscopy features of the cellular reorganisation of tone tissue in Paget's disease are well known. They result in the constitution of abnormal bone tissue. Ultrastructural studies have enabled a more detailed study of osteoblasts, osteocytes and osteoclasts. Osteoclasts are the only cells which present abnormalities. In particular, we observe intranuclear and intracytoplasmic inclusions composed of microcylinders. The morphological analysis is able to relate the presence of these formations with a viral infection of the paramyxovirus group. The results obtained by immunocytochemical techniques support this hypothesis. The osteoclast lesion could be a determining factor in the abnormal reorganization of bone tissue in Paget's disease, which may be considered to be a disease due to a slow acting virus.