Introduction to the gingival connective tissue
The connective tissue of the gums is called the lamina propria. It is thick collagen with some elastic fibers. It consists of two layers: papillae and reticular layer. The papillary layer borders the epithelium, and the reticular layer borders the periosteum of the alveolar bone. Papillary protrusions can be seen in the connective tissue papillary layer, which interlock with the epithelial pins of the network. Densely packed collagen fibers with specific orientations and connections can be seen in the connective tissue. Other important components of the gingival connective tissue are various cells and macromolecules. The detailed description of these components is given below.
In the gingival connective tissue, also called the gingival fiber apparatus, there is a specific orientation and organization of the collagen fiber bundles. These fiber bundles have been named according to their location, origin, and insertion as the dentogingival, alveologingival, dentoperiosteal, circular, semicircular, transmucosal, intergingival, intercircular, transseptal, and interpapillary groups.158-160.
Dentogingival fibers:These fibers are embedded in the cement near the cemento-enamel junction (CEJ) and fan out into the gingival connective tissue that attaches the gingiva to the teeth. On the buccal and lingual surfaces of the teeth, these fibers fan out outward from the cementum.
Alveological fibers:These fibers extend from the periosteum of the alveolar ridge into the gingival connective tissue. These bundles of fibers attach the gums to the bone.
periosteal fibers:These fibers extend laterally from the tooth to the periosteum of the alveolar bone. They attach the tooth to the bone.
round fibers:These fibers surround the tooth in a ring coronal to the alveolar ridge, which connects adjacent teeth. These fibers are not attached to the cementum of the tooth.
Semicircular fibers:The fibers originate from the cementum near the UCA, cross the marginal free gingiva and insert in a similar position on the opposite side of the tooth.
Transgingival fibers:These fibers extend from the cementum near the UCA of the teeth and run horizontally between the adjacent teeth, uniting them into a unitary arch.
Intergingival fibers:These fibers extend mesiodistally throughout the arch and around the last molars in the arch. In this way, they connect the neighboring teeth to form a dental arch unit.
Intercircular fibers:These fibers surround multiple teeth in the arch that connects the adjacent teeth in a unitary arch.
Transseptal Fibers:These fibers run from the cementum of one tooth across the crest of the alveolar bone to the cementum of the adjacent tooth. They connect adjacent teeth with each other and ensure their alignment in the dental arch.
Interpapillary fibers:These fibers are coronal to the transseptal fibers connecting the oral and buccal interdental papillae of the posterior teeth.
Cell components of the gingival connective tissue
There is a multitude of cells that reside in the gingival connective tissue, accounting for about 8% of its total volume. Fibroblasts are the primary cellular element of the gingival connective tissue responsible for the formation and breakdown of collagen fibers. They play an important role in healing after periodontal surgery. Other cells seen in the gingival connective tissue are macrophages, mast cells, lymph cells, and blood leukocytes. Below is a detailed description of the cellular components of the gingival connective tissue,
Gingival fibroblasts are the most abundant cells in periodontal connective tissue. They make up about 65% of the total cell volume of the gingival connective tissue. Depending on their functional status, they show significant differences in their morphology, size, and shape. They are spindle or stellate cells (active fibroblasts) with an oval or round central nucleus. These cells produce extracellular matrix products, ie collagen fibers and amorphous ground substance, which play an important role in the formation of the extracellular matrix. Fibroblasts are also responsible for the resorption of collagen fibers and thus play an important role in collagen homeostasis. Current evidence indicates considerable heterogeneity in fibroblast cell populations.161. When viewed under an electron microscope, these cells show abundant mitochondria, a prominent Golgi apparatus, and densely packed sheets of rough endoplasmic reticulum, indicating active synthetic cells. Fibroblast activity is controlled by the local cellular environment, both the extracellular matrix and soluble factors.
Polymorphonuclear cells (PMN)
These are primary cells that are abundant in an acute inflammatory response. These cells are usually found in the junctional epithelium where they migrate along the chemoattractant gradient to reach the site of bacterial accumulation. Under the influence of various pro-inflammatory mediators, these cells migrate out of the blood vessels by transendothelial migration to reach the site of injury. Under healthy conditions, when there is no inflammation, these cells are rarely found in the gingival connective tissue.
Lymphocytes and plasma cells:
Lymphocytes and plasma cells are the key cells involved in the immune response against infection. The majority of lymphocytes found in the gingiva are T-lymphocytes, which are mainly found in the area immediately adjacent to the junctional epithelium. These form the cell-mediated immune response against an infection. The humoral response or the antibody-mediated response is generated by plasma cells derived from mature B cells. Mature B cells have a………….. Content available in the hard copy of the website…………. Content available in the hardcopy of the site…..
Monocytes and macrophages:
Monocytes and macrophages are mononuclear phagocytes that play important roles in tissue homeostasis and immunity. Monocytes circulate in the blood, bone marrow and spleen and do not multiply in a steady state. These cells possess chemokine receptors and pathogen recognition receptors that mediate their migration from blood to tissues during infection. Macrophages are scavenger cells that reside in lymphoid and non-lymphoid tissues. These cells are characterized by the presence of a relatively small oval or toothed nucleus, abundant cytoplasm containing primary and secondary lysosomes, microfilaments, scattered rough endoplasmic reticulum, and numerous small vesicles distributed throughout the cytoplasm. Normally, only a small population of these cells is present in normal, non-inflamed gingiva. Inflammation leads to an increased number of these cells in the gingival connective tissue.
Mast cells are large granule cells that arise from multipotent progenitors in the bone marrow and are normally distributed throughout connective tissue 162. These cells are involved in numerous activities ranging from vascular control, tissue injury and repair, to allergic inflammation and host defense. These cells are located near blood vessels or nerve endings, where they release their products when stimulated. Its products include heparin, histamine, and a variety of other pro-inflammatory mediators, including the interleukins IL-1, IL-3, IL-4, IL-5, IL-6, IL-8, IL-10, IL-13, and IL -16, along with granulocyte-macrophage colony-stimulating factor (GM-CSF), platelet-activating factor (PAF), RANTES, macrophage inflammatory protein (MIP-1a), and the metabolites of arachidonic acid, prostaglandin E2, and leukotriene C4163. Inflamed and healing gums have been shown to increase mast cell counts.164. The cell membrane of mast cells has a relatively wide range of receptors that can mediate an interaction with components of the immune system and thus play an important role in the immune response. They are also an important source of growth factors such as vascular endothelial growth factor or nerve growth factor.
Osteoblasts and osteoclasts:
These cells are located near the alveolar process and are responsible for bone formation and resorption (described in Alveolar Bone). Cementoblasts and cementoclasts: These cells are responsible for the formation and resorption of cement (described in the alveolar bone).
Macromolecular components of the gingival connective tissue:
The gingival connective tissue contains collagen and non-collagenous glycoproteins. Non-collagenous glycoproteins include laminin, fibronectin, proteoglycans, a small amount of elastin and serum components.
It is the most abundant protein in connective tissue and is involved in forming the structural framework of both hard and soft connective tissue. The characteristic feature of collagen is its balanced structure. Collagen molecules are synthesized by resident fibroblasts. To date, 28 types of mammalian collagen are known to form triple helices by three polypeptide chains twisted around each other in a rope-like structure.165, 166. The triple helix structure is formed by the presence of glycine (Gly) at every third position of the amino acid chain. Therefore, the collagen domain has the repeating sequence X-Y-Gly [where X and Y can be any amino acid, but one of them is usually a proline (Pro)]. All different forms of collagen cannot form fibrils. Those capable of forming fibrils include collagen types I, II, III, V, XI, XXIV, and XXVII. In the gingiva, collagen makes up 60% of the total tissue protein.167. Type I and Type III collagen make up the majority of extractable collagen from gum tissue, accounting for approximately 99%. Type V collagen accounts for less than 1%168.
The collagen turnover rate is………. Content available in paper version of the website …………. Content available in the hardcopy of the site…..
Laminins are heterotrimeric extracellular matrix proteins composed of α, β, and γ subunits. There are multiple isoforms of each subunit that produce a variety (15) of laminin proteins cataloged to date. It binds to cell surface receptors as well as to various matrix components. It is an important component of the basal lamina due to its ability to form fibrils and networks itself and with type IV collagen.
Fibronectin is a ubiquitous and essential component of the extracellular matrix (ECM). It is also an important component of plasma due to its role in thrombosis. The name "fibronectin" is derived from the Latin words "fibra" for "fiber" and "nectere" for "to bind". Fibronectin is formed by joining two similar polypeptide subunits by a pair of disulfide bonds near the C-terminus of each subunit. It contains two major heparin-binding domains that interact with heparan sulfate proteoglycans. It also contains collagen and fibrin binding sites. Cellular fibronectin is secreted by fibroblasts and many other cell types and is organized into fibrils that contribute to the insoluble extracellular matrix. Fibronectin binds to many receptors in the integrin family. Besides this, it has a remarkably wide variety of functional activities. In addition to binding to cell surfaces via integrins, it also binds to several biologically important molecules including heparin, collagen/gelatin and fibrin. It plays an important role in wound healing. It facilitates the proliferation of platelets on the collagen fibers, their association with the fibrin clot, favors the opsonization of phagocytes and favors the migration of fibroblasts. It has specific domains for binding proteoglycans, making it an important binding material in the extracellular matrix.
Proteoglycans are proteins that are heavily glycosylated. Structurally, these are made up of glycosaminoglycans (GAGs) covalently linked to core proteins. They form an important part of the extracellular matrix. An important function of proteoglycans in the extracellular matrix is to provide elasticity and resistance to compression under pressure, which is attributed to their high degree of supramolecular organization.
The biosynthesis of proteoglycans requires several modifying enzymes in addition to glycosyltransferases. These can be classified based on the nature of their glycosaminoglycan chains. The protein part of proteoglycans is synthesized by ribosomes and then enters the lumen of the rough endoplasmic reticulum. The glycosylation of proteoglycan occurs in the Golgi apparatus in several enzymatic steps. A tetrasaccharide linkage is then attached to a serine side chain on the core protein to serve as a primer for the growth of the polysaccharide. Sugar units are then attached to this structure in the presence of the enzyme glycosyltransferase.
Table: Classification of proteoglycans according to the nature of their glycosaminoglycan chains
|Glycosaminoglycane||small proteoglycane||large proteoglycans|
|Chondroitinsulfat / Dermatansulfat||Decorin, kDa=36, Biglican, kDa=38||Versicano, kDa = 260–370|
|Heparinsulfat/Chondroitinsulfat||testicano, kDa=44||Perlecano, kDa = 400-470|
|Chondroitinsulfat||Bikunina, 25 kDa||Neurocano, kDa=136|
|sulfate cuttings||Fibromodulin, kDa=42, Lumican, kDa=38|
Fully formed proteoglycans are exported to the extracellular matrix space via secretory vesicles. In gum tissue, dermatan sulfate is the predominant glycosaminoglycan, accounting for up to 60% of the total. The remaining components include hyaluronic acid, heparan sulfate, and chondroitin sulfate 188,189. Proteoglycans were originally thought to be an inert cementing substance, but it is now clear that, due to their charged nature, they are highly hydrated and capable of interacting with a wide variety of matrix and cell surface components, which is essential for maintaining normal tissue function.
Elastin is another important structural protein found in the extracellular matrix (ECM) of gingival connective tissue. The formation of elastin, also called elastogenesis, occurs through the assembly and cross-linking of the tropoelastin protein. As discussed in collagen synthesis, expression of genes encoding elastin leads to the formation of tropoelastin. Tropoelastin inside the cell combines with elastin-binding protein (EBP) and this complex is secreted from the cell surface. The elastin-binding protein (EBP) then dissociates from the tropoelastin molecule and re-enters the cell. The interchain covalent crosslinking system present in elastin is similar to that of collagen. Tropoelastin aggregates are oxidized by lysyl oxidase to form crosslinked elastin. Elastin plays an important structural role in elastic tissues, including arteries, skin, ligaments, cartilage, and tendons, where it is found in high concentrations.190. It is also present in the connective tissue of the oral mucosa and attached gingiva. There is limited elastin content in the periodontal ligament.
The gingival connective tissue consists of cellular and extracellular components that are very important for maintaining homeostasis in the periodontal area. In the discussion above, we have examined various cellular and extracellular components of the gingival connective tissue. With this basic knowledge, we can now examine other important components of the periodontium such as periodontium, cementum and alveolar bone.
The references are available in the hard copy of the website.