Dentin is the calcified tissue which forms the bulk of the tooth and has pulp in its center. In the crown it is surrounded by enamel and in the root it is surrounded by cementum. It is formed by odontoblasts which are present in the pulp, but have odontoblastic processes running through the thickness of dentin. This morphology divides the dentin into few parts. Dentinal tubules which contains the odontobnlastic processes, peritubular dentin which makes the wall of the dentinal tubules, and inter tubular dentin, which forms the bulk of dentin between the tubules. Lets us start with dentinal tubule. Dentinal tubules are hollow tubules or canaliculi which run from the pulp where the odontoblasts are present to the dentinoenamel junction in crown or to the cementodentinal junction in the root. Shape of dentinal tubules – dentinal tubules are not straight but have a gentle s shape, or a sigmoid curve in their course, that is there are 2 curvatures in opposite directions. These 2 curvatures are called as primary curvatures. The curvature closer to pulp has its convexity towards the root apex, whereas the curvature away form the pulp has its convexity towards the occlusal or incisal surface. Why this s-shape? For that we should understand that each tubule actually shows the path followed by the odontoblast during the formation of dentin. So the s-shaped curvature is because of the crowding of this path as the odontoblasts move from outside, which has more area, to the pulp, which has less area. This s-shape curvature is more pronounced in the crown as the difference in the area is more whereas it is less in root where the difference is less. Moreover, there are two areas where dentinal tubules don’t show this curvature but run in a straight line- 1st is the cusp or the incisal ridge area. 2nd is the root apex. Apart from the 2 primary curvatures minute curvatures are present all along the course of the dentinal tubule called the secondary curvatures which can be well appreciated at higher magnification. Contour lines of Owen are lines formed when secondary curvature of adjacent tubules coincide together. All dentinal tubule in any part of the dentin, ends on both surfaces at 90 degrees, be it the pulpal end or the outer ends at dentino-enamel junction or cemento-dentinal junction. Dentinal tubules have branches all along its course called the lateral branches and at the outer end called the Terminal branches. Now let us seen the dimensions of dentinal tubules. the length of each tubules depends on the thickness of dentin. Moreover they are longer than that because of the primary and secondary curvatures. The diameter of the dentinal tubules is not constant they are wider at the pulpal end about 4 microns and narrower at the external end about 1 micron. Giving a ratio of about 1:4. Let us see what are the contents of dentinal tubules. Odontoblast process. Odontoblasts are located in the pulp just beneath dentin. Their cytoplasmic processes extends from the odontoblasts into the dentinal tubules called as odontoblastic process. They lack any major organells like mitochondria or endoplasmic reticulum but have abundance of tubules and filaments. These processes help in maintaining the dentin. The extent of odontoblastic process present in the tubules is still a controversial topic. Though sometimes odontoblastic processes may cross the dentinoenamel junction and reach enamel where they are called as enamel spindles. Dentinal fluid. Is a fluid present in the peri-odontoblastic space, located between the odontoblastic process and the wall of dentinal tubule. This fluid is formed by the transudate of plasma from the blood vessels in pulp. The movement of this dentinal fluid is thought to be the main reason of dentin sensitivity. Lamina limitans. Literally meaning the limiting membrane is an organic layer present on the inner side of the dentinal tubule.. It is rich of glycosaminoglycans. Now coming to Peritubular dentin. As the name suggests it is the dentine at the periphery of the dentinal tubule. It forms the wall of the dentinal tubule. Peri-tubular dentin has very less amount of collagen in its organic matrix but has abundant non collagenous proteins. It is more mineralized than intertubular dentin. The thickness of peritubular dentin increases as we move from pulp end to outwards. This is the reason why the dentinal tubules are narrower at the outer end than the pulpal end. Since the deposition continues inside the tubule a more appropriate name would be intratubular dentin but not a popular one. With age dentinal tubules become completely occluded with continuous deposition of peritubular dentin, This is also called as sclerotic dentin. It occurs mostly at apical third of root. This changes the refractive index of dentin and thus also called as the transparent dentin or translucent dentin. Finally the Inter-tubular dentin. Inter tubular dentin forms the main bulk of dentin, or it is the dentin between the dentinal tubules. its organic matrix is rich in type I collagen fibers which run at 90 degree to the dentinal tubules. It is less mineralized than peritubular dentin.