Aplasia: - Failure of organ to develop with only rudiment of organ present. It occurs, when precursor cells are there but they do not differentiate into the organ.  Agenesis: - Complete failure of organ to develop, i.e. e complete absence of the organ, with no rudiment. It occurs when there is not precursor cell of an organ.  Atresia: - Ducts in the body or their openings or any orifice or passage is abnormally closed or absent.  Atrophy: - Shrinkage in the size of an organ by a decrease in cell size and number.