Embryo and Fetus

Abstract In the first half of the 20th century, congenital malformations were believed to arise in nature from genetic or infectious causes. The thalidomide tragedy of the late 1950s and early 1960s showed that assumption to be incorrect, and as such sensitized both the biomedical community and the public to the potential hazards of exposing the developing conceptus to foreign substances. In retrospect, realization that xenobiotic exposures and chemical imbalances were capable of affecting embryonic and fetal growth should not have been surprising to members of the medical, veterinary, and general scientific communities. Epidemiological data indicate that in utero exposure to many xenobiotics can result in structural defects and/or functional deficits within progeny. Humans and animals are exposed to numerous agents (e.g., drugs, environmental contaminants, metabolic by products) during critical periods of development, and at least some of these agents have the capacity to act as teratogens. Accordingly, a more thorough grasp of the processes that drive both normal and abnormal development has assumed increasing importance. This chapter will describe normal developmental events in selected vertebrate species, detail methods for determining if and how much such processes may have been disrupted by teratogens, and describe common mechanisms of maldevelopment along with factors that may influence the extent and severity of such damage.