Periodontal Ligament Cells and Fibers

• Periodontal ligament is a dense fibrous connective tissue that occupies the periodontal space between the tooth root and alveolus.
• Width of PDL ranges from 0.15 to 0.38 mm, narrowest at the mid-root and widest at the alveolar crest, followed by the apical region.
• PDL width decreases with age.
• The PDL extends coronally up to the most apical connective tissue of gingiva.
• PDL consists of blood vessels, various cells, & extracellular matrix.

Functions of the Periodontal Ligament

• Physical function involves attachment of teeth to the bone.
• Physical function includes transmission of occlusal forces.
• Physical function includes resistance to impact forces.
• Physical function includes maintenance of gingival tissue in relation to the teeth
• Cells of the periodontal ligament participate in formation and resorption of cementum and bone (Formative and Remodeling).
• Sensory function of the PDL nerve fibers transmits touch, pressure, and pain to higher centers.
• Nutritive function of the blood vessels of ligament provides essential nutrients for the ligament's vitality and hard tissue of cementum and alveolar bone.

Development:

• PDL forms shortly after the start of root formation & outer dentinal layer formation.
• External & internal dental epithelia proliferate from cervical loop of dental organ to form Hertwigs epithelial root sheath which fragments into epithelial cell rests of Malassez.
• The enamel organ & root sheath are surrounded by the dental sac made of condensed cells.
• A thin layer of cells adjacent to the dental organ, known as dental follicle, divides & differentiates into cementoblasts, fibroblasts and osteoblasts.
• Fibroblasts synthesize the fibers & ground substance of the PDL.
• Fibers initially occupy the periodontal space without specific orientation, but acquire an oblique orientation toward the coronal portion just before tooth eruption.
• Similar fibers appear on the side of the bone.
• Fibers elongate, meet & fuse in the middle.
• Fibers mature progressively towards the root apex as eruption progresses; proper arrangement of the principle fibers of the PDL occurs with occlusal contact.
• Damaged periodontal fibers are replaced & remodeled by newly formed fibers.

Histological structure of the periodontal ligaments:

• The PDL consists of cells and extracellular substances

Cells

• Synthetic cells: Fibroblasts, Osteoblasts, Cementoblasts.
• Resorpative cells: Fibroblasts, cementoclasts, osteoclasts.
• Progenitor cells: undifferentiated mesenchymal cells.
• Epithelial cells: epithelial rest of malassez.
• Defensive cells: macrophages, lymphocytes and mast cells

Synthetic Cells

• Osteoblasts are derived from multipotent mesenchymal cells.
• Osteoblasts cover the periodontal surface of alveolar bone.
• Collagen fibers of ligament that penetrate the alveolar bone are present between the cells.
• Fibroblasts are considered the principal cells of PDL.
• Fibroblasts are ovoid or elongated, oriented along principal fibers & exhibit pseudopodia-like processes.
• Collagen turnover is regulated by fibroblasts.
• Fibroblasts have different functions include secretion of collagen or production of collagenase.
• Cementoblasts located on the cementum surface.
• Cementoblasts help in cementum formation.
• Cementoblasts produce cementoid layer which undergoes mineralization.
• Some cementoblasts become embedded in cementum as cementocytes.

Resorptive cells

• Osteoclasts are multinucleated cells which resorb bone.
• Osteoclasts help in remodeling, allows functional changes in tooth position.
• Fibroblasts help in both synthesis & resorption of collagen.
• Collagen fibers can be resorbed by mononuclear fibroblasts.
• Help in remodeling of PDL.
• They exhibit lysosomes which contain fragments of collagen undergoing digestion.
• Cementoclasts found on the surface of cementum.
• The cementum is not remodeled like alveolar bone & PDL

Progenitor Cells

• Undifferentiated mesenchymal cells.
• Have capacity to undergo mitotic division & replace differentiated cells at the end of their lifespan.
• Located in perivascular region.

Epithelial cell rests of Malassez

• Remnants of the epithelium of Hertwig root sheath.
• Found close to cementum.
• Arranged parallel to root surface
• Formed at the time of cementum formation.
• When stimulated, these cells undergo rapid proliferation & produce cysts & tumors.

Extracellular Substances

• Fibers of the periodontal ligament are mainly collagen, divided into principal, accessory, and oxytalan (elastic) fibers.
• Ground substances consist of Glycosaminoglycans and Glycoproteins

Principal Fibers

• Principal fibers of periodontal ligament are collagen bundles with wavy course arranged in three ligaments

Gingival fibers

• Dentogingival fibers extend from the cervical cementum into lamina propria of the gingiva.
• Alveologingival fibers extend from the alveolar crest into the lamina propria.
• Circular group fibers encircle the tooth and interlace with outer fibers of bone.
• Dentoperiosteal fibers extend from cementum over the crest and incline apically between periosteum of alveolar bone to lamina propria of the gingiva.

Functions of the gingival fibers

• Hold the marginal gingiva against the tooth.
• Provide marginal gingiva with enough rigidity to withstand the forces of mastication without distorting.

Transseptal Ligament

• Connects two adjacent teeth.
• Runs from cementum of one tooth over the crest of the alveolus to cementum of the adjacent tooth.

Alveolodental Ligament

• Alveolar crest group: Fiber bundles radiate from the alveolar process crest and attach to the cervical part of the cementum.
• Horizontal group: Fiber bundles run from cementum to bone at right angle to the long axis of the tooth.
• Oblique group: Fiber bundles run obliquely.
• Attachment in bone is coronal than the attachment in the cementum, and it has the greatest number of fibers.
• It performs main support of the tooth against masticatory force.
• Apical group: Bundles radiate from apical region of the root to the surrounding bone.
• Interradicular group: Bundles radiate from interradicular septum to the furcation of the multirooted tooth.

Accessory fibers

• Collagenous, run from bone to cementum in different planes, more tangentially to prevent rotation of the tooth, found in region of horizontal group.

Oxytalan Fibers

• Immature elastic fibers.
• Run in axial or oblique direction.
• One end is embedded in cementum or bone and the other end is in the wall of blood vessels.
• Suggested function to support blood vessels of the periodontal ligament during mastication, prevents sudden vessel closure under masticatory forces.

Interstitial Tissue

• Blood vessels, lymphatics, and nerves of the PDL are surrounded by loose connective tissue termed interstitial tissue.
• Fibroblasts histiocytes, UMC, and lymphocytes also found in interstitial tissue.
• Biological significance: unknown.

Blood Supply

• Arterial blood supply of the periodontal ligament is derived from: -Branches from the gingival vessels. -Branches from the intra-alveolar vessels; running horizontally and the main blood supply. -Branches from the apical vessels that supply dental pulp.

Nerve supply

• Comes from either inferior or superior dental nerves.
• Bundles of nerve fibers run from the apical region of the root towards the gingival margin.
• Nerves enter the ligament horizontally through multiple foramina in the bone.
• Small fibers mediate pain sensation.
• Large fibers mediate touch & pressure via mechanoreceptors.

The age changes of periodontal ligament

• The periodontal ligament through aging will decrease in vascularity, cellularity, and thickness.
• Presence of cementicles, appearing near the surface of cementum which may be free, attached or embedded in the cementum.
• Nidus favoring the concentric layers deposition of calcosphrite such as degenerated cells, hemorrhage areas, epithelial rests of Malassez.
• Cementicles are usually seen in the periodontal ligament by aging but sometimes they may be seen in a younger person after local trauma.

Explore the cells and fibers of the periodontal ligament (PDL). Understand their functions in synthesizing fibers, ground substance, and resorbing cementum. Learn about the origin of osteoblasts and the role of undifferentiated mesenchymal cells.