Development of Mammalian Circadian Rhythms

Abstract

The goal of developmental biology is to understand the origins of biological organization as it unfolds within each generation. Investigations of development often begin with describing how normal development proceeds, then ask how the observed processes or events are regulated. At the level of the whole organism, the most fundamental of such questions (even if oversimplified) is to ask whether the behavior, structure, or physiologic process of interest is inborn or is in some way shaped by the external environment under which the organism develops. This was one of the earliest questions to be asked about circadian rhythms. Charles Darwin contested the view he attributed to Wilhelm Pfeffer that the persistent rhythms of leaf movements in plants under constant conditions should be attributed to “ ‘Nachwirkung’ or the aftereffects of light and darkness.” Darwin concluded instead that “the periodicity of their movements is to a certain extent inherited” (Darwin, 1896). Seventy-five years later, as the modern field of circadian biology was being established, experiments with several different organisms investigated whether organisms needed to be exposed to 24-hour cycles in light and dark during development in order to express circadian rhythms when mature (Aschoff, 1960; Pittendrigh, 1954). The conclusion from these studies was that the expression of circadian rhythms is “independent of any ontogenetic learning process” (Pittendrigh, 1954). For the whole organism, this conclusion is still appropriate today, but as the regulation of circadian rhythmicity continues to be elucidated, the general question persists.Understanding the relative contributions of intrinsic programs and environmental effects in guiding the differentiation of specific features of circadian organization remains a goal of developmental studies.