無根歯の発生学的研究

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The development of lower incisors of white rats (Rattus norvegicus albinus) for each stage of embryonic, newborn, and adult lives was studied. The summary is as follows. 1) At the 13th day of the embryonic life, the oral epithelium lining the anterior portion of the mandibular process has thickened symmetrically on both sides. This is the primitive dental lamina. The lamina extends gradually outer and downwards in proportion to the development of the embryo. The periphery of the lamina is covered by a single layer of short columnar cells, and the inside is filled with cells containing spherical or spheroidal nuclei. 2) At the 14th day of the embryo, the lower free margin of the dental lamina swells roundly to shape the so-called bud stage. 3) From the 15th to the 16th day, posterior portion inside the bud becomes concave to form the so-called cap stage. On the 16th day, however, with progressing development, the posterior end of the enamel cap is separated into two parts, dorsal and ventral. Active cell division characterizes this stage so that the round cells concentrates densely at the central portion of the inner concavity to build the enamel knot which finally assumes the shape of the enamel organ. It is to be noted that a longitudinal streak is seen to traverse along the ventral surface of the enamel cap, which is presumed to have some relation to degenerative process in the dental lamina of permanent tooth. 4. On the 18 th and 19 th day, after further proliferation and increase of depth of the concave surface, the enamel organ forms the bell form. The enamel knot disappears gradually and this stage is characterized by the further differentiation of the cells of the enamel organ, each layer becomes distinct, i. e. the enamel organ consists of the outer and the inner epithelium, the stellate reticulum, and the stratum intermedium. At the same time the dental papilla becomes clear. The outer enamel epithelium arranges on the convex surface of the enamel organ. The mesenchym which is enclosed by the enamel organ consists of the young connective tissue, and the mesenchymal cells which face the inner enamel epithelium become odontoblasts and the dental papilla at this stage shows the figure of the primary pulp. 5. In the period of completion of hard tissue which occurs in the course from the 20 th day of the embryonic life to the 10 th day after birth, the cylinder of enamel organ becomes longer and longer and shows rudimentary shape of tooth. Enamel organ degenerates in turn of inner, outer, dorsal side, except backend and ventral side. The inner and outer enamel epitheliums unite and arrange as a layer of flat cells, and the inner enamel epithelium secretes a thin Haematoxyline stainable layer. But soon after this layer becomes Haematoxyline-unstainable and remains transparent. On the other hand, the ventral side grows further and the pre-enamel is heaped on the pre-dentin. The apex tissue of the dental papilla becomes fibrous and builds bone-like dentin. Pre-dentin and calcified dentin is heaped on the pre-enamel, and this heap is in turn of ventral, inner, outer, dorsal side. A mass of the epithelial cells in the enamel organ neck part remains also in this stage, but it disappears on about 7 th day after birth. The author perceives that this mass of epithelial cells plays the part of inducement of oral epitheliums into deep mesenchymal tissues. 6. In completion stage, the enamel, dentin and bone-like dentin increase in size, and in calcification degree. After formation of the hard tissue is completed, tooth germ begins to erupt. Formation of the enamel and dentin is nearly as well as human, but as 1 mentioned above, tranparent zone is the secretion of the inner enamel epithelium and the cement cannot be seen. 7. The area of the backend of completed tooth shows young enamel organ, but inner, outer, dorsal side of the forward part is about the same with degeneration type in the formation stage of the hard tissue. The enamel in the ventral side shows also atrophy according to forward, but the inner enamel epithelium grows yet in a back third-part. The enamel increases in thickness, in frontier part and the enamel organ degenerates, and at last, the inner and outer epithelium unite and pass over gingival epithelium. The area of backend of the dental papilla shows young type. The tissue in slightly forward formates pulp, and the odontoblasts buied thicker dentin according to forward. The apex of the pulp changes gradually to the bone-like structureless dentin.
九州歯科学会の論文
1954-12-30

無根歯の発生学的研究

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