The defective or under-development of an organ or tissue such as the underdevelopment of parts of a tooth (dental hypoplasia). Enamel hypoplasia is a defect in either matrix formation or mineralization of enamel during amelogenesis. It may be localized to one tooth or generalized to affect several or all teeth, the distribution being related to the extent of tooth formation at the time of the disturbance (chronological hypoplasia). Localized hypoplasia usually affects the upper central incisors as a result of infection or trauma to the primary predecessor. Hypoplastic enamel is characterized as yellow-brown, pitted, or irregular. It may also appear as isolated white opacities (white spots), particularly on the upper central incisors. Chronological hypoplasia is largely as a result of systemic illnesses such as measles, mumps, and viral infections, resulting in a linear horizontal band of ridged or pitted enamel corresponding to the period of the infection. This mostly occurs during the first few months of life and is therefore most commonly seen affecting the permanent first molars, upper central incisors, lower lateral incisors, and canines. In the prenatal period, diseases affecting the mother can be significant. See also neonatal.