Movement of entire cell populations during renewal of the rat incisor as shown by radioautography after labeling with 3H‐thymidine. The concept of a continuously differentiating cross‐sectional segment

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Abstract
Renewal of the rat incisor was studied in three dimensions by employing a serial cross‐sectioning technique to locate the boundary between labeled and unlabeled cells in the enamel organ and odontoblast layer at various times after a single injection of 3H‐thymidine. This boundary, or leading edge of the front of labeling, was graphically illustrated through point‐plotting reconstruction of the labial surface of the incisor. At one hour after the injection of 3H‐thymidine the front of labeled ameloblasts was located within the presecretory zone related to early predentin secretion. This front formed a “C”‐shaped curve stretching across the labial surface of the tooth from the lateral to the mesial cemento‐enamel junction. The “C” was open anteriorly and the lateral arm extended almost twice as far incisally as the mesial arm. The edge of the front of labeled odontoblasts was positioned apical to and parallel with this “C”‐shaped curve. The morphological appearance of all cells along each respective front was found to be similar. As the fronts of labeled ameloblasts and labeled odontoblasts moved forward with the erupting incisor, the cells along these fronts differentiated simulataneously and subsequently formed enamel and dentin. Throughout this movement the distance between fixed points along the leading edge of the front of labeled ameloblasts, and its positional relationship to the front of labeled odontoblasts, did not change appreciably. This indicated that cells of the tooth were being carried incisally at a uniform speed. It was concluded that renewal in the rat incisor consists of the generation by the bulbous part of the odontogenic organ of epithelial “U”‐shaped cross‐sectional segments which enclose a core of pulp. As this segment is transported towards the gingival margin, cellular differentiation and subsequent formation of hard tissue is seen to begin at the central labial side of the segment and to progress in a mesial and lateral direction towards the lingual side. In the process, the limits of the enamel organ at the mesial and lateral cemento‐enamel junctions are established and the entire circumference of the segment is eventually enclosed by a rim of dentin.