Some Aspects of the Development of the Dentition Before Birth
The first local changes leading toward tooth formation occur as early as the sixth week of prenatal life. The oral epithelium in the upper and lower jaws thickens, forming the dental lamina from which local buds arise at each point where a tooth will form.88 Individual tooth formation proceeds by means of mitotic activity, particularly in the inner enamel epithelium, until the ameloblasts and odontoblasts are differentiated. Predentin and enamel are subsequently formed; the amelodental border is determined.
Teeth are characterized by several typical features. They are composed of the hardest materials found in the human body. This applies particularly to the enamel. The time involved in the formation of enamel and dentin is extremely long in comparison with the formation of other tissues. For example, the calcification of the first permanent molars has already started by the time of birth. When they emerge in the oral cavity at six years of age, their root formation is not yet completed.
Another feature is the almost complete absence of natural repair mechanisms in the enamel and dentin to restore parts lost by decay or trauma. After crown formation is completed, no extension or replacement of enamel beyond mineralization and remineralization can occur. Comparable but less rigid restrictions apply to repair and secondary apposition of dentin.
Moreover, teeth are typical in the feature that their crowns are formed directly at their ultimate sizes. In consequence, their dimensional development runs as if it were ahead of the proportional increase of the surrounding and embedding structures, the jaws. The nonconcordance in increase in size of the forming crowns and that of their surrounding structures lead to several complex and not harmoniously appearing situations characteristic of the development of the dentition. Further, the crowns of the teeth are formed in the region that will contain their roots later. In the adult situation, the roots in the anterior region will require considerably less space than was needed for the crowns during their formation and in their prior-to-emergence period.
A distinct difference exists between the time span involved in the formation of the deciduous teeth and that of the permanent ones. The first deciduous tooth emerges at about six months of age, the first permanent tooth at about six years of age. Consequently, the mandibular first deciduous incisor has only about one year for its formation and calcification; the first permanent molar has six times as long. The difference in composition between the deciduous and permanent teeth is related to this difference in time available for formation and calcification. Deciduous teeth have a lower calcification level than the permanent ones. Their appearance is more white, hence the term “milk teeth.” Permanent teeth are more calcified and become more mineralized, hence they are darker and more yellow in color. The thicker the enamel cap, the more obvious the trait, as demonstrated by the permanent maxillary canines, particularly in males. The relation between calcification level and enamel color is also obvious from the observation that local enamel hypocalcifications show up in permanent teeth as white spots.
Because of the lower level of calcification, deciduous teeth have less resistance to wear than permanent ones. In normal use, deciduous molars soon lose their marked and sharp points and gradually their cusps are worn off. It is not uncommon to encounter exposed dentin in deciduous molars in 10-year-old children, even in Western societies where the feeding pattern induces little enamel wear. A comparable situation occasionally can be encountered in permanent teeth, however, then resulting from the higher calcification level after a considerably longer period of use than is the case for deciduous teeth.
The morphogenesis of incisors, canines, and molars is essentially the same. However, some differences exist in this respect and they have to be understood so that several typical aspects of the development of the dentition become clear.49 Forming molars gradually become larger until calcification starts (Figs. 1-1 and 1-2). Thereafter, they continue to increase in size on the basis of interstitial growth of the inner enamel epithelium and because calcification does not start on the different cusp tips at the same time. In a genetically determined sequence, the differentiation of ameloblasts and odontoblasts and the subsequent calcification start on the various cusp tips. Areas not yet calcified maintain their potency to increase in size. When coalescence is attained between the calcified parts of the crown, the distance between two cusps involved cannot increase any more and the intercuspal dimension is fixed. Subsequent formation of enamel still takes place, but it is limited to filling in the region between the two cusps. After coalescence has been completed around the occlusal surface, the size of the crown in mesiodistal and buccolingual directions can increase only by addition of new layers of enamel at the crown circumference (Fig. 1-3).
In contrast with molars, incisors start their calcification only at a single point, at about the center of the future incisal edge. From there, calcification spreads in a more horizontal direction than in molars. The combination of having only one calcification center and spreading in more or less one plane of space—parallel to the future incisal edge—results in a relatively early attainment of a large part of the ultimate mesiodistal crown dimension. This dimension soon can increase only by enamel apposition at the mesial and distal surfaces (Fig. 1-4).
Canines, like incisors, have only one calcification center. Their morphodifferentiation and calcification lag behind that of the incisors and molars. The calcification proceeds in mesiodistal direction at an angle of about 45° with the long axis of the future tooth. The increase in mesiodistal dimension of the calcifying region of canines is consequently slower than it is for incisors. The combination of these factors results in a difference in timing regarding the initial attainment of the ultimate dimension between the mesial and distal enamel dentin borders. Mandibular deciduous canines and first molars reach that stage in week 28 after conception; central incisors in week 20.
Fig. 1-1 Overview showing the precalcification and early calcification stages of the mandibular first deciduous molar, based on stages in morphodifferentiation. From the first macroscopic indication of tooth development (stage I), growth occurs and morphodifferentiation proceeds. In stage V, calcification starts at one point. The second calcification center does not appear before stage X is reached. The size of the tooth bud crown increases considerably between stages V and X. This increase in intercuspal dimensions continues and does not stop until coalescence between the approaching calcified parts has been realized. Variation in morphodifferentiation occurs after stage IX has been reached, leading to variations in final crown morphology. (Kraus, B.S., and Jordan, R.E/>