Dentinogenesis is the processes of formation of dentin. And its formation is carried out by cells called Odontoblasts. Dentin is the calcified tissue which forms the bulk of tooth, whereas the odontoblasts are the present in pulp and there processes extend through the thickness of dentin called odontoblastic processes. If we take a look at the chronology of teeth, formation of primary dentin begins just before the beginning of enamel formation, then it continues even after tooth eruption and ends with root completion. However, now the secondary dentin formation begins, which continues during the whole life span of the tooth and ends only when the tooth dies. Differentiation of Odontoblasts. The dentin of a tooth can be divided into two part, dentin in the crown called coronal dentin and dentin in the root the root dentin. Let us first see how odontoblasts of the coronal dentin are formed. During tooth development, in the enamel organ, the cells of the inner enamel epithelium are separated from the cells of the dental papilla by a 'cell free zone'. The cells of the inner enamel epithelium then transform into Ameloblast. During the life cycle of Amelobalsts, as the cells enter the Organizing stage they become tall columnar. This eliminates the cell free zone and the Ameloblasts now come in direct contact with the cells of the dental papilla. The Ameloblasts then release signalling molecules and growth factors because of which the undifferentiated cells of the dental papilla, differentiate in to Odontoblasts. Odontoblasts are tall columnar cells, around 7 microns in diameter, 40 microns in length and with nucleus positioned towards the dental papilla. Now the differentiation of root dentin odontoblasts. This also occurs during tooth development but at a later stage after the formation of Hertwigs Epithelial Root Sheath. Hertwigs epithelial root sheath is a bilaminar layer made of inner enamel epithelial cells and outer enamel epithelial cells. It is this sheath which releases signalling molecules and growth factors to the undifferentiated cells of the dental papilla and differentiate them in to Odontoblasts. Formation of odntoblastic processes. This is a unique feature of Dentinogensis. As Odontoblasts deposit dentin and move backwards they leave an extension of cells in the forming dentin. As cells continue moving backwards these extensions continue growing in length. These cell extensions are odontoblastic processes, which extend from odontoblasts to the dentino enamel junction. So, till now we have studied about the formation of odontoblasts and formation of odontoblastic processes. let us now study about the formation of the calcified part of dentin. Dentinogenesis occurs in two steps. Step 1 is formation of organic matrix. Step 2 is mineralization. First the formation of organic matrix. The organic matrix is made of proteins, of which collagen forms 90%. In this collagen, majority is formed by type 1 collagen and small amount by type 3 collagen and type 5 collagen. In non collagenous proteins there are, denitn phosphoprotein (DPP), dentin sialoprotein (DSP), dentin glycoprotein (DGP), dentin matrix protein (DMP-1). The odontoblasts, release all these proteins into the stroma. A scaffold is formed by collagen type 1 which accommodates large portion of minerals. Whereas, The non collagenous proteins, regulate this mineral deposition. They may act as an inhibitor or promoter of mineralization. Predentin. As odontoblasts start formation of dentin, the first layer that is deposited, the layer nearest to the odontoblast is made only of organic matter, called PREDENTIN. It has no mineral deposition as yet. It is similar to the organic layers seen in other mineralized tissues. So, as Dentinogensis goes on, this predentin layer will always be present. No matter if its the primary dentin formation, secondary dentin formation or tertiary dentin formation. The zone from which mineralizaton starts, is called the mineralization front. It is because of the fact that, the mineralization always lags behind organic matter formation. Mineralization The main inorganic portion of dentin is formed by the hydroxyapetite crystals. They are about 3nm wide and 100nm long. These crystals are larger than the ones present in bone but way smaller than the ones in enamel. Mineralization mainly occurs by matrix vesicles. In which, Odontoblasts release membrane bound vescicles, into the organic matrix, they are 25nm to 250 nm insize,. They contain hydroxyapetite crystals, calcium ions phospholipids and many such chemicals which initiate and promote mineralization. There are 2 patterns of mineralization- Globular or Calcoshperic pattern and Linear pattern. Globular pattern is mostly seen in in coronal dentin, near mantle dentin. Linear pattern, more in root dentin and near the pulp. The rate of mineralization is faster in globular pattern than in linear pattern. In Globular mineralization the Odontoblasts deposit the hydroxyapatite crystal in several areas of collagen matrix. These crystals then grow centrifugally with deposition of more minerals. Eventually these enlarging globules fuse to form a single mineralized mass. In case, there is hinderance in the fusion of these globules, the produced hypocalcified areas are called interglobular dentin. In linear mineralization the crystals get deposited along the collagen fibers. These crystals fuse together early, at a smaller size, and so make the mineralization front appear in a straight line This was, the process of dentinogenesis in primary dentin which is same in secondary dentin and tertiary dentine, the difference is only in the rate of dentinogenesis. Secondary dentin formation is much slower compared to primary dentin deposition. However Rate of Tertiary dentin formation is inconsistently rapid due to which irregularities are commonly seen in its structure..