How repair cementum becomes attached to the resorbed roots of human permanent teeth

Human premolars extracted for orthodontic reasons were fixed with a mixture of formaldehyde and glutaraldehyde and processed for light and transmission electron microscopy. About 15% of all available teeth showed signs of superficial root resorption. Quantitatively, the resorption lacunae were more frequent at the apical third of the root, when compared to the mid-root and the cervical levels. Qualitatively, the resorption areas appeared as shallow irregular concavities. The degree of resorption and/or repair varied considerably both within and between sites, ranging from arrested resorption to advanced repair. An about 1- to 2-microns-thick seam of exposed collagen fibrils of the residual dentinal matrix lined the resorbed root surfaces in the arrested stage of resorption. A particular class of mononuclear cells repopulated these surfaces and commenced to attach newly produced collagenous matrix fibrils to the exposed dentinal matrix. Increasing fibril formation and bundling resulted in a collagenous fiber fringe oriented perpendicular to the bottom of Howship's lacunae. Following the implantation and establishment of this initial fiber fringe, the junctional zone between the two matrices and further portions of the repair matrix became progressively obscured by a fine granular and electron-dense material, indicating the advancing front of mineralization. In addition, the junctional zone later attained a high degree of electron density and basophilia. Subsequently formed matrix derived from cementoblasts was structurally heterogeneous and resembled cellular intrinsic fiber cementum (CIFC). It is suggested that the initially produced matrix along resorbed root surfaces closely resembles that seen during initial formation of acellular extrinsic fiber cementum (AEFC), whereas further apposition results in a tissue with the characteristics of CIFC.