The development of cellular cementum in rat molars, with special reference to the fiber arrangement.

To elucidate how intrinsic and extrinsic fibers are arranged during cementogenesis, rat cellular cementum was observed in various developmental stages by light and electron microscopy. As cementogenesis progressed, the periodontal ligament cells showed characteristics of collagen-secreting cells, wing-like processes, and delimited extracellular compartments. The principal fibers were organized into substantial bundles in these compartments. The organization of extrinsic and intrinsic fibers progressed in parallel with the extracellular compartmentalization. The main results suggest the following. (1) The extracellular compartments formed by periodontal ligament cells regulate the three-dimensional architecture of the principal fibers. (2) The compartments formed by cementoblasts are essential for the formation of the typical intrinsic-extrinsic fiber structures. When the cementoblasts move away from the cementum surface or are embedded in the cementum, these cells retract the processes that have encircled the principal fibers. At the same time, these cells secrete intrinsic fibers around the principal fibers. This results in the typical intrinsic-extrinsic fiber structures. (3) In the early stage when the cellular compartmentalization is not established, both the intrinsic and extrinsic fibers are irregularly arranged.