Dentine

Dentine

Gray's Anatomy

Dentine is yellowish avascular tissue forming the bulk of the tooth. It is a tough and compliant composite material, about 70% by weight mineral and 29% organic matrix. Its conspicuous feature is the regular pattern of microscopic dentinal tubules, about 1-2 μm in diameter, extending from the pulpal surface to the enamel-dentine junction. Tubules have a single sinuous primary curvature oriented apically and more pronounced in the crown. A spiral secondary curvature, less regular, has a periodicity and amplitude of a few microns.

Near the enamel-dentine junction tubules bifurcate, some with short extensions into enamel. Abundant lateral branches interconnect adjacent tubules. Each tubule encloses a single cytoplasmic process of an od 22222j924w ontoblast, containing microtubules, microfilaments but few ribosomes or mitochondria.

Odontoblast cell bodies are in a pseudostratified layer lining the pulpal surface.

In newly erupted teeth, processes are believed to extend the full thickness of dentine but in older teeth may be partly withdrawn so as to occupy only the pulpal third, the outer regions containing extracellular fluid. Lining most tubules is a heavily mineralized cylinder of peritubular dentine, devoid of collagen fibres, separated from the plasma membrane of the process by a glicosaminoglycan-rich lamina limitans. It is uncertain weather the process directly abuts the lamina limitans or weather there is a fluid-filled periodontoblastic space separating them.

Between the odontoblasts and the dentine is a layer of non-mineralized matrix, the predentine. The predentine-dentine border is irregularly scalloped because dentine mineralizes as microscopic spherical aggregates of crystals. The enamel dentine junction is more regularly scalloped, with convexities towards the dentine, a pattern unrelated to mineralization. Next to enamel-dentine junction is a 30-40 μm layer (mantle dentine) which is less mineralized and has collagen fibres arranged parallel to the tubules. In the remaining circumpulpal dentine, fine collagen fibres are perpendicular to and interwoven around the tubule.

Dentine like enamel is deposited incrementally and is not remodeled. Both tissues carry a permanent record of changing shape, rhythmical formation and disturbances during development. Fine incremental lines of von Ebner record diurnal alteration in the orientation of collagen fibres and enter by the apical foramen to run longitudinally, giving branches to an extensive subodontoblastic plexus. Capillary loops may also occur in the odontoblast layer. Blood flow in terms of rate per unit volume, is grater in the pulp than in other oral tissues.

Micropuncture measurements of interstitial fluid pressure indicate that it is much lower than hitherto supposed. Several small veins and lymphatic vessels emerge from the pulp.

Unmyelinated postganglionic sympathetic nerve fibres from the superior cervical ganglion enter the pulp with the arterioles. Myelinated and unmylinated sensory nerve fibres from the trigeminal ganglion traverse the pulp longitudinally giving branches to ramify in the plexus of Raschkow in the cell-rich parietal zone. Here fibres lose their myelin sheaths and continue into the odontoblast layer, some entering the dentinal tubules. Intratubular nerves are distinguishable from odontoblast processes because the former contain many mitochondria; they are more numerous beneath the cusps (where one in four tubules is occupied) than elsewhere.

Ultrastructural studies have failed to show nerve fibres beyond 100 μm into human dentine but autoradiography of rat's teeth following injection of special substances into the trigeminal ganglion has revealed innervation in dentinal tubule near the enamel dentine junction.

Stimulation of dentine weather by thermal, mechanical or osmotic means evokes a pain response. The mechanism of stimulus transduction is unknown but is unlike to involve a direct stimulation of nerve endings in dentine. Newly erupted teeth are sensitive yet do not have a plexus of Rashkov, although in contrast to earlier studies, some nerve fibres have been found in dentinal tubules before tooth eruption. Pain-producing chemicals and local anaesthetics show little ability to stimulate or anaesthetize exposed dentine. One possibility is that the odontoblasat process can propagate some kind of impulse and excite nerve endings in contact with the proximal part of the process or the cell body. But neither synapses nor gap junction have been definitely identified between nerves and odontoblasts; although their cell membranes occasionally come into close approximation, the nature and functional significance of such junction are unknown.

An alternative

hypothesis suggests that stimuli generate movement of intracellular fluid or extracellular fluid along the dentinal tubules, causing in turn a local distortion of the pulp, sensed by free nerve endings in the plexus. Evidence that odontoblasts are joined together by continuous tight junctions suggests that the odontoblasts may be directly involved in relaying intratubular fluid movements to nerve endings. This "hydrodynamic" theory would explain the ineffectiveness of neuroactive agents and why pain is produced by drying and by solutions of high osmotic pressure. Solution equally effective in producing pain, however, create very different rates of flow.

Enamel

Gray's Anatomy

Enamel is an extremely hard and rigid material covering the crown of teeth. It is a heavily mineralized cell secretion containing 95-96% by weight crystalline apatites and less than 1% organic matrix. Since its formative cells are lost from the surface during eruption, it is incapable of further growth; repair is limited to the remineralization of minute incipient carious lesions. It reaches a maximum thickness of 2.5mm cusps and thins to knife edge at the cervical margines.Enamel is composed of closely packed enamel prisms(or rods), U-shaped in cross section, extending from close to the enamel-dentine junction to within 6-12 μm of the surface. Each prism is delineated by a matrix-rich prism sheath and is separated from neighbouring prisms by a continuous interprismatic region.

Prisms are about 3-4 μm wide in inner enamel, increasing 6 μm near the surface. Prisms are packed with flattened hexagonal crystallites. Hexagonal transverse profiles of ribbon-like crystallites are randomly oriented. In the cuspal region of a prism these are almost parallel to the prism's long axis and may be as long as the enamel is thick; but in the cervical region of a prism and in ineterprismatic regions, the crystallites have a pronounced cervical inclination and end at the cervically-adjacent prism sheath. A sudden change always exists between crystallite orientations on the two sides of a prism sheath. In surface enamel crystallites are packed with their long axes parallel so that prism sheaths do not form.

At intervals of about 4 μm along its length, each prism is crossed by a dark striation the light microscope manifestation of a rhythmic swelling and shrinking of prism diameter during one day's growth. Higher order incremental lines in enamel are striae of Retizius, passing from the enamel-dentine junction to the surface where they end in shallow furrows visible on newly erupted teeth. Each stria represents a period of 7-8 days' enamel growth.

Striae are produced by a sudden double right-angle translocation of the prisms in the longitudinal plane and may be clear or brown in transmitted light.

A prominent stria, the neonatal line, is formed in teeth whose mineralization spans birth. Neonatal line in enamel and dentine are of forensic importance, indicating that an infant has survived for a few days.

Each prism is sinuous in the tooth's transverse plane with a wave length of about 1.5mm, undulations of one prism being match by those lateral to it but slightly out of phase with those above or below. Prism sheaths are comparatively weak interfaces in enamel.

Decussation of prisms in the tooth's longitudinal plane is an adaptation which increases the toughness of enamel by enlarging the surface area of potential cracks between prisms in that plane. Similar regular undulations over cusps produce the appearance of gnarled enamel in section.

Prisms sheaths in the inner enamel are considerably thickened to form tuft-like projections from the enamel-dentine junction, extending from a considerable distance in the longitudinal plane of the tooth. Longitudinal sheaths of organic material penetrating the full thickness of enamel are enamel lamellae. Extensions into the enamel of dentinal tubules are enamel spindles, prominent over cusps.