Chapter 15. Periodontal ligament (including oral and periodontal mechanoreception and the tooth support mechanism)
Non-collagenous matrix or ground substance 204
Blood supply 206
Nerve fibres 206
Periodontal ligament as a specialized connective tissue 210
Development of the root and periodontal ligament 210
The periodontal ligament is a fetal-like connective tissue that is the tissue of ‘attachment’ of the tooth to the alveolar bone. It is involved in the tooth support mechanism (resisting masticatory loads) and in the generation of the force(s) of eruption (see page 120). It is a highly vascular and well-innervated tissue. The mechanoreceptors within the periodontal ligament convey sensory information to the brain stem and the trigeminal nuclei, and they are also involved in the reflexes of mastication and salivation. Clinically, the ligament is involved in inflammatory periodontal disease and is the tissue that reacts to orthodontic loads. Being fetal-like, it could be a source of embryonic stem cells.
In terms of the tooth support mechanism, the periodontal ligament is often thought of as a ‘suspensory ligament’ whereby masticatory loads are resisted by the periodontal oblique collagen fibres, being placed in tension. The evidence, however, suggests a more multifactorial mechanism with the tissue being placed under compression with masticatory loads.
• know the structural components (including vasculature and innervation) and biochemical composition of the periodontal ligament, and be able to relate these to the functions of the tissue (particularly the tooth support and tooth eruptive mechanisms; see also page 120)
• be able to describe the development of the periodontal ligament and understand the ‘specialities’ of the tissue.
The periodontal ligament is the dense fibrous connective tissue that occupies the periodontal space between the root of the tooth and the alveolus. It is derived from the dental follicle (see page 114). The average width of the periodontal space is 0.2 mm. Functionally, the periodontal ligament is:
• the tissue of attachment between the tooth and alveolar bone; it is thus responsible for resisting displacing forces (the tooth support mechanism) and for protecting the dental tissues from damage caused by excessive occlusal loads (especially at the root apex)
• responsible for the mechanisms whereby a tooth attains, and then maintains, its functional position; this includes the mechanisms of tooth eruption (see page 120), tooth support (particularly the recovery response after loading) and drift
• involved in the formation, maintenance and repair of alveolar bone and cementum
• via its mechanoreceptors, involved in the neurological control of mastication.
The fibres of the periodontal ligament are mainly collagenous but there may be small amounts of oxytalan, reticulin fibres and, in some species, elastin fibres.
There are two main types of collagen in the periodontal ligament , type I and type III. These are categorized as fibrous collagens:
• The major type is type I, accounting for approximately 70% of the periodontal ligament collagen. It contains two identical α1 chains and a chemically different α2 chain. It is low in hydroxylysine and glycosylated hydroxylysine.
• Type III collagen represents approximately 20% of the periodontal ligament collagen. This molecule consists of three identical α1 III chains and is high in hydroxyproline but low in hydroxylysine, and contains cysteine. The function of type III collagen is not properly understood, although it is associated in other sites of the body with rapid turnover.
• Small amounts of type V , VI and XII collagens have also been identified in the periodontal ligament, along with trace amounts of basement membrane collagens (types IV and VII).
Principal fibres of the periodontal ligament
Much of the collagen is gathered together to form bundles approximately 5 mm in diameter. These bundles are termed the principal fibres of the periodontal ligament. The principal collagen fibres are comprised of:
• dento-alveolar crest fibres
• horizontal fibres
• oblique fibres
• apical fibres
• inter-radicular fibres.
Within each collagen bundle, subunits of structure called collagen fibrils can be seen. The collagen fibrils are formed by the packing together of individual tropocollagen molecules. The collagen fibrils of the periodontal ligament are small and of uniform diameter (approximately 40–45 nm). This pattern is reminiscent of collagen in connective tissues placed under compression and differs markedly from the bimodal distribution with large fibrils usually associated with tissues under tension (e.g. tendon).
Although controversy has existed concerning the extent to which individual fibres across the width of the periodontal ligament, it is now known that the fibres cross the entire width of the periodontal space and there are no separate tooth-related and bone-related fibres merging at an intermediate fibre plexus. The principal fibres of the periodontal ligament do not run a straight course as they pass from the alveolar bone to the tooth, being ‘wavy’. A specific type of waviness seen in the fibrils of collagenous tissues (including the periodontal ligament) is crimping, and it has been proposed that the crimps are gradually pulled out when the ligament is subjected to mechanical tension. The principal fibres of the periodontal ligament that are embedded into cementum and the bone lining the tooth socket are termed Sharpey fibres.
Functional roles of the collagens
The periodontal ligament connective tissue architecture is regulated by collagen type XII, interacting with the periodontal cells. This collagen is a non-fibrous collagen and may function by linking together the other collagens within the periodontal ligament. Evidence suggests that type XII is present only when the periodontal ligament is fully functional. The rate of collagen turnover is faster than other connective tissues and differs in different parts along the root of the tooth. The high rate of turnover is probably associated with the functional demands of the periodontal ligament in terms of remodelling in response to occlusal loads. However, turnover from teeth subjected to greater masticatory loads is not much different from teeth subjected to normal loads. Furthermore, the rate of turnover may not reflect total protein turnover and there may be several protein pools which have different turnover rates. Additionally, matrix turnover may be dependent on extracellular processing, rather than rate of synthesis.
Depending upon species, the periodontal ligament contains either oxytalan fibres or elastin fibres. The ultrastructural characteristics of oxytalan suggest that they are immature elastin fibres (pre-elastin). Oxytalan fibres are attached into the cementum of the tooth and course out into the periodontal ligament in various directions, rarely being incorporated into bone. In the cervical region, they follow the course of gingival and trans-septal collagen fibres but, within the periodontal ligament proper, they are more longitudinally oriented, crossing the oblique fibre bundles more or less perpendicularly. In the outer part of the ligament, they are said often to terminate around blood vessels and nerves. Unlike collagen, oxytalan fibres are not susceptible to acid hydrolysis. The functions of the oxytalan fibres remain unknown. Elastin fibres are restricted to the walls of the blood vessels, although in some animals (e.g. herbivores) they replace the oxytalan fibres. Reticulin fibres are related to basement membranes within the periodontal ligament (i.e. associated with blood vessels and epithelial cell rests) and are a variety of collagen.
Non-collagenous matrix or ground substance
Concerning the non-collagenous matrix or ground substance of the periodontal ligament, little detailed information about this important component is available because of its relative inaccessibility and complex biochemical nature. Although we are used to thinking of the ligament as a collagen-rich tissue, in reality it is a tissue rich in ground substance. The ground substance of the periodontal ligament consists mainly of hyaluronate glycosaminoglycans, proteoglycans and glycoproteins. All components of the periodontal ligament ground substance are presumed to be secreted by fibroblasts. The two proteoglycans identified within the PDL are proteodermatan sulphate and a proteoglycan containing chondroitin sulphate/dermatan sulphate hybrids, designated PG1.
The ground substance of the periodontal ligament is thought to have many important functions (ion and water binding and exchange, control of collagen fibrillogenesis and fibre orientation). Tissue fluid pressure is high in the periodontal ligament, about 10 mmHg above atmospheric pressure, and the tissue fluid has been implicated in the tooth support and eruptive mechanisms (see page 121).
Fibronectin is a glycoprotein that is thought to promote attachment of cells to the substratum, especially to collagen fibrils. Furthermore, cells also preferentially adhere to fibronectin and it may be involved in cell migration and orientation. Fibronectin is uniformly distributed throughout the periodontal ligament (in both erupting and fully erupted teeth) and is localized over collagen fibres and at certain sites on the cell–collagen interface. It is known that loss of fibronectin expression is a marker of tissue maturation in many connective tissues, and the fact that the periodontal ligament retains fibronectin expression may be indicative of the ligament’s perceived fetal-like characteristics.
Tenascin is another glycoprotein found within the periodontal ligament and, like fibronectin, is more characteristic of a fetal-like connective tissue than a fully ‘mature’ connective tissue. Unlike fibronectin, tenascin is concentrated adjacent to the alveolar bone and the cementum. The role of this glycoprotein in the functions of the periodontal ligament awaits clarification.
The predominant cell in the periodontal ligament is the fibroblast.
The periodontal ligament fibroblasts are responsible for regeneration of the tooth support apparatus and have an essential role in the adaptive responses to mechanical loading of the tooth (including orthodontic loading). The periodontal ligament fibroblasts appear as flattened, disc-shaped cells with many fine cytoplasmic processes. Periodontal fibroblasts are rich in the intracytoplasmic organelles associated with the synthesis and export of proteins: rough endoplasmic reticulum, Golgi apparatus and mitochondria. There is evidence, however, that, in addition to synthesizing and secreting proteins, the cells are responsible for collagen degradation. This contrasts with earlier views that degradation was essentially an extracellular event involving the activity of proteolytic enzymes such as collagenases. The main evidence indicating that the periodontal fibroblasts are also ‘fibroclastic’ is the presence of organelles termed intracellular collagen profiles. These profiles show banded collagen fibrils within an elongated membrane-bound vacuole. It is thought that the intracellular collagen profiles are associated with the degradation of collagen that has been ‘ingested’ from the extracellular environment. Nevertheless, the degradation of collagen may be expected to include both extracellular and intracellular events (see below).
The periodontal ligament fibroblasts have cilia and many intercellular contacts, a feature that is not common in the fibroblasts of other fibrous connective tissues. The significance of the cilia in fibroblasts is unknown. The intercellular contacts comprise simplified desmosomes and gap junctions. There is little information concerning the functional significance of these organelles in the periodontal fibroblast.
Role of the periodontal fibroblasts in tissue remodelling
Because of the high rate of turnover of collagen in the periodontal ligament, any alteration in fibroblast cell function will produce a loss of this tissue. Evidence exists that not only do fibroblasts synthesize and secrete collagen but also that they are responsible for collagen degradation; however, it is more widely accepted that periodontal fibroblasts secrete matrix metalloproteinase-1 (MMP-1), which degrades extracellular matrix collagen, and can also secrete tissue inhibitors of metalloproteinases (TIMPs). These TIMPs are found in high concentrations in healthy periodontal tissues. Collagenase secretion can be upregulated in response to cytokine exposure. Because fibroblasts are induced to secrete cytokines (including prostaglandin) in response to applied mechanical loads (such as orthodontics), the periodontal fibroblasts may have intrinsic mechanisms for remodelling the matrix. Importantly, inflammation associated with periodontal disease may cause increased expression of MMPs and aggressive loss of collagen within the periodontal ligament, leading to tissue destruction.
Cementoblasts, cementoclasts, osteoblasts and osteoclasts
In addition to fibroblasts, the connective tissue cells of the periodontal ligament also include cementoblasts and cementoclasts, and osteoblasts and osteoclasts:
• Cementoblasts are the cement-forming cells lining the surface of cementum. They are squat cuboidal cells; they are rich in cytoplasm and have large nuclei. Like fibroblasts, they contain all the intracytoplasmic organelles necessary for protein synthesis and secretion.
• Osteoblasts are the bone-forming cells lining the tooth socket, closely resembling cementoblasts. The layer of osteoblasts is prominent only when there is active bone formation. When bone is not forming, its surface is occupied by flattened, inactive bone-lining cells. Like the periodontal fibroblasts, active osteoblasts contain an extensive rough endoplasmic reticulum and numerous mitochondria and vesicles, although their Golgi material appears more localized and extensive.
• Osteoclasts and cementoclasts (or odontoclasts) are found in areas where bone and cementum are being resorbed. These cells arise from blood cells of the macrophage type. When osteoclasts resorb alveolar bone, the surface of the alveolar bone shows resorption concavities termed Howship’s lacunae, in which lie the osteoclasts. Osteoclasts show considerable variation in size and shape, ranging from small mononuclear cells to large multinuclear cells. The part of the cell that lies adjacent to bone often has a striated appearance, the so-called ‘brush border’. The brush border comprises many tightly packed microvilli which may be coated with fine, bristle-like structures. The cytoplasm contains large numbers of vesicles of different sizes and types; some contain acid phosphatase.
Epithelial cell rests
Aggregations of epithelial cell rests, the rests of Malassez, are a normal feature of the periodontal ligament. They are said to be the remains of the developmental epithelial root sheath of Hertwig (see page 116). The rests lie about 25 µm from the cementum surface. In cross-section, the epithelial cells appear cluster-like, though tangential or serial sections show a network of interconnecting strands parallel to the long axis of the root. The cluster arrangement of the cells is reminiscent of a duct-like structure. The cells are separated from the surrounding connective tissue by a basal lamina. Studies reveal little activity in the epithelial cells and cell turnover is slow.
Defence cells within the periodontal ligament include macrophages, mast cells and eosinophils. These are similar to defence cells in other connective tissues. Macrophages are responsible for phagocytosing particulate matter and invading organisms, and for synthesizing a range of molecules with important functions such as interferon (the antiviral factor), prostaglandins and factors that enhance the growth of fibroblasts and endothelial cells. Macrophages are derived from blood monocytes. Mast cells are often associated with blood vessels. They show a large number of intracytoplasmic granules. Other cytoplasmic organelles are relatively sparse. Numerous functions have been ascribed to the mast cell, including the production of histamine, heparin and factors associated with anaphylaxis. Eosinophils are only occasionally seen in the normal periodontal ligament. Characteristically, they possess granules called peroxisomes. The cells are capable of phagocytosis.
The rich blood supply to the periodontal ligament is derived from the appropriate superior and inferior alveolar arteries, although arteries from the gingiva (such as the lingual and palatine arteries) may also be involved. The arteries supplying the periodontal ligament are not primarily derived from those entering the pulp at the apex of the tooth, but from a series of perforation arteries passing through the alveolar bone. The major vessels of the periodontal ligament lie between the principal fibre bundles, close to the wall of the alveolus. The volume of the periodontal space occupied by blood vessels and by blood may be so great that we should perhaps think of the periodontal ligament as a ‘blood space’ as much as a ‘connective tissue’!
Specialized features of the vasculature of the periodontal ligament vasculature are:
• a crevicular plexus of capillary loops
• the presence of large numbers of fenestrations in the capillaries.
A crevicular plexus of capillary loops completely encircles the tooth within the connective tissue beneath the region of the gingival crevice. The functional significance of this is not fully understood, although it may be related to the provision of a dentogingival seal.
Fenestration of capillaries within the periodontal ligament is unusual because fibrous connective tissues usually have continuous capillaries. It is possible that the fenestrations are related to the high metabolic requirements of the periodontal ligament (high rate of turnover). The number of fenestrations also relates to the stage of eruption.
The veins within the periodontal ligament do not usually accompany the arteries. Instead, they pass through the alveolar walls into intra-alveolar venous networks. Anastomoses with veins in the gingiva also occur. A dense venous network is particularly prominent around the apex of the alveolus.
The nerve fibres supplying the periodontal ligament are functionally of two types: sensory and autonomic:
• The sensory fibres are associated with nociception and mechanoreception.
• The autonomic fibres are associated mainly with the supply of the periodontal blood vessels.
Compared with other dense fibrous connective tissues, the periodontal ligament is well innervated. The nerve fibres entering the periodontal ligament are derived from two sources. Some nerve bundles enter near the root apex and pass up through the periodontal ligament; others enter the middle and cervical portions of the ligament as finer branches through openings in the alveolar walls.
Periodontal nerve fibres are both myelinated and unmyelinated:
• The myelinated fibres are on average about 5 μm in diameter (although some are as large as 15 μm) and are sensory fibres only.
• The unmyelinated fibres are about 0.5 μm in diameter and are both sensory and autonomic.
At the light microscope level, a plethora of forms that are assumed to represent nerve endings have been described within the periodontal ligament. These forms vary from simple free endings to more elaborate arborizing structures, although they still only mediate two sensory modalities — pain or pressure.
Most attention has been paid to the periodontal mechanoreceptors (the detection of mechanical stimuli being the modality of mechanoreception). There are a number of different morphological types of mechanoreceptor nerve ending, each responding to a different type of mechanical stimulus. Mechanoreceptors in and around the mouth perform a major role in the transmission of touch and textural information when eating. The sensations of texture, such as smoothness, crunchiness, crispiness and chewiness are all transduced by mechanoreceptors situated on the tongue and oral mucosa and within the periodontal ligament. Mechanoreceptors also provide afferent feedback essential in the control of mastication, swallowing and salivation. Much of the work on mechanoreceptors has been carried out on hairy and non-hairy skin and the adjacent subcutaneous connective tissues, and comparatively little has been carried out on the oral tissues. Nevertheless, the literature reveals many morphological and functional similarities between mechanoreceptors found in the mouth and those found in non-hairy skin and related connective tissues.
Like cutaneous mechanoreceptors, oral mechanoreceptors may be classified on the basis of their response properties into two main categories: slowly adapting (SA) types and rapidly adapting (RA) types. These two categories are associated with the sensations of touch-pressure (SA types) and flutter-vibration (RA types). Slowly adapting receptors discharge repetitively for as long as the stimulus is maintained and rapidly adapting receptors discharge only when the stimulus is applied (and sometimes when it is removed). This selectivity of the mechanoreceptors to different types of stimuli depends mainly on their structure. Anatomically, the structures that mediate mechanosensation in the mouth include Merkel cell complexes, Ruffini type endings, Meissner endings and possibly Pacinian corpuscles:
• The Merkel cell complex and Ruffini type endings are associated with slowly adapting responses and in the cutaneous tissues have been designated slowly adapting type I (SAI) and slowly adapting type II (SAII) receptors respectively.
• The Meissner endings are associated with rapidly adapting responses and, in the glabrous cutaneous tissues, have been designated rapidly adapting type I (RAI) receptors.
• Pacinian corpuscles appear to be rare in oral tissues; however, in cutaneous tissues these receptors are associated with very rapidly adapting responses and have been designated rapidly adapting type II (RAII) receptors.
Neurones involved in mechanoreception are usually myelinated fibres of the Aβ type with axons ranging between 5 and 14 μm in diameter which conduct impulses at a velocity of 30–70 m sec −1. If these receptors are classified according to their functions, they can be divided into:
• intensity and duration detectors (SA types)
• velocity detectors (RA and SA types)
• acceleration and vibration detectors (RA types).
Although each of the types of receptor has a unique morphological and physiological characteristic, any mechanical stimulus applied to the oral cavity could simultaneously excite some or all of these receptor types to varying degrees. The few studies that have been carried out on the mechanoreceptors within the mouth lead us to believe that these receptors do not have any particular properties that are unique to their location in the oral tissues. Even those within the unique periodontal tissues appear to be typical Ruffini-like, slowly adapting receptors.
SAI receptors signal information about the intensity and duration of the mechanical stimulus applied to the tissues in which they are located. Because of both their dynamic and static sensitivities they are also able to signal the velocity of the applied stimulus. They respond with an irregular frequency of firing and adapt slowly to a sustained force. These very slowly adapting receptors are associated with Merkel cell neurite complexes, they respond to perpendicular ramp-and-hold-type displacements of the tissues as low as 15 μm or constant force-type stimuli of as low as 1 mg. They signal an initial velocity-dependent nervous discharge followed by a slowly adapting discharge of action potentials in the afferent nerve. They do not, however, respond to horizontal stretching of the tissues. Their frequency of firing can be greater than 1 kHz when excited by their optimal surface pressure. They have been found in the oral mucosa, the epithelium of the fungiform papillae of the tongue, hard palate, incisive papilla, attached gingivae, and unattached oral mucosa. SAI-type responses have been recorded from mucous membranes of the tongue, lips and cheeks of human beings.
SAII receptors are slowly adapting receptors that signal information about the intensity and direction of mechanical stimuli to the tissues in which they are located, as well as the velocity of the applied stimulus. Unlike SAI-type receptors, they respond with a regular frequency of firing but, in common with SAI types, they also display both dynamic and static sensitivities. These low-threshold, slowly adapting receptors have been identified as Ruffini-like endings. They not only respond to perpendicular low-force indentations of the tissues (threshold approximately 15 μm), but can also be stimulated by stretch of the tissues in which they lie. There is generally one low-threshold spot for each fibre innervating the SAII receptor. They can follow vibration stimuli of up to 400 Hz. The Ruffini-like endings are sometimes encapsulated by fibroblast or perineural cells; however, the presence of a capsule depends on the structure of the surrounding tissues. Those in the skin, and particularly those associated with hairs, mucosa and some joints, appear to be encapsulated, whilst those in the periodontium lack a capsule. The presence or lack of a capsule appears to have no effect on the function but may be associated with whether or not the connective tissue in which they lie is already aligned or not. Ruffini-like endings have been found in the oral tissues, including hard palate, incisive papillae, periodontal ligament and oral mucosa.
Periodontal ligament mechanoreceptors (Ruffini-like SAII receptors)
Concerning periodontal ligament mechanoreceptors, when a horizontal force is applied to the tooth crown, the whole tooth rotates about a fulcrum and mechanoreceptors, which lie in the periodontal ligament, are stimulated. If recordings are made from afferent nerve fibres from these mechanoreceptors, the response properties range from very rapidly adapting to very slowly adapting. Many studies have attributed the response properties to many of the morphological types of receptor described in other tissues. However, there is now compelling evidence that there is not a range of receptor types within the periodontal ligament, but that there is only one single type, whose response characteristics depend on its position in the ligament as well as on the rate, magnitude and direction in which the stimulating force is applied to the tooth. Rapidly adapting responses are seen from receptors found close to the fulcrum of the tooth, and slowly adapting responses are seen from receptors close to the apex of the tooth. There appears to be a grading of adapting properties for receptors lying between these two extreme locations. Furthermore, the thresholds of the receptors also appear to be related to their position in the ligament, with those found close to the fulcrum of the tooth having higher thresholds and those found closer to the apex having very low thresholds, again with a graded force threshold between the two sites. When morphological studies have been made on receptors that have been located in relation to their position relative to the fulcrum and apex of the tooth, only Ruffini-like endings have been identified. The characteristics of these endings were that they were branched, unencapsulated and incompletely surrounded by terminal Schwann cells with extensions projecting towards the collagen bundles. The Ruffini-like ending is considered to be the primary mechanoreceptors in the periodontal ligament. The physiological response characteristics of the periodontal ligament mechanoreceptors are consistent with them being type II slowly adapting (SAII) receptors. Given optimal stimulation, the receptors located at the apex of the tooth exhibit typical slowly adapting type II dynamic and static responses with a regular discharge of impulses. On the other hand, the receptors located close to the fulcrum exhibit responses typical of a slowly adapting receptor that is receiving a stimulus that is just above threshold, and they give what appears to be a rapidly adapting response. The lower thresholds exhibited by those closer to the apex can also be explained by the degree of stimulation being greater at the apex than at the fulcrum of the tooth. Periodontal ligament mechanoreceptors respond maximally when the area in which they lie is put into tension (i.e. stretch). They exhi/>