Stern, IB. An Electron Microscopic Study of the Cementum, Sharpey's Fibers, and Periodontal Ligament in the Rat Incisor. Amer. J. Anat. 115:377, 1964.

Aim

• The present study provides a description of the ultrastructure of rat cementum just prior to its eruption. The periodontal fibers and the cementoblasts were also studied to gain a further insight into the relationship between the cementum and the periodontal ligament in this continuously erupting tooth.
• 14 white rats were examined. Specimens were examined by light and electron microscopes.

Light Microscopic Observation

The labial surface of the rat incisors is coated with enamel and on its lingual surface and lateral surfaces with cementum. The rat incisor has 2 CEJs which course the length of the tooth as the lateral borders of a single stratum of acellular cementum. The inner surface of this layer is smooth and relatively regular while the outer surface is more irregular because of the extending fibers and small protruding tubercles may be evident. Fiber bundles course from the surface of the cementum around the cells and structures of the periodontal ligament to the alveolar bone or to the gingival epithelium. The ligaments also contains numerous capillaries and nerve fibers lying between the fiber bundles.

Electron Microscopic Observations

• Cellular structure- the cells adjacent to the cementum have numerous cytoplasmic extensions. The cytoplasmic processes only occasionally come into contact with the cementum.
• Periodontal ligament- the periodontal fibers extend from the cemental surface and in wavy course pass around the cells and their processes. Each collagen fibril is distinct and separate from adjacent fibrils. Fibrils may cross each other in different planes and produce a palisade-like arrangement. As they insert into the cementum, the periodontal fibrils are arranged perpendicular to the surface. Even when they run parallel or tangential to the cemental surface they turn, before inserting, and enter the cementum in an approximately perpendicular position.
• Cementum- the dentin and cementum are clearly delineated. There is never any indication of a space between the two tissues and the collagen fibrils do not cross the dentinocementum junction. The cemental surface is studded with projections of varying size. The small conical projections are associated with individual periodontal fibrils. The fibrils are embedded in the cementum. They may be traced inward for a variable distance from the cemental surface. Near the surface they tend to be parallel and to continue in the orientation of their insertion. As they are traced deeper into the cementum they turn and may have a more diagonal and divergent orientation. At times the fibrils appear to end in the vicinity of a discontinuous, somewhat diffuse, zone of density which may be evident near the center of the cementum. The deeper underlying intrinsic fibrils course parallel to the dentinocemental junction. Irregularly arranged thin dense lines are found in the position usually occupied by the intrinsic fibrils

Part of the Authors Abstract

During the course of eruption of the rat incisor, the periodontal fibers which are attached to the cementum move with tooth until they somehow undergo dissolution prior to the entry of the tooth into the oral cavity. A similar picture is obtained during the more limited period of eruption of rooted teeth. The mechanisms of formation, reorganization and dissolution of the periodontal fibers are all matters of biologic importance. Moreover, investigations of these phenomena may have some bearing upon understanding those pathologic processes which involve the detachment of the gingiva from teeth and the exfoliation of teeth from their sockets.

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