Imprinting and critical periods in early development

The review addresses the fundamental process of 'imprinting'. In his classical studies on newly hatched goslings Konrad Lorenz analysed the development of social binding and established the term 'imprinting' to describe this process. One of his major ideas was that imprinting occurs in 'critical periods', which are limited and severely restricted to the animal's very early life. For some time past, the term 'imprinting' is also used for an epigenetic mechanism, the 'genomic imprinting', which can be simply defined as gamete-of-origin dependent modification of genotype. Furthermore, in the course of the perinatal period 'imprinting' of physiological control systems occurs. Functional systems of the organism develop from open loop systems without feedback control into closed systems controlled by feedback mechanism. During 'critical periods', the actual environment influences the development of the respective physiological control systems for the entire life period, especially by changes in neuroorganization and expression of related effector genes. On the one hand, these mechanisms may cause perinatal malprogramming, which has been related to, e.g., metabolic disorders and cardiovascular diseases during later life in humans as well as in animals. On the other hand, knowledge on these mechanisms might be specifically used to induce long-term adaptation of the organism, for instance, to the postnatal climatic conditions (epigenetic temperature adaptation). Furthermore, the question if 'critical period' and 'sensitive period' are synonymous or different, and problems of identifying these developmental windows are discussed. Environmental manipulation of immature physiological mechanisms may be a physiological tool for characterization of 'critical periods'.