Ameloblasts are cells present only during tooth development that deposit tooth enamel, which is the hard outermost layer of the tooth forming the surface of the crown. Structure Each ameloblast is a columnar cell approximately 4 micrometers in diameter, 40 micrometers in length and is hexagonal in cross section. The secretory end of the ameloblast ends in a six-sided pyramid-like projection known as the Tomes' process. The angulation of the Tomes' process is significant in the orientation of enamel rods, the basic unit of tooth enamel. Distal terminal bars are junctional complexes that separate the Tomes' processes from ameloblast proper. Development Ameloblasts are derived from oral epithelium tissue of ectodermal origin. Their differentiation from preameloblasts (whose origin is from inner enamel epithelium) is a result of signaling from the ectomesenchymal cells of the dental papilla. Initially the preameloblasts will differentiate into presecretory ameloblasts and then into secretory ameloblasts which lay down the tooth enamel. The differentiation from preameloblasts to ameloblasts occurs during the first stage of amelogenesis, called the pre-secretory (or inductive) phase. The ameloblasts will only become fully functional after the first layer of dentin (predentin) has been formed by odontoblasts. The cells are part of the reduced enamel epithelium after enamel maturation and then subsequently undergo apoptosis before or after tooth eruption. These stages occur during the third and final stage of amelogenesis, called the maturation phase. There are various factors which can affect the differentiation and development of ameloblasts, causing abnormalities to form within the tooth structure. One example is the BMP (bone morphogenetic protein,) which has an important role in ameloblast differentiation. When Follistatin, a BMP inhibitor, is over expressed in the epithelium of developing teeth, the ameloblasts do not differentiate and no enamel forms. Another example includes the conditional deletion of Dicer-1 in the epithelium of developing teeth may cause impaired differentiation of ameloblasts which results in deficient enamel formation. Life cycle The life cycle of ameloblasts consists of six stages: Morphogenic stage Organizing stage Formative (secretory) stage (Tomes' processes appear) Maturative stages Protective stage Desmolytic stage The murine ALC (ameloblast like cell) cell line is of ameloblastic origin. 1. Morphogenic Stage In this morphogenic stage the morphology of the cell is short, columnar with large oval nuclei. The golgi apparatus and centrioles are located in the proximal end of the ameloblast and mitochondria are dispersed throughout the cytoplasm. 2. Organizing Stage In this stage