Structure of periodontal tissues in health and disease* A NTONIO N ANCI &D IETER D. B OSSHARDT The periodontium, defined as those tissues support- ing and investing the tooth, comprises root cemen- tum, periodontal ligament, bone lining the tooth socket (alveolar bone), and that part of the gingiva facing the tooth (dentogingival junction). The wide- spread occurrence of periodontal diseases and the realization that lost tissues can be repaired and, perhaps, regenerated has generated considerable interest in the factors and cells regulating their for- mation and maintenance. It is important to under- stand that each of the periodontal components has its very specialized structure and that these structural characteristics directly define function. Indeed, proper functioning of the periodontium is only achieved through structural integrity and interaction between its components. In recent years, a number of detailed descriptions of the structural and compositional features of periodontal tissues have been published (3, 5–7, 9, 15, 17, 46, 50, 56, 58, 61); we refer the reader to these for a comprehensive description of the develop- ment, formation, and structure of periodontal tissues. The present review will focus on structure–function relationships pertinent to understanding periodontal tissue breakdown and the repair ⁄ regeneration of affected structures. Healthy periodontal tissues Dentogingival junction The dentogingival junction (gingiva facing the tooth) is an adaptation of the oral mucosa that comprises epithelial and connective tissue components. The epithelium is divided into three functional com- partments – gingival, sulcular, and junctional epithelium – and the connective tissue into super- ficial and deep compartments. The junctional epi- thelium plays a crucial role since it essentially seals off periodontal tissues from the oral environment. Its integrity is thus essential for maintaining a healthy periodontium. Periodontal disease sets in when the structure of the junctional epithelium starts to fail, an excellent example of how structure determines function. The junctional epithelium The junctional epithelium arises from the reduced enamel epithelium as the tooth erupts into the oral cavity. It forms a collar around the cervical portion of the tooth that follows the cementoenamel junction (Fig. 1). The free surface of this collar constitutes the floor of the gingival sulcus. Basically, the junctional epithelium is a nondifferentiated, stratified squa- mous epithelium with a very high rate of cell turn- over. It is thickest near the bottom of the gingival sulcus and tapers to a thickness of a few cells as it descends apically along the tooth surface. This epi- thelium is made up of flattened cells oriented paral- lel to the tooth that derive from a layer of cuboidal basal cells situated away from the tooth surface that rest on a basement membrane. Suprabasal cells have a similar ultrastructure and, quite remarkably, maintain the ability to undergo cell division. The cell layer facing the tooth provides the actual attachment of the gingiva to the tooth surface by means of a structural complex called the epithelial attachment. This complex consists of a basal lamina-like structure that is adherent to the tooth surface and to which the superficial cell layer is attached by hemidesmosomes. The basal lamina-like structure is a specialized extracellular matrix in which typical basement membrane constituents have not been immuno- detected in any significant quantity but which is *Parts of this article are adapted from Reference 50. 11 Periodontology 2000, Vol. 40, 2006, 11–28 Printed in the UK. All rights reserved Copyright Ó Blackwell Munksgaard 2006 PERIODONTOLOGY 2000