Henry Gray (18251861). Anatomy of the Human Body. 1918.
formation of the papillæ, these cells undergo a differentiation into three layers. Those which are in immediate contact with the papilla become elongated, and form a layer of well-marked columnar epithelium coating the papilla. They are the cells which form the enamel fibers, and are therefore termed enamel cells or adamantoblasts. The cells of the outer layer of the special dental germ, which are in contact with the inner surface of the dental sac, presently to be described, are much shorter, cubical in form, and are named the external enamel epithelium. All the intermediate round cells of the dental germ between these two layers undergo a peculiar change. They become stellate in shape and develop processes, which unite to form a net-work into which fluid is secreted; this has the appearance of a jelly, and to it the name of enamel pulp is given. This transformed special dental germ is now known under the name of enamel organ(Fig. 1011).
While these changes are going on, a sac is formed around each enamel organ from the surrounding mesodermal tissue. This is known as the dental sac, and is a vascular membrane of connective tissue. It grows up from below, and thus encloses the whole tooth germ; as it grows it causes the neck of the enamel organ to atrophy and disappear; so that all communication between the enamel organ and the superficial epithelium is cut off. At this stage there are vascular papillæ surmounted by caps of epithelial cells, the whole being surrounded by by membranous sacs.
Formation of the Enamel.The enamel is formed exclusively from the enamel cells or adamantoblasts of the special dental germ, either by direct calcification of the columnar cells, which become elongated into the hexagonal rods of the enamel; or, as is more generally believed, as a secretion from the adamantoblasts, within which calcareous matter is subsequently deposited.
The process begins at the apex of each cusp, at the ends of the enamel cells in contact with the dental papilla. Here a fine globular deposit takes place, being apparently shed from the end of the adamantoblasts. It is known by the name of the enamel droplet, and resembles keratin in its resistance to the action of mineral acids. This droplet then becomes fibrous and calcifies and forms the first layer of the enamel; a second droplet now appears and calcifies, and so on; successive droplets of keratin-like material are shed from the adamantoblasts and form successive layers of enamel, the adamantoblasts gradually receding as each layer is produced, until at the termination of the process they have almost disappeared. The intermediate cells of the enamel pulp atrophy and disappear, so that the newly formed calcified material and the external enamel epithelium come into apposition. This latter layer, however, soon disappears on the emergence of the tooth beyond the gum. After its disappearance the crown of the tooth is still covered by a distinct membrane, which persists for some time. This is known as the cuticula dentis, or Nasmyths membrane, and is believed to be the last-formed layer of enamel derived from the adamantoblasts, which has not become calcified. It forms a horny layer, which may be separated from the subjacent calcified mass by the action of strong acids. It is marked by the hexagonal impressions of the enamel prisms, and, when stained by nitrate of silver, shows the characteristic appearance of epithelium.
Formation of the Dentin.While these changes are taking place in the epithelium to form the enamel, contemporaneous changes occurring in the differentiated mesoderm of the dental papillæ result in the formation of the dentin. As before stated, the first germs of the dentin are the papillæ, corresponding in number to the teeth, formed from the soft mesodermal tissue which bounds the depressions containing the special enamel germs. The papillæ grow upward into the enamel germs and become covered by them, both being enclosed in a vascular connective tissue, the dental sac, in the manner above described. Each papilla then constitutes the formative pulp from which the dentin and permanent pulp are developed; it consists of rounded cells and is very vascular, and soon begins to assume the shape of the future tooth. The next step is the appearance of the odontoblasts, which have a relation to the development of the teeth similar to that of the osteoblasts to the formation of bone. They are formed from the cells of the periphery of the papillathat is to say, from the cells in immediate contact with the adamantoblasts of the special dental germ. These cells become elongated, one end of the elongated cell resting against the epithelium of the special dental germs, the other being tapered and oftened branched. By the direct transformation of the peripheral ends of these cells; or by a secretion from them, a layer of uncalcified matrix (prodentin) is formed which caps the cusp or cusps, if there are more than one, of the papillæ. This matrix becomes fibrillated, and in it islets of calcification make their appearance, and coalescing give rise to a continuous layer of calcified material which covers each cusp and constitutes the first layer of dentin. The odontoblasts, having thus formed the first layer, retire toward the center of the papilla, and, as they do so, produce successive layers of dentin from their peripheral extremitiesthat is to say, they form the dentinal matrix in which calcification subsequently takes place. As they thus recede from the periphery of the papilla, they leave behind them filamentous processes of cell protoplasm, provided with finer side processes; these are surrounded by calcified material, and thus form the dental canaliculi, and, by their side branches, the anastomosing canaliculi: the processes of protoplasm contained within them constitute the dentinal fibers (Tomes fibers). In this way the entire thickness of the dentin is developed, each canaliculus being completed