Reprinted from THE AMERICAN JOURNAL OF ANATOMY

The epithelial tissue forming the posterior aspect of the apical foramen in the upper incisor of the rat was reconstructed from 1 μm thick serial cross sections. Like the lower incisor, this portion of the odontogenic organ in the upper incisor was composed of a bulbous and a "U"-shaped part. However, the bulbous part was considerably blunter and the "U"-shaped part much larger in circumference in comparison to the lower incisor. Although no differences were found between the upper and lower incisor regarding the contents and the basic organization of cells within each part of the odontogenic organ, specific differences were found within the bulbous part in the upper incisor. There was a more definitive boundary between the outer dental epithelium and stellate reticulum, a more intimate relationship of cell streams to the stellate reticulum, and a noticeable lack of swirling of cells as part of the streams. These features suggest that the activity inside the bulbous part is less intense in the upper incisor than it is in the lower incisor. In addition, the relationships between the bulbous part, the "U"-shaped part and the root sheath part of the odontogenic organ and the enamel organ were described for the upper incisor. Fundamentally, the upper and lower incisors of the rat are built according to the same plan. This similarity is seen microscopically along the length of each tooth, where exact parallels exist in the morphology of cells, their organization and the sequence of their differentiation and function (Warshawsky and Smith, '74). In a specific structural sense, however, an upper incisor is not exactly the same as a lower incisor. Grossly, the upper can easily be distinguished from the lower by the upper's sharper curvature, shorter apical-incisal length, and wider labiallingual diameter (Addison and Appleton, '15; Schour and Massler, '42; Smith and Warshawsky, '73). A basic question for which no information is currently available in the literature asks simply, do comparable morphological differences exist in the undifferentiated epithelial regions of these continuously renewing teeth? Thus, the purpose of this report is to describe the three dimensional shape and the cellular organization of the odontogenic organ in the upper incisor of the rat and compare it to the odontogenic organ of the lower incisor. To do this, appropriate reference must be made to the illustrations contained in the preceding paper on the lower incisor (Smith and Warshawsky, '75). This investigation again made use of serial cross and longitudinal sections of Epon embedded material from which a three dimensional reconstruction was made. MATERIALS AND METHODS The rats used and the method of preparation employed were similar to those described in the previous report (Smith and Warshawsky, '75). For the three dimensional model of the upper incisor, every eighth section in a 450 μm cross sectional series was photographed and prints were made to a final magnification of x 200. Clear plexiglass plates 1.6 mm in thickness were used to keep the model in scale. The process of transcribing information from photographs onto plexiglass has also been described (Warshawsky and Accepted November 27, '74. AM. J. ANAT., 142: 431-456. 431