The odontoblasts are differentiated from certain cells of the pulp by the induction of ameloblasts, which are also differentiated from the inner enamel epithelium. Thus, there are many evidences on the epitheliomesenchymal interactions during the development of the tooth, although the morphological grounds are hitherto poorly established. In the porcine amelogenesis, 2 kinds of processes are derived from the ameloblast. One is known as an apical process (Kallenbach, 1976), which originates from the basal end of the cell and proceeds into the dental papilla penetrating the basal membrane in the early stage of ameloblast differentiation. The other is termed as an ameloblast fiber (Lester, 1970) and originates from the secretory end of Tomes' process entering into the enamel at a much later stage. The apical process contacts with the odontoblastic process in the early stage of enamel formation, and the phenomenon may be considered to be the morphological expression of the epitheliomesenchymal interactions. The ameloblast fiber channels the enamel tubule, which is similar to those of the tubular enamel in the marsupials. The tubules may be the precursor of the enamel spindle.
The purpose of this study was to investigate the relation between the initiation of occlusion and the bone formation after the implantation of the apatite ceramics. P3, P4 and M1 were extracted from fourteen adult mongrel dogs. After two months, one-piece and two-piece cylindrical implants measuring 5.0-6.5mm in diameter and 11mm in length of the buried portion were implanted. The two-piece implant consisted of the apatite-root and Ti-core. After 2, 4 and 8 weeks, both implants were put under occlusal function by cementing the metal crowns. The dogs were sacrificed at 2, 4 and 8 weeks after the initiation of occlusion. The specimens of the mandibles with the implants and the crowns were histologically examined. In the case of the implants under occlusion at 2 weeks after implantation, the greater part of the implant surface was covered with a fibrous tissue. In the case of the implants under occlusion at 4 weeks after implantation, one-half of the implants was covered with a fibrous tissue and the other one-half was in contact with the bone. In the case of the implants under occlusion at 8 weeks after implantation, all the implants were in contact with the bone. The bone around the implant increased after occlusion.
In the present study, the normal root formation of the molars and the tissue reactions to the infection or the subsequent endodontic procedures on the developing molars in the rats were studied histologically. The pulp cavity of the developing molars in the 3- week-old rats were exposed and allowed to remain open to the oral environment for a period of 1 week. The infected root canals were then covered with cement after the endodontic treatment. Microscopic examination of the teeth in the normal animals revealed that in the 3-week-old animals the root length of the lower first molars was about 1/2 of that of the matured teeth. The root of the lower first molars matured at 9 weeks after birth. After 1 week of pulp exposure, the epithelial sheath was still observed in the region of the root apex despite the necrosis of the pulp tissue. In the later stage, two types of periapical tissue reactions to the infection were observed. One was the destructive reaction in which the periapical inflammation progressed in intensity and severity. The other was the repairtive reaction in which an apical closure by the osteoid tissue and a cementum-like tissue were seen. When the infected root canals were covered with cement after the endodontic procedure, the periapical tissue showed also two types of reaction, i. e., one was the destructive and the other was the repairtive. In the latter type, however, continued root growth and closure of the foramina with hard dental tissue and osteoid tissue were much more progressive.
The role of the mandibular condyle in the cranio-facial growth and development still remains unknown in spite that it is considered to be very important in orthodontics. The author made observations on the mandibular condyle of prenatal rabbits with light microscopy and transmission electron microscopy to learn about the characteristic of this structure and to clarify the mechanism of the growth and development of the mandibular condyle. The results were as follows: 1. The fibrous and subfibrous cell zones which form the surface of the mandibular condyle continue to the periosteum of the mandibular ramus. 2. The osteo-chondro progenitor cell zone is observed beneath the subfibrous cell zone at the tip of the mandibular condyle. This zone continues to the osteoprogenitor cell zone in the lateral area of the condyle and furthermore preosteoblasts and osteoblasts are observed in the direction of the mandibular ramus. 3. In the profound layer of the osteo-chondroprogenitor cell zone the chondroblastic zone, the chondrocytic zone and hypertrophic cell zone are observed in the cartilage area. 4. In the lateral area of the mandibular condyle three zones are identified: the osteoprogenitor cell zone, preosteoblastic zone and osteoblastic zone, inward from the subfibrous cell zone. And the collar bone is formed adjacent to the calcified cartilage of the hypertrophic cell zone of the cartilage area.
In a series of studies to investigate the basic features and characteristics of the bone, the present study was made on the structural features of the resting line and reversal line in the bone tissue, using light microscopy, scanning electron microscopy and transmission electron microscopy. From the result, it was recognized that the zones of the resting line and reversal line were the regions susceptible to dissolution by acid-etching, as compared to the lamellar bone. Both zones of the resting line and reversal line consist of a small number of fine collagen fibrils and a large number of small rod-like crystals. The hydroxyapatite crystals were deposited on the surface of the collagen fibril and inside the collagen fibril. In the intercollagen fibril area and non-collagen fibril area, small rod-like crystals were seen deposited densely. These rod-like crystals were seen deposited as a bundle. These bundles of crystals were arranged mainly in two directions. In the inner structure of the rod-like crystal, the lattice image could not be observed. The electron diffraction pattern was also not obtained on the crystals so clearly. Therefore, it was considered that at least the rod-like crystal was not a hydroxyapatite crystal.
The purpose of this study was to investigate the structure of the dentinal tubules in the superficial layer of the human coronal dentin. It was observed by light microscope, scanning electron microscope and transmission electron microscope. The findings were as follows: 1. The width of the intratubular layer in the superficial layer was generally narrow. In some cases, the dentinal tubules may lack partially the intratubular layer. 2. The longitudinal fibers outside of the intratubular layer were observed on the dentinal tubules of the superficial layer. 3. Generally speaking, the dentinal tubules of the superficial layer were smaller than those of the middle layer. But if the size of the dentinal tubules outside of the longitudinal fibers is measured, the size of the dentinal tubules of the superficial layer will be very near to that of the middle layer. 4. The tubular structure of the dentinal tubules of the superficial layer were generally fine. Among these there are also those in which the existence of the tubular structure can not be confirmed clearly. 5. Because of crystallization, stricture or obturation of the dentinal tubules in the superficial layer was observed very often.