2. Development of radicular cementum
Root formation is dependent on the orderly growth of Hertwig's epithelial root sheath which ends as the epithelial diaphragm. The latter also governs the formation of multiple roots. As root formation proceeds, Hertwig's epithelial root sheath becomes perforated by ectomesenchymal cells of the dental follicle which traverse the sheath to reach the dentin surface. These cells lay down a collagenous matrix, which enlarges the perforations of the sheath and gradually displaces it away from the dentin surface. Hertwig's epithelial root sheath eventually breaks up into a network of more or less interconnected epithelial strands, located within the future periodontal ligament. These epithelial remnants are known as the epithelial cell rests of Malassez.
Prior to tooth eruption the collagen adjacent to the dentin surface becomes remodeled into thin fibers, perpendicular to the dentin surface. These fibers slowly mineralize from the dentin surface outward. The non-mineralized end of the fibers extending into the future periodontal ligament space eventually form the principal fibers of the periodontal ligament.
Fig. 86: As pointed out earlier (see fig. 80),
ectomesenchymal cells from the dental follicle (DF) pass through Hertwig's epithelial root
sheath to assume a position close to the dentin of the root surface. These cells
serve as precursors of the cementoblasts (CB) responsible for the production of radicular
cementum. Their main functions are (1) to produce intrinsic collagen fibers that are
orientated more or less parallel to the root surface, and (2) to produce the ground
substance that will imbed the intrinsic and extrinsic fibers of cementum and allow the
tissue to mineralize.
Fig. 87: Early stage of cementogenesis. Cementogenesis begins with the production of
small bundles of collagen fibers (EF) orientated perpendicularly to, and interdigitated
with, the superficial, non-mineralized collagen fibrils of the dentin (D). At this
early stage, the fibroblasts in the dental follicle (DF) are orientated with their long
axis parallel to the root surface. Their orientation will change later, as the periodontal
ligament matures, to one that is more or less perpendicular to the root surface. The
periodontal fibroblasts contribute the extrinsic fibers that become incorporated into the
cementum.
Fig.
88: Higher magnification of early stage of cementogenesis. Extrinsic fibers (EF)
become interdigitated with the superficial collagen fibers of the root dentin (D).
Cementoblasts (CB) provide the ground substance that will imbed the fibers and allow
mineralization to proceed.
As the dental follicle (DF) becomes remodelled into a periodontal ligament, most of the
collagen fibers that become embedded in cementum originate from the periodontal ligament
fibroblasts as extrinsic fibers. The cementoblasts provide the ground substance, in
which these fibers are embedded prior to mineralization, and some intrinsic fibers that
surround and fill the space between the extrinsic fibers.
As new cementum is deposited on the surface, the cementoblasts are
displaced toward the periodontal ligament, thereby avoiding entrapment. This gives rise to
the acellular, fibrillar cementum, most commonly found on the cervical two-thirds of the
root. This cementum is laid down comparatively slowly. The collagen fibrils are
densely and regularly packed, frequently with adjacent collagen banding patterns in
register. With the light microscope, appositional (or cementing) lines are readily
discernible.
Fig. 89: Junction of root dentin (D) and cementum (C). With transmission
electron microscopy, the dentino-cemental junction is readily identified (arrows), as the
orientation and arrangement of the matrix fibrils is quite different in the two tissues.
In dentin, individual fibrils run a haphazard course, whereas in the
cementum, the fibrils are arranged in bundles that are orientated more or less
perpendicularly to the dentino-cemental junction.
Fig. 90: Junction of acellular, extrinsic fiber cementum (AEFC) with the
periodontal ligament (PDL). The principal fibers (EF) of the periodontal ligament
are continuous with the Sharpey's fibers (SF) of the cementum. Because the principal
fibers of the ligament are produced by ligament fibroblasts, rather than cementoblasts,
they are considered as extrinsic fibers of
cementum. These fibers become part of the cementum after they become mineralized and
incorporated into the cementum as Sharpey's fibers.
Fig. 91: Junction of acellular, extrinsic fiber cementum (C) with the periodontal ligament (PDL). As can be seen in this and the previous figure (Fig. 90), adjacent Sharpey's fibers in acellular, extrinsic fiber cementum tend to blend with one another, so that the fibers lose their identity as distinct, individual fibers.
