Periodontium is a collective term describing tooth supporting tissues that develop and function effectively as a unit (Hoffman, 1960; Schroeder, 1986). These include the alveolar bone, cementum, periodontal ligament (PDL), and gingiva. Cementum is a specialized mineralized tissue covering the entire root surface and forms the interface between root dentin and the PDL and has a unique function of attachment of PDL collagen fibers to the root surface. Traditionally, cementum has been classified as cellular and acellular cementum depending on the presence and absence of cementocytes in cementum but further grouped into intrinsic or extrinsic fiber cementum depending on the presence of the collagen fibers formed by cementoblast or by fibroblasts (i.e., Sharpey’s fibers), respectively (Jones, 1981; Schroeder, 1986). Formation of acellular extrinsic fiber cementum (AEFC) has been investigated during root formation in rodents (Lester, 1969; Selvig, 1964; Stern, 19641, dogs, and man (Bosshardt and Schroeder, 1991a,b, 1992; Owens, 1974,1978; Schroeder, 1992; Sequeira et al., 1992). These studies indicate that cementogenesis in rats differs from that in the dog and human. A discussion of the structure and development of all types of cementum in the many different animals studied to date is beyond the scope of this article. The focus of this paper is on the formation of the initial layer of AEFC by cells originating in the dental follicle proper during root formation and on the role of epidermal growth factor (EGF) and its receptor (EGF-R) in maintaining the fibroblast phenotype in the PDL. The PDL, a dense connective tissue located between the alveolar bone and cementum, is composed of cellular as well as extracellular matrix components. The PDL contains numerous actively secreting fibroblasts and a small number of undifferentiated paravascular fibroblast precursors. Epithelial cells of the rests of Malassez, derived from the breakup of the epithelial root sheath, endothelial cells, and macrophages are also present. In addition, a small number of cementoblastic cells are located adjacent to the root surface, while osteoblastic cells such as preosteoblasts and osteoblasts are present along the alveolar bone surface. The collagen fibers, consisting of type I and I11 collagen, are the major structural components of the ligament. They are organized into principal fibers that sus-