The formation of intrinsic fibers was examined in the advanced stage of rat cellular cementogenesis by light and electron microscopy. Using scanning electron microscopy, cementoblasts showed wing-like processes, partly encircling principal fibers. At the cementum-facing side of the cells these processes showed segmentation into finger-like processes, arranged in parallel with the cementum surface. Transmission electron microscopy showed many cytoplasmic fragments around intrinsic fibers at the cementum surface. These fragments contained microtubules and collagenous secretory granules that were arranged in parallel with the cementum surface and the intrinsic fibers. The wing-like processes contained microtubules and secretory granules that were arranged perpendicularly to the cementum surface and in parallel with the principal fibers. These observations suggest that: (1) the cytoplasmic fragments are cross-sectioned finger-like processes; (2) cementoblasts secrete intrinsic fibers from the finger-like processes and additional principal fibers from the wing-like processes; (3) cementoblasts constantly shorten their wing-like processes by forming finger-like processes. This development starts at the side facing the cementum and proceeds towards the periodontal ligament. With the segmentation, the cementoblasts change the arrangement of secretory granules to secrete intrinsic fibers around preformed principal fibers.