| SECTION IX | TOOTH DEVELOPMENT FROM LOBES |
Tooth crowns develop from lobes or primary growth centers (Fig. 1-62). Most normal teeth show evidence of having developed from three to five lobes. As a general rule, the facial portion of anterior teeth (incisors and canines) forms from three lobes, and the lingual cingulum area forms from one lobe. Evidence of three facial lobes can sometimes be seen as a labial ridge separated from the rest of the facial surface by two shallow depressions dividing the facial surface into three parts (seen clearly on a maxillary central incisor in Fig. 1-63A) or three mamelons on an incisal edge (Fig. 1-63B). To summarize, anterior teeth normally develop from four lobes: three facial lobes and one lingual lobe forming the cingulum.
FIGURE 1-62. Lobes or primary anatomic divisions on teeth. Drawings (A), (B), and (C) show the facial, mesial, and incisal views of a maxillary central incisor that, like all anterior teeth, forms from four lobes. The lingual cingulum develops from one lobe (labeled 4) seen in views (B) and (C). Mamelons may appear on the incisal edge of newly erupted incisors, an indication of the three labial lobes. Drawings (D) and (G) are the mesial and occlusal view of a two-cusped premolar that also forms from four lobes. As with anterior teeth, the facial cusp forms from three lobes, and one lingual lobe forms the lingual cusp. The divisions between the facial and lingual lobes are evidenced by the marginal ridge developmental grooves. Each cusp of a molar is formed by one lobe. Drawing (E) is a mandibular first molar with five lobes, three buccal, and two lingual, which is one lobe per cusp. Drawing (F) is a maxillary first molar with three larger lobes and one smaller lobe, or one per cusp. A very small fifth (Carabelli) cusp, when large enough, may have formed from a separate lobe.
FIGURE 1-63. A. The light reflecting off of this maxillary central incisor reveals the three bulges reflecting the formation of the facial surface by three facial lobes. The three bulges are separated by two depressions. B. This mandibular incisor with three mamelons reflects its formation from three facial lobes.
As on anterior teeth, the facial portion of the facial cusp of a premolar forms from three lobes, often evident by a buccal ridge and a depression on either side dividing the facial surface into three parts. Each lingual cusp forms from one lobe. Therefore, a two-cusp-type premolar forms from four lobes: three facial and one lingual, the same as for an anterior tooth. However, a three-cusp-type premolar with two lingual cusps forms from five lobes: three facial and two lingual, one for each lingual cusp.
As a general rule, each molar cusp forms from one lobe. For example, maxillary or mandibular molars with five cusps form from five lobes, and those with four cusps form from four lobes. Some maxillary molars have as few as three cusps and form from three lobes. A small fifth cusp (of Carabelli) may also be present on some maxillary molars, and when it is large, it may have formed from a separate lobe.
Two types of tooth unusual occurrences (called anomalies), peg-shaped maxillary lateral incisors (seen later in Chapter 11) and some extra teeth (also called supernumerary teeth), form from less than three lobes. Guidelines for determining the number of lobes that form each tooth are presented in Table 1-5.
| TABLE 1-5 | Guidelines for Determining the Number of Lobes Forming Adult Tooth |
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