V Dhakray, M Mittal, P Khanna, M Jain, B Yadav
V Dhakray, M Mittal, P Khanna, M Jain, B Yadav. Evolution And History Of The Periodontal Ligament - A Review. The Internet Journal of Medical Technology. 2012 Volume 6 Number 1.
The periodontium is defined as those tissues supporting and investing the tooth, comprises of root cementum, periodontal ligament, bone lining the tooth socket and that part of the gingiva facing the tooth. The widespread occurrence of periodontal diseases and the realization that lost tissues can be repaired and perhaps regenerated has generated considerable interest in the factors and cells regulating their formation and maintenance. It is important to understand that each of the periodontal components has its very specialized structure and these structural characteristics directly define function. Indeed, proper functioning of the periodontium is only achieved through structural integrity and interaction between its components.
Although the various methods by which teeth are fixed in their position upon the bones which carry them pass by gradational forms into one another, so that a simple and at the same time absolutely correct classification is, impossible, yet for the purpose of description for principal methods may be enumerated, namely, attachment by means of fibrous membrane, by a hinge, by ankylosis and by implantation in bony sockets.
There is a fundamental difference between the attachment of reptilian and mammalian teeth. In the ancestral reptiles the teeth are ankylosed to the bone. In mammals they are suspended in their sockets by ligaments. The evolutionary step from reptile to mammal included a series of co-ordinated changes in the jaws. The central point of these changes is the radical “Reconstruction” of the mandible. In reptiles the mandible consists of a series of bones united by sutures. Only the upper most bones, the dentary, carry the ankylosed tooth. The change from the many – boned reptiles to the single-¬boned mammalian mandible brought with it a radical change in the mode of growth. In the reptile, the mandibular and maxillary teeth “move” with the bones to which they are fused. In the mammal the teeth have to “move” as units independent of the bones, and this movement is made possible by the remodelling of the periodontium. The evolutionary change from the reptiles to the mammals replaces the ankylosis of tooth and bone to a ligamentous suspension of the tooth. This change permits movement of mammalian teeth and the continued repositioning necessitated by jaw growth or tooth wear.
Cells Of The Periodontal Ligament
The fibroblasts lie between the collagen fibers and although various shapes have been described
The periodontal ligament fibroblast contains a prominent nucleus, which has single distinct nucleolus and clearly defined nuclear pores. When stained with colloidal silver
The nucleus is of a flattened disc shape, and it has a diameter of approximately 10 µm
As fibroblasts produce the extra-cellular matrix of the periodontal ligament, which demonstrates a very high rate of turnover
Rough Endoplasmic Reticulum αGolgi complex αSecretory vesicles α Cell membrane.
And a period of approximately 30 min is required for the completion of this process. The rough endoplasmic reticulum is dispersed throughout the cytoplasm, except for the finest cell processes, and occupies approximately 5% (In the human periodontal ligament
The periodontal ligament fibroblasts possess a Golgi complex found primarily in juxta nuclear position
Lysosomes are present in periodontal ligament fibroblasts in the form of large membrane bound vesicles containing a homogenous matrix that is more electron dense than the surrounding cytoplasm. Their numbers are considerably less than in actively phagocytic cells such as macrophages.
The structures so far described are typical of those seen in fibroblasts in connective tissues in general. However periodontal ligament fibroblasts also contain significant numbers of other organelles only infrequently seen in adult connective tissues.
These are small fragments of Collagen fibrils within membrane bound vesicles
Three broad types can be observed within the cell:
Degradation Of Collagen Fibrils• By Lysosome
So, the degradation of periodontal ligament collagen is indeed an intracellular process, and the question arises as to why this is so as normally the degradation of collagen has historically been regarded as an “extra-cellular process”, in which a specific enzyme, collagenase (Matrix metalloproteinase – ¬I – MMP – I) is thought to be responsible for cleaving the triple helical portion of the molecules within the fibrils into 1/4-3/4 fragments and together with MMP-IV, it leads to spontaneous denaturation under physiologic conditions. The rest of the molecule is then responsive to further proteolysis by gelatinise (MMP-II) and MMP-V. However, before any of this collagenase activity can occur, the glycoproteins such as fibronectin & proteoglycans residing on the fibril surface masking the collagenase binding site must first be removed by stromelysin (MMP-III).
However even though collagen degradation is an extra-cellular process, degradation of periodontal ligament collagen is an intra-cellular process due to the following reasons.
The microfilaments and microtubules are present in the cytoplasm of the cell either as a network that fills the cell processes
Their presence within the periodontal ligament, together with the apparent migration of cells in the occlusal direction has led to the proposal that the fibroblasts generate the eruptive force
Microtubules have also been linked to fibroblast motility, and disruption of microtubules leads to internal accumulation of pro collagens
Centrioles are structures, which consists of a hollow tube of microtubules. A structure frequently associated with centriole of periodontal Iigament fibroblasts is a solitary cilium.
The final element of the cytoskeleton that must be considered are the “intermediate filament” (IF) so called because of their size (approximately 11 mm) which is intermediate between that of the thin (actin) and thick filaments of muscle. There are five classes of Ifs, depending on the tissue of origin.
Mesenchymal tissues typically express Vimentin, a subtype of class III Ifs
One further feature of Periodontal ligament fibroblasts is the presence of numerous intercellular contacts not normally found in adult connective’ tissues
In the Periodontal ligament, two major types of contacts are seen:
The gap junctions vary in size about 0.1 pm & 0.5 pm in diameter and macula adherence are generally smaller ranging from 0.1 – 0.4 pm.
The third type of contact reported in rat periodontal ligament is the ‘close contact’
Periodontal ligament fibroblasts also have cell surface receptors for –
The cells of periodontal ligament also shows positive staining reaction to cellular retinoic acid binding protein (CRABP-l)
They have accumulation of numerous glycogen granules and contain significant quantities of both intermediate and actin filaments. The cell membrane may demonstrate numerous intercellular contacts of both the gap junction and simplified desmosome type
These cells within the periodontal ligament are found on the surface of the alveolar bone. Their gross appearance and ultrastructure is same as that seen in osteoblasts anywhere in the body. In active state they form a layer of cuboidal cells, which exhibit strong basophilic cytoplasm. A prominent nucleus lies towards the basal end of the cell and a pale juxta nuclear area indicates the site of Golgi complex.
Like fibroblasts they are seen to contain a prominent rough endoplasmic reticulum and numerous mitochondria and vesicles. The Golgi complex however appears more localized and, extensive than the fibroblast. Microtubules and microfilaments are present. The cells do not appear to posses receptors for EG. Cell to cell contact is via gap junctions and also via simplified desmosomes. They also contact via gap junctions with osteocytes lying within lacunae in the adjacent bone, thus forming a co-ordinated system through the bone tissue.
Osteoblast precursor cells are often seen beneath the osteoblast layer in the vicinity of adjacent blood capillaries. These cells have a reduced cytoplasm and few organelles. During stages of differentiation from
Precursor – committed osteoprogenitor – pre osteoblast
They first migrate away from the bone surface into the body of periodontal ligament before eventually taking up their functional position
Although it has been claimed that bone resorption is mediated via osteocytes
The features are same as cells elsewhere in the skeleton, in that:
They are found within the resorption lacunae
They are large multinucleated.
They have a ruffled border adjacent to resorbing surface, enclosed by a smooth (annular or ‘clear’ zone
The cytoplasm adjacent to ruffled border has numerous mitochondria suggesting extreme metabolic activity.
Tightly packed infolding of the cell membrane, coated with fine’ bristle like structure
cytoplasm stains intensely than adjacent active osteoblast, suggesting the presence of only small amounts of rough endoplasmic reticulum.
Numerous free ribosomes are present suggesting considerable protein synthesis for internal use
bone matrix is primarily fibrillar collagen but there is no evidence of presence of intercellular collagen profiles. This may be related to the controlled extracellular environment available to the osteoclast or the removal of collagen by subjacent periodontal ligament fibroblasts
Epithelial cell aggregates are a normal feature of the periodontal ligament. They represent the remains of the developmental Hertwig’s epithelial root sheath which is involved in differentiation of root odontoblasts
The Epithelial cell rest (ECR) can be distinguished from the fibroblasts of the periodontal ligament in routine histological sections by the close packing of the cuboidal cells and tendency to stain more deeply. They are unique in being completely surrounded by connective tissue cells .
Initially Epithelial Cell Rests is found as one or two cells with partial basal lamina and subsequently the epithelial rests become more cellular and are contained within an almost complete basal lamina with narrow intercellular spaces. As laminin is chemotactic to epithelial cells the basal lamina may therefore play a role in formation differentiation and maintenance of the epithelial cell rest
Epithelial Cell Rests have a high nuclear – cytoplasmic ratio & exhibit basal cell-like undifferentiated and hyper proliferative characteristics as indicated by expression of cytokeratins 5, 6,14,16 & 19
Epithelial Cell Rest are located closer to cementum than to alveolar bone surface, the average distance being 27 µm in the apical region, gradually increasing cervically to 41 µm
Epithelial, Cell Rest-Role In Root Resorption
Ultrastructure Of Epithelial Cell Rest
Ultrastructurally, epithelial cell rest is separated from connective tissue by a basal lamina, which may be fragmented
The Scanty cytoplasm is characterized by the presence of filaments, which insert into the desmosomes frequently found between adjacent cells and into the hemidesmosomes between the cells and the basal lamina. Tight junctions are also found between cells. Mitochondria are abundant and distributed throughout the cell while scarcity of rough endoplasmic reticulum and golgi complex indicated lack of significant protein secretion
Cellular Changes With Age
A decrease in cellularity of the periodontal ligament with age has been reported in
The periodontal ligament contains defence cells, including macrophages, mast cells and eosinophils. These cells achieve more importance during inflammatory periodontal disease.