Henry Gray (18251861). Anatomy of the Human Body. 1918.
Development of the Deciduous Teeth.The development of the deciduous teeth begins about the sixth week of fetal life as a thickening of the epithelium along the line of the future jaw, the thickening being due to a rapid multiplication of the more deeply situated epithelial cells. As the cells multiply they extend into the subjacent mesoderm, and thus form a ridge or strand of cells imbedded in mesoderm. About the seventh week a longitudinal splitting or cleavage of this strand of cells takes place, and it becomes divided into two strands; the separation begins in front and extends laterally, the process occupying four or five weeks. Of the two strands thus formed, the labial forms the labiodental lamina; while the other, the lingual, is the ridge of cells in connection with which the teeth, both deciduous and permanent, are developed. Hence it is known as the dental lamina or common dental germ. It forms a flat band of cells, which grows into the substance of the embryonic jaw, at first horizontally inward, and then, as the teeth develop, vertically, i. e., upward in the upper jaw, and downward in the lower jaw. While still maintaining a horizontal direction it has two edgesan attached edge, continuous with the epithelium lining the mouth, and a free edge, projecting inward, and imbedded in the mesodermal tissue of the embryonic jaw. Along its line of attachment to the buccal epithelium is a shallow groove, the dental furrow.
About the ninth week the dental lamina begins to develop enlargements along its free border. These are ten in number in each jaw, and each corresponds to a future deciduous tooth. They consist of masses of epithelial cells; and the cells of the deeper partthat is, the part farthest from the margin of the jawincrease rapidly and spread out in all directions. Each mass thus comes to assume a club shape, connected with the general epithelial lining of the mouth by a narrow neck, embraced by mesoderm. They are now known as special dental germs. After a time the lower expanded portion inclines outward, so as to form an angle with the superficial constricted portion, which is sometimes known as the neck of the special dental germ. About the tenth week the mesodermal tissue beneath these special dental germs becomes differentiated into papillæ; these grow upward, and come in contact with the epithelial cells of the special dental germs, which become folded over them like a hood or cap. There is, then, at this stage a papilla (or papillæ) which has already begun to assume somewhat the shape of the crown of the future tooth, and from which the dentin and pulp of the tooth are formed, surmounted by a dome or cap of epithelial cells from which the enamel is derived.
FIG. 1012 Longitudinal section of the lower part of a growing tooth, showing the extension of the layer of adamantoblasts beyond the crown to mark off the limit of formation of the dentin of the root. (Röse.) ad. Adamantoblasts, continuous below with ep.sch., the epithelial sheath of Hertwig. d. Dentin. en. Enamel. od. Odontoblasts. p. Pulp. (See enlarged image)
In the meantime, while these changes have been going on, the dental lamina has been extending backward behind the special dental germ corresponding to the second deciduous molar tooth, and at about the seventeenth week it presents an enlargement, the special dental germ, for the first permanent molar, soon followed by the formation of a papilla in the mesodermal tissue for the same tooth. This is followed, about the sixth month after birth, by a further extension backward of the dental lamina, with the formation of another enlargement and its corresponding papilla for the second molar. And finally the process is repeated for the third molar, its papilla appearing about the fifth year of life.
After the formation of the special dental germs, the dental lamina undergoes atrophic changes and becomes cribriform, except on the lingual and lateral aspects of each of the special germs of the temporary teeth, where it undergoes a local thickening forming the special dental germ of each of the successional permanent teethi. e., the ten anterior ones in each jaw. Here the same process goes on as has been described in connection with those of the deciduous teeth: that is, they recede into the substance of the gum behind the germs of the deciduous teeth. As they recede they become club-shaped, form expansions at their distal extremities, and finally meet papillæ, which have been formed in the mesoderm, just in the same manner as was the case in the deciduous teeth. The apex of each papilla indents the dental germ, which encloses it, and, forming a cap for it, becomes converted into the enamel, while the papilla forms the dentin and pulp of the permanent tooth.
The special dental germs consist at first of rounded or polyhedral epithelial cells; after the