 So, now we have the structure of periodontal ligament. So, we have basically two categories, one is cells and the next one is extracellular substance. So, under cells we have basically five categories. The first category is synthetic cells, the cells which produces other cells, that is osteoblast which produces bone, fibroblast, cementoblast. The second one is resoptive cell which distracts the cell, osteoclast, fibroblast and cementoclast which distracts bone, fibres and cementum. Third one is progenitor cell, then epithelial rest of molasses and defencil. So, the basic defencil, muscle, macrophages and snuffles. So, synthetic cells we have osteoblast, fibroblast, cementoblast, resoptive cell, the destructive cell osteoclast, fibroclast and cementoclast, this is blast means create, clast means distract. Progenitor cell, epithelial rest of molasses, we have seen when the sheath was disrupted, the hard wicks epithelial root sheath, once it loses its continuity, the remnants will be on the periodontal ligament as epithelial rest of molasses and the common defencil. And in extracellular substances, that means main bulk of substances, we have fibres and ground substances. In fibres, we have collagen fibres and oxytalin fibres. In ground substances, we have glycosaminoglycans and glycoproteins. Now, we will move on to the, our principal or the chief component of cells sector, that is synthetic cell, that is fibroblast, which is the architect, which is a builder, which is a caretaker of periodontal ligament, which is very predominant in the periodontal ligament. So, basically it originates from cemental surface and also from alveolar bone surface. Cemental surface, it originates from ecto mesenchym of investing layer of dental papilla and dental follicle from the cemental surface, whereas the alveolar bone side, it is originated from very vascular mesenchym. So, both sides it originates towards the cemental surface, it is originates from the ecto mesenchym, we have seen in dental papilla and dental follicle and in alveolar bone side, it is originated from peri vascular mesenchym. So, these cells, which is oriented with their long axis parallel to the direction of collagen fibres. So, how the collagen fibres are oriented? So, it just follows the long axis parallel to the collagen fibres and which is aligned along between collagen fibres and appearance governed by the surrounding matrix. So, these fibroblasts of periodontal ligament generate an organizational pattern as they have the ability to both synthesize and shape the proteins of extracellular matrix. So, it has both the properties that is the ability to synthesize and shape the proteins of extracellular matrix. So, these proteins also it can shape and it can synthesize other elements also. So, the certain fibres or the fibrils form bundles to get inserted into tooth and bone, which is known as sharp piece fibres. This is very important sharp piece fibres. Commonly asked short note, so what is sharp piece fibres? So, certain fibres or fibres in fibroblast it gets bundled or it gets forms in a bundle and get inserted into tooth and bone. So, once it is embedded in the wall of alveolus or tooth, these fibres calcify to certain degree. Which fibres? Sharpies fibres calcify to certain degree and are associated with an abundance of non-collagenous proteins found in the bone. So, these proteins are known as osteopontin and bone silo protein. So, these are associated with sharpies fibres. So, what are the functions of fibroblast? Okay, not osteoblast, functions of fibroblast. So, the first function is to synthesize collagen. First is a collagen synthesize, then synthesize fibrils. Organize fibrous network and generate force on tooth eruption, produce extracellular matrix of pyridontal ligament which has capacity to give rise to cementoblast and osteoblast. So, fibroblast can give rise to osteoblast and also cementoblast. It maintains a normal width of pyridontal ligament which is synthesized and shaped the proteins of extracellular matrix I mentioned earlier in which collagen fibres form bundles and insert into the bone or tooth as sharpies fibres. So, it can shape the extracellular matrix proteins. So, it regulates collagen turnover by phagocytosis, phagocytosing the old collagen fibres. Now, we will move on to the second synthetic cell that is osteoblast. These cells covers the pyridontal surface of alveolar bone which line the tooth socket and our cuboidal in shape with prominent round nucleus at the basal end of the cell which has rough endoplasmic reticulum, mitochondria and vesicles which are very active and abundant in osteoblast. Microfilaments are prominent beneath the cell membrane and this cell conducts one another through desmosomes and tight junction. What about cementoblast? Cementoblast aligns the surface of cementum which are cuboidal with large vesicle nucleus with one or more nucleole and all the organelles required for protein synthesis and secretion are present in cementoblast. There are two types of cells that is cells with cytoplasmic process and cells without cytoplasmic process. So, cells actively depositing cellular cementum exhibit the cytoplasmic processes and basophilic cytoplasm but whereas a cellular cementum producing cells which doesn't have prominent cytoplasmic processes. So, these are the two types of cells which you seen in cementoblast type cells. Now, let's see the resopty cells. Those are osteoblast, fibroclast and cementoblast. So, next we have resopty cells. Resopty cells are nothing but which distracts the cells which is doing the function just opposite of synthetic cells. They create these resopty cells, they distract the cells. So, the most common one is osteoclast which basically they resorb on and tend to be very large and multi-nucleated but can also be small and mononuclear. So, these multi-nucleated osteoclast are formed by fusion of precursor cells to circulating monocytes. The part of plasma membrane lying adjacent which is being resorbed is raised in characteristic folds. So, you can see the folds over here, the folds. So, this is known as ruffled or striated border. So, the part of plasma membrane lying adjacent to bone that is being resorbed is raised in characteristic folds which is known as ruffled or striated border. So, these are found against the bony surface occupying shallow depression which is known as house-ships lacunae. So, there are many short notes will be coming from this one. The ruffled or striated border, house-ships lacunae and one more thing we have that is clear zone. So, you can see this area which is devoid of all the organelles. So, this ruffled border is separated from the rest of plasma membrane by a zone specialized membrane that is closely applied to the bone. So, it is separated from the rest of plasma membrane by a zone of specialized membrane which is very closely applied to the bone. And the underlying cytoplasm, this is a cytoplasm which tend to be devoid of organelles and that is known as clear zone. So, in osteoclast, we have learned what is ruffled or striated border that is this particular appearance that plasma which is plasma cells, plasma membrane which is close to the bone and it is being resorbed which gives that because resorption will not happen in a linear fashion. It will be foldable fashion it happens. And this seen in particular depression, this osteoclast seen in particular depression known as house-ships lacunae and there will be a clear zone which is a cytoplasm which is devoid of organelles. Whereas a cemento-clast, cemento-clast the peculiar thing about cemento-clast there does not remodel. So, these cemento-clast are not usually found in the periodontal ligament because it does not remodel. So, these cells seen when there is pathological conditions or during resorption of deciduous teeth and when regressive forces are applied because we forcefully apply forces that the forces are being applied on orthodontic therapy. So, in orthodontic therapy we apply forces that is intentional forces and that cases we can see cemento-clast. So, these cemento-clast resemble osteoclast and are located in depression and cementum resembling house-ships lacunae. So, this is not usually seen in periodontal ligament. It comes when there is a pathological problem, resorption of deciduous bone and the orthodontic forces. And one more thing, these cells not only just resolve cementum but also they can destroy dentine and enamel. So, this is also known as ordento-clast. So, ordento means tooth. So, clast is something which is distracting structure. So, this is also known as ordento-clast because it distract enamel and dentine. Now, we have our third category. So, we finished fibro-clast is nothing but cell switch, destruct fibros or collagen. So, osteoblast creates bone, fibro-blast creates collagen, cemento-blast creates cementum, osteoclast distract bone and fibro-clast distract collagen, cemento-clast distract cementum. Now, we are moving on to our third cell which is progenitor cell. So, progenitor cell is all connected tissue including PDL which contains progenitor cells that have the capacity to undergo mitotic divisions. So, mitosis is the basis of replication of cells. So, these are undifferentiated mesenchymal cells that have a perivascular location within around 5 micrometers of blood vessels. So, when stimulated appropriately the cell undergo mitosis and what are the cells will be formed? So, we may have fibro-blast, osteoblast or cemento-blast that in turn produces collagen, bone and cementum. So, progenitor cells are the cells which produces osteoblast, fibro-blast and cemento-blast. So, this should be here. Progenitor as a primitive cells that itself give rise to osteoblast, fibro-blast, cemento-blast. So, that is the synthetic cell of synthetic cell we can say. And we have epithelial rest of molasses. We already discussed it. There will be epithelial roachy that is half weeks epithelial roachy which undergo lysis and there will be epithelial rest of molasses because it loses its continuity and epithelial rest will be seen along the root surface as networks, strands or islands or tubule-like structures which will be parallel to the surface of root which has around 25 micrometer in diameter. So, their function is not yet clear but they could be involved in periodontal repair and regeneration. So, it is most numerous in apical and cervical areas. Children, it is very numerous as the number reduces. These cells may proliferate to form arm tumors. And one more thing, we have the cells that the epithelial rest of cells undergo calcification to become cementicals. Okay. So, what all we learnt? We learnt ruffled or striated border, how ships lack on a clear zone, odendoclast and cementicals. These all are shock nodes. So, osteoclast, osteoplast, fibroblast, fibroclast, progenitor cell, epithelial rest of molasses, half weeks epithelial root sheath, cementicals, all will be asked for shock nodes. So, this is a very common SA periodontal ligament. So, we have not done. So, we are into ourselves. Now, we go to the different cells. So, different cells are common different cells. We have mast cell, esophil and macrophages. Now, we need to study the extracellular substances. So, we had seen the basic structure, the basic origin, origin and its shape. After that, we moved to the structural cell. Structural cell, we have structure, we have cells and extracellular proteins. We finished cells. Now, we have extracellular substances. Now, let's see the extracellular substances. So, extracellular substances, we have fibres and ground substances. In fibres, we have collagen, elastic reticular, secondary fibres, oxitalin fibres and also indifferent fibres, plexus. So, the collagen fibres are the main fibres, which is the basic types are type 1 and type 3. The 70 percentage belongs to type 1 and it is uniformly distributed in the ligament. Whereas, the type 3, which accounts for around 20 percentage, found in periphery of sharpies fibres. And type 4 and type 7, which are associated with epithelial cell rest and blood vessels. So, type 13, which is associated with PDL when the ligament is completely functional. And the collagen is gathered to form bundles and approximately 5 micrometer diameter. And these bundles are termed as principal fibres. So, principal fibres are very important. The next video I will be doing the principal fibres. So, within each collagen bundle, the subunits are present, which is known as collagen fibrills. So, fibrills combine to form fibres. Now, we have the turnover rate of collagen. This is faster than all other collagen present in the connective tissue, because it is highestly seen in pedodontal ligament, the turnover of collagen. And the rate appears to be highest towards root apex. And the collagen on both side has lower turnover rate than that on the bone side, when it shows, where it shows higher turnover rate. So, towards the bone side, it has having higher turnover rate. And now we are moving on to ground substances. Principal fibres will be dealt in next video. The major glycosaminoglycans are contributed in sulphate, dermatone sulphate, heparin sulphate, hyaluronic acid and keratin sulphate. So, glycoproteins. Glycoproteins we have various type, fibronectin. These glycoproteins are densely packed proteins with extracellular matrix, and they are being localized in the calcified sections of human periodontal ligament. The most common is fibronectin. It promotes attachment of cells to substratum, especially to collagen. It is expressed strongly along attachment sites of PDL collagen fibres to cement them, but not born. In addition to its function, it is also having functions such as blood coagulation, wound healing and chemotaxis. So, it promotes cell addition to collagen. It is associated with collagen fibres to cement them. Next, we have tenacin. It is also known as cytotactin. It is the other glycoprotein identified in the periodontal ligament. It is found mostly in healing wounds. Unlike fibronectin, it is not uniformly distributed throughout the PDL, but is concentrated between the less densely packed collagen fibres near cementum and alveolar bone. So, laminin is a glycoprotein component of basement membrane of epithelial cell rest of molasses, which has properties like cell adhesion, migration and differentiation. So, these are the basic extracellular substances, which are glycosaminoglycans. We have many types and also the glycoproteins. Now, let us move on to the principal fibres of perinodontal ligament.