Outlines of external development

Abstract

At stage 11, the heart tube has looped completely and despite the fact that it has lost its left-right symmetry, it can still be considered a homogeneous structure from both morphological and functional points of view. The embryonic cardiomyocytes, derived from the cardiogenic plate mesoderm, form a layer, which is dubbed primary myocardium. Impulse propagation and the subsequent contraction waves, which initially have a peristaltoid form, run from the inflow to the outflow end of the heart tube, i.e. in postero-anterior direction. The presence of cardiac jelly guarantees adequate propulsion of blood. From the outset on, pacemaker activity is dominant at the inflow end of the heart tube, although coupling of excitation and contraction is first achieved in the future ventricular area [Van Mierop 1967]. Which mechanisms are responsible for the pacemaker dominance of the inflow end is as yet unknown but it may well be related to the anteroposterior differentiation of the heart tube as a whole, in which retinoic acid signaling plays a quintessential role [Xavier-Neto et al. 2001]. Moreover, many molecular factors involved in early cardiac development [reviewed by e.g. Franco et al. 1998 and Xavier-Neto et al. 2001] exhibit an antero-posterior expression gradient and thus a gradual change of electrochemical properties along the heart tube.