Comparative cardiovascular development: improving the conceptual framework.

Immature vertebrates-either as an embryo in an egg, as free-living larva, or as an in utero fetus, are clearly not just small versions of adults. Their cardiovascular physiology (and doubtlessly other aspects of physiology) differs from that of adults both qualitatively and quantitatively. Yet, comparative cardiovascular physiologists have been relatively conservative in constructing a new (or at least modified) conceptual framework for the understanding of developmental cardiovascular physiology. We recommend that this framework rely less on the established cardiovascular truisms for adult cardiovascular physiology that are proving to be less useful and in instances even inaccurate for interpreting development of the heart and vasculature. We have suggested that three methodologies in particular be incorporated to a greater extent in studies of comparative cardiovascular development: (a) emphasis on multivariate approaches; (b) differentiation between absolute (extrinsic) and relative (intrinsic) time for development, and; (c) employment of time lines for both intra- and interspecific comparisons of the ontogeny of cardiovascular processes. While certainly none of these approaches are novel and others have previously dwelt at length on their importance in other contexts, we feel that the emerging framework for investigating cardiovascular physiological development would benefit from incorporating these and other approaches into experimental design as well as data analysis. Failing to do so results in a heavy dependence on analytical approaches typically used for adults, and thus under-appreciates the novelty and complexity of the developing vertebrate cardiovascular system.