Dental pulp of a tooth is the centrally placed soft connective tissue which is surrounded by dentin from all sides. It can be divided into two parts, the part within the crown, coronal pulp also called the pulp chamber and the second part which lies in the root called radicular pulp also called the pulp canals. It communicates with the outer tissues by apical foramen. Development. Dental pulp develops from dental papilla. During the stages of tooth development, in the enamel organ, once the inner enamel epithelial cells fully differentiate into Ameloblasts, they send signals to the cells of the dental papilla to differentiate into odontoblasts. As odontoblasts deposit dentin and move backwards, the part of the dental papilla which remains unmineralized forms the dental pulp. Histologically, the dental pulp can be divided into 4 distinct zones. Odontoblastic zone, Cell free zone, Cell rich zone and Pulp core. Now if we take a look at all these zones in the pulp, the first three zones, that is, odotoblastic zone, cell free zone and cell rich zone, are present at the periphery of the pulp. These three layers collectively form the 'Odontogenic zone of the pulp', whereas the major central portion of the pulp is formed by the pulp core. Now let us start with the first zone, odontoblastic zone. Odontoblastic zone contains only a single layer of odontoblasts at the periphery of the pulp just adjacent to the dentin. Though it may appear pseudostratified, due to crowding in the pulp. Odontoblasts are the most distinctive cells and are the second most common cells. In number they are about 60,000 to 75,000 per square mm in coronal pulp and lesser in radicular pulp. Fully differentiated odontoblasts in coronal pulp are columnar in shape, in mid portion of root they are cuboidal in shape and in apical part they are spindle shaped. This change in size can be attributed to their functional activity. If we take a look at them closely, each odontoblast has a cytoplasmic process extending into the dental tubule called odontoblastic process. The cell body is approximately 50 microns long and 5 to 10 microns in width. On looking inside the cell, we see the nucleus is spherical and located at the base or towards the pulp. Adjacent to the nucleus are most of the cell organelles, including golgi bodies and rough endoplasmic reticulum. The apical part is devoid of any organelles. Though during dentinogenesis, it is filled with secretory granules. Odontoblasts are fully differentiated cells and therefore cannot divide. Numerous cell junctions are seen in odontoblasts. Desmosomes, which provide mechanical union to the cells. Tight junctions, which form permeability barrier between the pulp and dental tubules and gap junctions which help in communication between odontoblasts. Next zone, cell-free zone also called zone of Weil or sub-odontoblastic zone. In the dental pulp, this zone lies just beneath the odontoblastic zone. Although this zone is seen only in coronal pulp and not in radicular pulp. As the name suggests, in this zone, no cells are present. The next zone in the dental pulp is cell rich zone. Cell rich zone is the zone next to cell free zone. This zone has a high density of cells. On looking closely, there are mainly two cell populations. First, fibroblasts which are ubiquitous cells of unmineralized connective tissue. They may be spindle shaped or star shaped. They are the most numerous cells in the pulp and their function is to produce extracellular fibers. Second, undifferentiated mesenchymal cells. There is a population of cells which are undifferentiated, pluripotential, primitive, mesenchymal cells capable of differentiating into variety of cells. They are larger than fibroblasts and are spindle or polyhedral in shape with peripheral processes and oval nuclei. These cells, when required may differentiate into odontoblasts or fibroblasts or macrophages. What chemical stimulus exactly differentiates these cells into odotoblasts, is still a topic of research. As during tooth development, the cells of the dental papilla get signals from the ameloblasts, after which they differentiate into odontoblasts. But now that in pulp the ameloblasts are gone, which cell, sends what signal to this differentiation to happen, is still debatable. The final zone is the pulp core. Pulp core is the biggest zone and forms the center of the pulp chamber and pulp canals. It has numerous cells, but not as densely packed as in the cell rich zone. Similar to cell rich zone, fibroblasts and undifferentiated mesenchymal cells are present in pulp core. Additionally, it has many defense cells cells like macrophages, dendritic cells, mast cells, neutrophils, eosinophils, basophils, lymphocytes and monocytes. During inflammation their number increases. Apart from these, variety of cells. We see a network of blood vessels, lymphatic vessels and nerves in the pulp core. They all enter and exit through the apical foramen. Now if we go to the pulp core and take a closer look we see that the blood vessels form an extensive capillary network and their branches pass into the peripheral layers, reaching up to the odontoblasts. Coming to the nerves. The dental pulp is heavily innervated. Majority of nerves are non-myelinated. These nerves form a plexus just beneath the cell rich zone, called the parietal layer of nerves or plexus of rashkow or the subodontoblastic plexus. Nerves from this plexus cross the cell rich zone, cell free zone and extend up to the odontoblastic zone and may enter dental tubules. According to the Hydrodynamic theory, it is these nerves which cause dentine sensitivity.