III. Periodontal ligament

D. CELLS, VESSELS AND NERVES

D.  Cells, vessels and nerves of the periodontal ligament

The periodontal ligament contains a unique assortment of cells that are capable of generating and maintaining three distinct tissues, namely the ligament itself as well as the mineralized tissues on either side of it, i.e. the cementum and the alveolar bone. 

The major cell types of the periodontal ligament include the following:

Fibroblasts, macrophages and undifferentiated ectomesenchymal cells.
Cementoblasts and cementoclasts.
Osteoblasts and osteoclasts.
Cell rests of Malassez.
Vascular and neural elements.

Fig. 69:   Transmission electron micrograph of periodontal ligament fibroblasts (FB).  These cells are responsible not only for the synthesis of collagen and its assembly into collagen fibers (CF), but also for the removal of collagen fibers during the continuous remodeling that takes place in the ligament.  Collagen fibrils are removed, in part, through intracellular degradation by fibroblasts in digestive vacuoles. ICC, intracellular collagen in digestive vacuole.


Fig. 70:    Transmission electron micrograph of intracellular collagen fibrils (ICC) in digestive vacuoles of a periodontal ligament fibroblast.  Collagen fibrils are digested by lysosomal enzymes that have been added to the vacuoles.  The presence of these enzymes has been confirmed by cytochemical localization in tissue sections.

Fig. 71:   Transmission electron micrograph of cementoblasts (CB) in the periodontal ligament in close proximity to the cementum(C) surface.  Morphologically, the cementoblasts do not differ to any extent from the periodontal ligament fibroblasts.   They are identified primarily by their intimate relationship to the cemental surface and the characteristic cytoplasmic projections that extend toward the cementum, frequently surrounding collagen fibers (F) that originate in the ligament and insert into the cementum layer.  The cementoblasts produce so-called intrinsic collagen fibers, i.e. fibers originating from cementoblasts,  as contrasted to the extrinsic collagen fibers produced by the periodontal ligament fibroblasts.  Intrinsic collagen fibers are orientated with their long axis more or less parallel to the tooth surface, whereas the extrinsic fibers are orientated perpendicular to the tooth surface.   Cementoblasts also secrete the cementum matrix in which the collagen fibers are imbedded.  Unlike bone, which is continuously remodelled, the cementum grows slowly in thickness, throughout life, by apposition of new cementum at the surface.  While cementum can be resorbed (see below), it is not continuously remodelled like bone.



Fig. 72:   Transmission electron micrograph of osteoblasts next to a bone-forming surface.  A non-mineralized osteoid seam (O) is sandwiched between the plump osteoblasts (OB) in the periodontal ligament and the mineralized alveolar bone (B).   An osteoblast trapped in a lacuna in the newly formed bone is about to become incorporated into the bone as an osteocyte (OC).


Fig. 73:   Histologic section through a resorbing tooth surface.  Resorbing cells include multinucleated odontoclasts (OC), which in all respects resemble osteoclasts, and  mononuclear cells (M) that resemble macrophages.  External tooth resorption is often of limited duration with repair by deposition of a new cementum layer that provides a new attachment for the periodontal ligament fibers.  In contrast to cementum, bone surfaces are subjected to alternating cycles of bone deposition and bone resorption that result in the gradual replacement of old bone by new bone.  This continuous remodeling also allows for changes in  bone architecture that may be needed for tooth movement and adaptation to changing mechanical stresses.

Fig. 74: Periodontal ligament cells also include endothelial cells which line the numerous blood vessels (BV) and lymphatics, non-myelinated nerve endings, the cell rests of Malassez (M), as well as undifferentiated ectomesenchymal cells that are able to give rise to fibroblasts, cementoblasts and osteoblasts.  These pluripotential cells are unique to the periodontal ligament.  Therefore, the periodontal ligament serves as the primary source of new cells when attempts are made at regenerating lost periodontal attachment.       

Fig. 75 (From unknown source):  Blood supply of the periodontal ligament.  The periodontal ligament has a rich blood supply that originates primarily from the dental arteries that enter through the apical foramen and from blood vessels in the adjacent bone marrow spaces.  Anastomoses between these vascular supplies are numerous throughout the ligament.  The ligament vessels also communicate with the supraperiosteal vessels that supply the adjacent gingiva.

Neural elements tend to be myelinated near the apical end and unmyelinated more coronally. They terminate within the periodontal ligament as free endings that are sensitive to pain, or as endings with specialized receptors for pressure. They closely follow the distribution of the arterial blood supply.

Fig.76: Cross-section through the apical tip of a single-rooted tooth.  The periodontal ligament in the apical region of the alveolus is richly supplied with nerves that enter through the apical foramen and then branch out to supply the pulp and the periodontal ligament.  Most of the branches near the apical end are mixed nerves (N) consisting of myelinated and non-myelinated axons.  As the nerves ascend coronally, they lose their myelin sheath so that most nerves in the ligament are non-myelinated.  Nerve endings in the periodontal ligament register only pain and pressure.


Fig. 76A: Magnified view of nerve in Fig. 76.  Myelinated axons (MA) are readily identified by their thick dark-blue outline, the result of preferential dye uptake by the myelin sheath.  Some nerve branches contain a mixture of myelinated and non-myelinated axons (MN).  The distribution of nerve branches is closely associated with that of blood vessels (BV).    

University of Pennsylvania and Temple University © 1999. All rights reserved. Created: May 8, 1999  Revised: URL: Max A. Listgarten: Comments to author: max@listgarten.com (Technical Support: Center for Dental Informatics: Heiko Spallek)