ANALYSIS OF THE ENAMEL FORMATION IN THE CONTINUOUSLY GROWING TEETH OF NORMAL AND VITAMIN C DEFICIENT GUINEA PIGS

¹ Walter G. Zoller Memorial Dental Clinic and the Department of Anatomy of the University of Chicago, Chicago, Ill.

(1) The purpose of a histophysiologic analysis of enamel formation is to envisage the complex process as a series of changes within all the tissues concerned and to understand the causal relations of these changes with respect to initiation, maintenance and limitation of amelogenesis (p. 463).

(2) In continuously growing teeth the development of the enamel-producing tissues can be followed from the beginning to the end in a single tooth. Three stages can be distinguished: (1) the embryonic period; (2) the period of production; (3) the period of maturation. The transitional changes between the different periods are of special interest (p. 467).

(3) During the embryonic stage, which is the homologue of any mammalian tooth germ, the enamel epithelium, prior to the formation of dentin, maintains its original relationship to the pulp which is its primary source of supply. The rest of the enamel organ is supplied by the secondary or outer source of supply, viz. the periodontal tissue (p. 468).

(4) In the beginning of amelogenesis, the ameloblasts become oriented toward the stellate reticulum and the periodontal vascular mesenchyme which now become their source of supply. This is the first turning point in the history of the ameloblasts. The structural changes, accompanying the beginning of amelogenesis (fig. 9), are indicative of the reversal of nutrient supply of the ameloblasts.

(5) The decisive change is not due to the vascularization of the enamel organ, but to the formation of dentin which replaces the former basement membrane between the ameloblasts and the pulp, thereby separating the inner enamel epithelium from the primary source of supply (p. 470).

(6) The influx of fluid from the outside is indicated by the gap between enamel epithelium and dentin opening at the very beginning of amelogenesis. It is kept open as long as production lasts (p. 471).

(7) The abundant uptake of fluid from the outside by the ameloblasts has been demonstrated by the vital staining method (p. 473).

(8) The demarcation between enamel epithelium and stratum intermedium is another sign of absorption going on at the outer boundary of the epithelium (p. 473). When this process comes to an end, the demarcation disappears (fig. 23).

(9) Since the protoplasmic bridges between the ameloblasts and the stratum intermedium cells are maintained during production, the transfer of material into the ameloblasts is carried across both the cell processes and the limiting membrane (p. 474).

(10) In scorbutic animals the formation of the dentin may be entirely suppressed in certain areas. Enamel forming tissues there remain in the embryonic stage (p. 474, fig. 15). The causal relation of dentin and differentiation of the enamel-forming tissues are proved by this coincidence.

(11) The building up of the enamel matrix consists of (1) the secretion of globular material, (2) of the formation of a structured ground substance (p. 476).

(12) The secretion of globular material is a function of both the ameloblasts and the stratum intermedium cells.

(13) Production of globular material, without its consequent incorporation into the ground substance, leads to the accumulation of amorphous masses laid down on top of the enamel matrix (fig. 18). This is the result of a hypersecretion under abnormal conditions and should not be called enamel hyperplasia as has been done.

(14) The reverse disturbance, that is the formation of the ground substance without its simultaneous impregnation with globular material, reveals the structure of the ground substance (figs. 19, 20). The attempt was made to trace this structure back to that of the earliest ground substance in the beginning of amelogenesis (fig. 22). The conclusions reached do not confirm the traditional opinion that each enamel prism corresponds to one particular ameloblast (p. 478).

(15) The growth of the ground substance does not coincide with a gradual shortening of the receding ameloblasts (fig. 5). Thus, the theory of the direct transformation of the ameloblasts into the enamel rods does not correctly describe the formation of the ground substance, which seems to be elaborated as an exoplasmic derivative of the cells (p. 481).

(16) The cessation of production is not due to a limited life span of the ameloblasts but to environmental conditions (p. 484).

(17) Ending of production and beginning of maturation are accompanied by regressive and progressive changes in the enamel epithelium. The polarity of the ameloblasts, the space between enamel epithelium and matrix, the demarcation of the enamel epithelium toward the stratum intermedium disappear while the ameloblasts are decreasing in height (p. 484). The reestablishment of the columnar epithelium marks the new organization necessary for the maturation (fig. 23).

(18) Loss of water by the ameloblasts (p. 486), retention of secreted material within the cells (fig. 23), and soon afterward the absorption of this material, indicate that the flow of fluid is reversed in the beginning of maturation. This is the second turning point in the history of the ameloblasts.

(19) The finding that vitally stained enamel matrix decolorizes regularly during maturation suggests an absorptive activity of the ameloblasts beginning with the reversal of the flow of material.

(20) After the destruction of the short ameloblasts, the enamel matrix does not reach complete maturity which, however, proceeds to a certain stage (fig. 30). It follows that the ameloblasts are charged with a function which is indispensable for one phase of maturation. This function is certainly different from that in the preceding period of production (p. 488).

(21) The concept of the twofold nature of enamel maturation is confirmed by (1) the activity of the ameloblasts and (2) the results of recent chemical analyses of developing enamel. The 2 processes on which maturation depends are the uptake of minerals, in addition to those contained in the enamel matrix from the beginning, and the removal of water and organic substances.

(22) According to recent studies by the aid of radio-active phosphorus, calcium is taken in by the enamel throughout life via the dentino-enamel junction. The same pathway will be used by the maturing enamel when the flow of material from the outside ceases. The analogy is strongly supported by the observation that the first changes typical of maturation are found at the dentino-enamel junction (fig. 32). The retention of calcium within the periodontal tissues in guinea pig molars at the time when maturation is not complete also points to the dentin and the pulp as the source of calcium (p. 491).

(23) Since the uptake of minerals during maturation does not depend on the ameloblasts, their indispensable function must be related to the removal of fluid and of organic substances. A further evidence for the absorptive activity of the short ameloblasts can be derived from the irreparability of the dysplasia of the enamel (p. 491).

(24) It has been found necessary to distinguish between calcification and crystallization of the enamel matrix. Crystallization, and, therefore, the final hardening of the enamel, is the result of both calcification and the removal of water, as well as of a certain amount of the organic material from the enamel. The 2 processes involved in maturation seem to be chemico-physically interlocked so that calcification cannot be completed when absorption has been stopped.