Teratology on the crossroads: historical aspects and modern approaches.

Teratology is the science of congenital developmental disorders (CDDs), overt or latent defects of the organism resulting from the effect of internal and external factors on developmental processes. In this article the significance and position of present-day teratology is discussed in the context of development of this branch of science and related disciplines. The authors present an updated overview of the most important milestones and stages of the development of teratology. Based on the analysis of the historical development of theses and theories that represent a decisive contribution to this field, we present a survey of the fundamental principles of experimental and clinical teratology. The aim of observing these principles is to get insight into developmental relations and to understand mechanisms of action on the level of cell populations (elementary morphogenetic processes), tissues and organs. It is important to realize that any negative intervention into the normal course of these processes, either on genetic or non-genetic basis, inevitably leads to a sequence of subsequent changes resulting in the development of congenital developmental disorders. Despite modern approaches of molecular biology and genetics, along with top diagnostic techniques, we are still not able to identify the actual cause in more than 50% of all congenital defects. One-half of the unidentified cases are referred to as "multifactorial", a term that is rather ambiguous. It either means that some of the basic principles of teratogenesis still escape our attention, or the interpretation of some of the well known principles might be misleading. A third possibility is rather pessimistic. The development of the individual is so sophisticated and dependent on a delicate network of a multitude of factors mutually affecting each other that it is extremely prone to give rise to a plethora of spontaneous errors which are unpredictable and impossible to prevent. Nevertheless, the long and complicated history of scientific endeavour has yielded considerable present-day knowledge on causes and mechanisms of CDDs, a history whose beginnings date back to antiquity.