1. PERIODONTAL LIGAMENT

3. (a) Cementum. (b) Periodontal Ligment. (c) Bone that lines the alveolus. (d) Deeper part of the gingiva. The cementum & the alveolar bone are the mineralized tissues while the periodontal ligament & the part of the gingiva are fibrous tissues. " The PDL is that soft, specialized connective tissue situated between the cementum covering the root of the tooth & the bone forming the socket wall.

5. * Width of PDL = 0.15 to 0.38 mm  PDL width decreases with age.  The PDL extends coronally up to the most apical of the connective tissue of the gingiva.  The PDL is a cellular connective tissue.  It consists of the blood vessels, various cells & extracellular matrix.

7. Other terms previously used for PDL are "Desmodont, Gomphosis, Pericementum, Dental periosteum, Alveolo dental ligament & Periodontal membrane.

8. EVOLUTION:-  There is a replacement of the ankylosis of the tooth & bone to a ligamentous suspension of the tooth. Because of this, movement of the mammalian teeth is made possible, resulting in continual repositioning as required by the jaw growth & also toothwear . DEVELOPMENT:-  Shortly after the beginning of root formation & the formation of the outer dentinal layer of root, the PDL is formed.  The external & internal dental epithelia proliferate from the cervical loop of dental organ to form a Hertwigs epithelial root sheath, which then fragments to form the discrete clusters of the epithelial cells called as " epithelial cell rests of Malassez"  The enamel organ & root sheath are surrounded by dental sac which is formed by condensed cells.

10.  A thin layer of cells lying adjacent to the dental organ, known as dental follicle, divide & differentiate into cementoblasts, fibroblasts & osteoblasts.  The fibroblasts synthesize the fibers & ground substance of the PDL.  These fibers initially occupy the periodontal space without any specific orientation, but just before tooth eruption acquire an orientation oblique to the tooth towards the coronal portion.

17. 3. Cementoclasts :-  found on surface of cementum.  cementum is not remodeled like alveolar bone & PDL Progenitor Cells  are the undifferentiated mesenchymal cells.  have the capacity to undergo mitotic division & replace the differentiated cells dying at the end of their lifespan.  located in perivascular region. Epithelial cell Rests of Malassez: -  remnants of the epithelium of Hertwig's epithelial root sheath.  found close to the cementum.  arranged parallel to the root surface.  formed at the time of cementum formation. when stimulated these cells undergo rapid proliferation & produce a variety of cysts & tumours.

19. Mast Cells :  contain numerous cytoplasmic granules which contains heparin & histamine.  role of heparin not clear.  the release of histamine into the extracellular compartment causes proliferation of the endothelial & mesenchymal cells. Macrophages :  help in phagocytosing dead cells & secreting growth factor, which helps to regulate the proliferation of adjacent fibroblasts.  they can be differentiated from fibroblasts by the phagocytosed material present in its cytoplasm. EXTRA CELLULAR SUBSTANCE:- Fibers Ground Substance (a) Collagen (a) Glycosaminoglycans (b) Elastic fibers (b) Glycoproteins

26. Nerve Supply: - - branches of second & third division of fifth cranial nerve (Trigeminal nerve) - provide sense of touch, pressure, pain & proprioception (mechano receptors). Cementicles:- - cementicles are the small calcified bodies present in PDL, they are not the cells of cementum. - may be fused with cementum or may be free.

27. FUNCTIONS OF PDL:- 1. Physical function:-  Provision of a soft tissue "casing" to protect the vessels & nerves from injury by mechanical forces.  Transmission of occlusal forces to the bone.  Attachment of the teeth to the bone.  Maintenance of the gingival tissues in their proper relationship to the teeth.  Resistance to the impact of occlusal. forces (shock absorption). Three theories to explain the mechanism of tooth support – 1. Tensional theory- acc. to this , when a force is applied to the crown the principal fibers first unfold & straighten & then transmit forces to the alveolar bone ,causing an elastic deformation of the bony socket . Finally when the alveolar bone has reached its limit,the load is transmitted to the basal bone. 2. Viscoelastic system theory- based on the concept that when forces are applied to the tooth ,movement of the fluid content of PDL occurs.The ECF is pushed from PDL into the marrow spaces through the cribriform plate.The periodontal fibers may only have a minor role in the transmission of forces. 3. Acc.to third concept ,the PDL acts like a thixotropic gel .When the gel is stirred or shaken ,as when forces are applied to the tooth,it becomes fluid,but it return to gel state when the forces are removed. _

29. 5. Homeostatic Mechanism. AGING OF LIGAMENT:- - The number of cells & their activity decrease, scalloping occurs in cementum & alveolar bone. - Some fibers are attached at the peaks of those scallops only & not in the depressions .  Thus, it helps in protecting the supporting structures of tooth & substance of crown from effects of the masticatory force.  Pain sensation is transmitted through small diameter nerves, temperature by intermediate type & pressure by large myelinated fibers 4. Nutritive function :-  PDL has rich vascular supply  It provides nutrition to the cementum, bone & gingiva.