J Periodontol • January 2000 Employing a Transgenic Animal Model to Obtain Cementoblasts In Vitro John A. D’Errico,* Janice E. Berry,* Hongjiao Ouyang,* Christopher L. Strayhorn,* Jolene J. Windle, † and Martha J. Somerman* ‡ 63 Background: Proper formation of cementum, a mineralized tis- sue lining the tooth root surface, is required for development of a functional periodontal ligament. Further, the presence of healthy cementum is considered to be an important criterion for predictable restoration of periodontal tissues lost as a con- sequence of disease. Despite the significance of cementum to general oral health, the mechanisms controlling development and regeneration of this tissue are not well understood and research has been hampered by the lack of adequate in vitro experimental models. Methods: In an effort to establish cementoblast cell popula- tions, without the trappings of a heterogeneous population con- taining periodontal ligament (PDL) cells, cells were obtained from the root surface of first mandibular molars of OC-TAg transgenic mice. These mice contain the SV40 large T-antigen (TAg) under control of the osteocalcin (OC) promoter. There- fore, only cells that express OC also express TAg and are immor- talized in vitro. Based on results of prior in situ studies, OC is expressed by cementoblasts during root development, but not by cells within the PDL. Consequently, when populations are isolated from developing molars using collagenase/trypsin diges- tion, only cementoblasts, not PDL cells, are immortalized and thus, will survive in culture. Results: The resulting immortalized cementoblast population (OC/CM) expressed bone sialoprotein (BSP), osteopontin (OPN), and OC, markers selective to cells lining the root surface. These cells also expressed type I and XII collagen and type I PTH/PTHrP receptor (PTH1R). In addition to expression of genes associated with cementoblasts, OC/CM cells promoted mineral nodule for- mation and exhibited a PTHrP mediated cAMP response. Conclusions: This approach for establishing cementoblasts in vitro provides a model to study cementogenesis as required to enhance our knowledge of the mechanisms controlling devel- opment, maintenance, and regeneration of periodontal tissues. J Periodontol 2000;71:63-72. KEY WORDS Periodontal ligament/growth and development; periodontal regeneration; dental cementum/growth and development. * Department of Periodontics/Prevention/Geriatrics, University of Michigan, Ann Arbor, MI. † Cancer Therapy and Research Center, University of Texas Health Science Center, San Antonio, TX. ‡ Department of Pharmacology, University of Michigan. C ementum was first described in 1835, 1 and research through the years has demonstrated that it is a unique tissue histologically, but shares many properties with other mineralized tissues, particularly bone. 2,3 Despite many years of research in which the importance of cementum in the repara- tive process following periodontal dis- ease has been explored, 4 very little is known about the cells responsible for for- mation of cementum, cementoblasts. The wealth of what is known about cementum comes from numerous stud- ies on the histology and composition of this tissue. Light and electron microscopy have enabled classification of cementum into 5 different subtypes based on the presence or absence (cellular versus acellular) of cells and the source of col- lagen fibers (extrinsic versus intrin- sic). 3,5,6 All of these subtypes are quite different from bone, in that they are not innervated, exhibit little or no remodel- ing, and are avascular. Despite these dif- ferences, cementum does have many similarities to bone. First, diseases that affect the properties of bone often alter cementum’s properties as well. For example, Paget’s disease results in hyper- cementosis; hypophosphatasia results in no cementum formation with exfoliation of teeth; decreased cementum is associ- ated with hypopituitarism; and defective cementum is seen in patients with clei- docranial dysplasia. Second, the com- position of cementum is similar to that of bone. Cementum is approximately 50% hydroxyapatite and 50% collagen and