AN ELECTRON MICROSCOPIC STUDY OF THE CEMENTUM, SHARPEY'S FIBERS AND PERIODONTAL LIGAMENT IN THE RAT INCISOR.

Abstract

AbstractThe rat incisor is a continuously erupting, so‐called “rootless,” tooth. It differs from human or similar “rooted” teeth in that its generative cells persist at the base of the tooth. These cells are derived from the invagination of oral epithelium which forms the dental lamina and the developing dental organ. This epithelium becomes a vase‐shaped structure surrounding a condensation of connective tissue elements called the dental papilla. The cells on the labial aspect of the epithelial dental organ specialize to form enamel. The cells on the lateral and lingual aspects remain undifferentiated and are called Hertwig's epithelial sheath. The odontoblasts are derived from the dental papilla. The hard tissues of the tooth form continuously at the base, and the tooth erupts into the oral cavity where the incisal edge is worn by abrasive action during gnawing (v. Brunn, 1887; Addison and Appleton, '15; Schour and Massler, '42).Hertwig's sheath extends from the base of the tooth to a short distance beyond the point where dentin formation commences. At this level the epithelial cells separate and the sheath disintegrates. As these cells separate, cementoblasts and collagen fibers appear adjacent to the newly forming dentinal surface. Apatite crystals are deposited in the matrix of collagen fibers and a thin layer of cementum results (Selvig, '63). Since Hertwig's sheath is persistent at the base of the tooth, this initial phase of cementogenesis is continuous.In rooted teeth, on the other hand, initial cementogenesis is completed when the root becomes fully formed, and Hertwig's sheath has been expended. There may be subsequent progressing depositions of cementum to form one or several strata. This initial stratum is called “primary cementum” and the successive layers are called “secondary cementum.” Primary cementum contains fine fibrils arranged radially (fiber cementum). Secondary cementum generally contains cells (osteocementum) but it may be acellular. Regardless of category, cementum contains matrix (intrinsic) fibers, and also, in most cases, Sharpey's (extrinsic) fibers (Lehner and Plenk, '36; Egli, '46; Held, '51; Schmid, '51; Sicher, '62).During the course of eruption of the rat incisor, the periodontal fibers which are attached to the cementum move with the 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.