What is the purpose of the embryonic heart beat? Or how facts can ultimately prevail over physiological dogma.
Embryonic physiology is often viewed as merely those processes understood for the adult but conducted on a smaller physical scale. Yet striking examples of the inaccuracy of this perspective can be identified in the embryonic cardiovascular system. For example, dogma holds that the embryonic heart begins to beat to pump blood for convective transport, just like that of the adult. This is the major assumption inherent in the hypothesis we have called "convective synchronotropy"; that is, the embryonic heart starts to beat synchronously with the need for convective blood flow. However, there is compelling evidence on many fronts that the convective flow of blood generated by the early embryonic vertebrate heart is simply not required for transport of oxygen, nutrients, metabolic wastes, or hormones, all of which can be achieved entirely by diffusion. In fact, fish, amphibian, and bird embryos lacking a functional heart (either through surgical intervention or mutation) or whose oxygen-hemoglobin transport has been chemically eliminated nonetheless continue to function and grow in size for extended periods up to the point at which diffusion alone can no longer serve oxygen transport needs. We advocate the alternative hypothesis of "prosynchronotropy" (i.e., the heart starts to beat well before convective blood flow is needed for bulk transport). So, what is the purpose of the early embryonic heart beat? Evidence is presented herein in support of a morphogenic rationale for prosynchronotropy. Specifically, it appears that the initial rationale for the beat of the vertebrate embryonic heart may be two-fold: to aid in subtle but significant aspects of cardiac growth, shaping, and maturation, and to facilitate cardiac maturation angiogenesis--the formation of new vessels by sprouting from vessel tips. Ultimately, the embryonic cardiovascular system provides a graphic demonstration of how adult physiological functions should not, without verification, be interpolated back to the embryo of that species.
Burggren
Warren W
WW
Department of Biological Sciences, University of North Texas, Denton, Texas 76205, USA. burggren@unt.edu
D000818:Animals; D001775:Blood Circulation / Q000502:physiology*; D001794:Blood Pressure / Q000502:physiology; D004058:Diffusion; D004622:Embryo, Mammalian / Q000502:physiology*; D004625:Embryo, Nonmammalian; D006321:Heart / Q000196:embryology*; D009024:Morphogenesis; D009200:Myocardial Contraction / Q000502:physiology*; D018919:Neovascularization, Physiologic / Q000502:physiology*