Tooth whitening enamel layer may also be gradually thinned or even perforated by the various forms of tooth wear. Teeth may be darkened by a buildup of surface stains (extrinsic staining), which hides the natural tooth color; or the tooth itself may discolor (intrinsic staining). Extrinsic stains can become internalised through enamel defects or cracks or as a result of dentine becoming exposed but most extrinsic stains appear to be deposited on or in the dental pellicle. Causes of extrinsic staining include: Changes in the thickness of the dental hard tissues would result in intrinsic discolouration. There are a few causal factors
Human tooth development the canine area. The other dominant hypothesis, the "clone model", proposes that the epithelium programs a group of ectomesenchymal cells to generate teeth of particular shapes. This group of cells, called a clone, coaxes the dental lamina into tooth development, causing a tooth bud to form. Growth of the dental lamina continues in an area called the "progress zone". Once the progress zone travels a certain distance from the first tooth bud, a second tooth bud will start to develop. These two models are not necessarily mutually exclusive, nor does widely accepted dental science consider them to be so: it