The formation of cellular cementum was studied by electron microscopy at the advancing edge of developing roots of molars of 25-day-old rats. It was found that the basement lamina about Hertwig's epithelial root sheath first becomes discontinuous on the dental sac side, collagen and cementoblast processes appearing within the formerly intact epithelial compartment. As an increasing bulk of dentinal and cemental collagen is deposited, some sheath cells become reoriented perpendicular to the surface of the dentinal collagen and then migrate from it toward the future periodontal space. Those sheath cells remaining in place after the commencement of cementum mineralization are subsequently embedded there by the mineralizing collagen located external to them. Ultrastructural proof is provided that two cell types, epithelial root sheath cells and cementoblasts, are incorporated by the forming cementum. Rate of formation is discussed as a basis for this nonspecific incorporation of cells by a mineralized connective tissue. © 1969 Academic Press, Inc.
