Yoshioki Hamamoto

Production of amelogenin by enamel epithelium of Hertwig's root sheath

OBJECTIVE To study the multiple potentials of differentiating odontogenic epithelial cells. STUDY DESIGN Bilateral first and second maxillary molars of 30 immature rats were perforated into the pulp chambers with a round bur. The pulps were observed histologically and immunohistochemically for amelogenin 3, 7, and 14 days after the perforation. RESULTS On day 7, the enamel epithelium of Hertwigs root sheath migrated and formed oval-shaped epithelial islands that resembled epithelial rests of Malassez. On day 14, the islands consisted of epithelial cells with large nuclei and cytoplasm partially surrounded by a subsequently formed osteodentin and cementum. Immunoreactivity for amelogenin was observed in the large epithelial cells and in the area between the cells as well as the calcified tissues. Some of the enamel epithelium resembling columnar ameloblasts of the enamel organ were also positive for amelogenin. CONCLUSIONS The enamel epithelium of Hertwigs root sheath appeared to differentiate into ameloblasts and produce amelogenin.

Ultrastructure of epithelial rests of Malassez in human periodontal ligament

The cell rests in healthy periodontal ligament were studied by transmission electron microscopy. They consisted of clusters of cells surrounded by a basal lamina. These cells were characterized by an irregular nucleus with condensed heterochromatin, tonofilaments, relatively abundant mitochondria and poorly developed rough endoplasmic reticulum. They were connected to each other by desmosomes and gap junctions. Coated pits and vesicles were also seen at the periphery. In addition, a primary cilium accompanied by a centriole was often observed in association with the Golgi complex and rootlet-like structures. The findings suggest that the epithelial rests are more than a vestigial structure.

Morphological changes of epithelial rests of Malassez in rat molars induced by local administration of N-methylnitrosourea

The objective was to examine the possibility that epithelial rests of Malassez can give origin to odontogenic tumours. A mixture of N-methylnitrosourea (MNU) and alginate impression material for dental use was injected onto the periosteum of the buccal side of the left mandible of 5-week-old, male Wistar rats (300 mg/kg body wt). The mixture was left at the site for several months. The rats were killed 1, 3, 5, and 8 months after the injection. After 5 and 8 months, the epithelial rests of Malassez in the cervical and bifurcational regions of the first, second, and third left mandibular molars were significantly enlarged and the alveolar bone around the lesion was resorbed by multinucleated cells in all rats. The epithelial masses were characterized by enamel organ-like structures, deposition of eosinophilic amorphous material, duct-like structures, and squamous metaplasia. In addition to these masses in the molar regions, odontogenic tumours were induced in the incisal region and carcinomas and sarcomas in the buccal region, knee, bladder, and skin. Local administration of a mixture of MNU and alginate impression material can induce odontogenic tumours from the epithelial rests of Malassez at high incidence.

Histological, histocytochemical and ultrastructural study on the effects of surface charge on bone formation in the rabbit mandible.

The osteogenic potential of different surface charges was studied by implanting uncharged, negatively changed and positively charged Sephadex beads subperiosteally on the buccal aspect of the mandible. The implant sites were examined 1, 2, and 4 weeks later. New bone formation was observed around the positively charged beads at 2 weeks. An amorphous collagen-free layer seen around the beads consisted of anionic substances and contained needle-like crystals. Tartrate-resistant acid phosphatase-positive, multinucleated giant cells, which had typical fine structural characteristics of osteoclast-like ruffled borders and a clear zone, were observed on the surface of positively charged beads, with osteoblastic cells juxtaposed to them. Bone formation was still going on at 4 weeks in this group. Around the uncharged or negatively charged beads, new bone formation and osteoclast-like cells were not observed at any time. The findings suggest that the surface charge of implant materials has a definite influence on the differentiation of osteoclastic cells and subsequent occurrence of osteoblastic cells leading to formation of bone.

Immunocytochemical localization of laminin in the epithelial rests of Malassez of immature rat molars.

Immediately after disruption of Hertwigs root sheath, epithelial cells were found near the root apex, singly or in groups of two or three cells. They were irregular in shape with only a partial lining of basal lamina. Immunoreactivity for laminin was intense in the basal lamina and weak between projections of the epithelial cells. In 5-week-old rats, the epithelial rests consisted of about five cells and still had an incomplete basal lamina with collagen fibrils in the intercellular spaces. Immunoreaction products were seen in the basal lamina and diffusely in the intercellular spaces. The epithelial rests of 9-week-old rats had more cells, an almost complete lining of basal lamina, and narrowed intercellular spaces. Immunoreaction products were seen in the basal lamina but not in the intercellular spaces. These findings indicate the basal lamina is involved in the formation of the epithelial rests.