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 edges—an 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.	51
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 part—that is, the part farthest from the margin of the jaw—increase 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.	52