ABSTRACT
Only over a narrow range of dosesnot so high as to cause embryolethality and not so low as to have no effectteratogens have deleterious effects on histogenesis or organogenesis. Examples include: agenesis, the complete absence of an organ (e.g., unilateral kidney agenesis); hypoplasia, reduced size of all or part of an organ (e.g., microcephaly); dysraphic anomalies, failure of apposed structures to fuse (e.g., spina bifida); division failures (e.g., syndactyly, the fusion of fingers); atresia, incomplete formation of a lumen (e.g., esophageal atresia); ectopia, presence of an organ outside its normal site (e.g., ectopic heart, with the heart outside the thoracic cavity); and developmental syndromes that involve multiple, but related, anomalies (e.g., fetal alcohol syndrome).
