The primary (deciduous) dentition consists of 20 teeth, five in each quadrant (Fig 1-22).74,110 The permanent incisors, canines, and premolars form from successional laminae that extend lingually from the primary precursors toward the midline (see Fig 1-22). The permanent molars develop from a distal extension of the dental lamina, the accessional lamina (Fig 1-23). Some dental embryologists consider the permanent molars to be members of the first dentition. Their microscopic successors undergo an aborted development.
Fig 1-22 Developing primary teeth and the primordia of the permanent teeth in a 28-week human fetus. Maxillary quadrant. (i1) Primary central incisor; (i2) primary lateral incisor; (c) primary canine; (m1) primary first molar; (m2) primary second molar; (I1) permanent central incisor; (I2) permanent lateral incisor; (C) permanent canine; (P1) permanent first premolar; (P2) permanent second premolar; (M1) permanent first molar. (Adapted from Ooe74 with permission.)
Fig 1-23 Mandibular molar region in a 159-mm fetus (at 20 weeks old), depicting the formation of the permanent first molar (M1) from a distal extension of the primordia of the primary second molar (m2). (m1) Primary first molar. (Adapted from Ooe74 with permission.)
During the development of primary teeth, the central incisor and canine are positioned labial to the lateral incisor (Fig 1-24). This arrangement is noted very early in the formation of the enamel organ from the dental lamina. The buds of the permanent teeth have a similar position, so that the lateral incisor is positioned lingual to the central incisor and canine. During normal postnatal development, space is created in the dental arch for the alignment of all anterior teeth. Often, the space created is insufficient, and the central incisor and the canine crowd out the lateral incisor.
Fig 1-24 Epithelial portion of the anterior tooth germs and adjacent structures in a 144-mm fetus. (i1) Primary central incisor; (i2) primary lateral incisor; (c) primary canine. (Adapted from Ooe74 with permission.)
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