title: Symmetry breakage in the vertebrate embryo: when does it happen and how does it work?
abstract: Asymmetric development of the vertebrate embryo has fascinated embryologists for over a century. Much has been learned since the asymmetric Nodal signaling cascade in the left lateral plate mesoderm was detected, and began to be unraveled over the past decade or two. When and how symmetry is initially broken, however, has remained a matter of debate. Two essentially mutually exclusive models prevail. Cilia-driven leftward flow of extracellular fluids occurs in mammalian, fish and amphibian embryos. A great deal of experimental evidence indicates that this flow is indeed required for symmetry breaking. An alternative model has argued, however, that flow simply acts as an amplification step for early asymmetric cues generated by ion flux during the first cleavage divisions. In this review we critically evaluate the experimental basis of both models. Although a number of open questions persist, the available evidence is best compatible with flow-based symmetry breakage as the archetypical mode of symmetry breakage.
authors: University of Hohenheim, Institute of Zoology (220), Garbenstrasse 30, D-70593 Stuttgart, Germany. Electronic address: martin.blum@uni-hohenheim.de.
authors: Martin
authors: M
authors: Blum
authors: University of Hohenheim, Institute of Zoology (220), Garbenstrasse 30, D-70593 Stuttgart, Germany.
authors: Axel
authors: A
authors: Schweickert
authors: University of Hohenheim, Institute of Zoology (220), Garbenstrasse 30, D-70593 Stuttgart, Germany.
authors: Philipp
authors: P
authors: Vick
authors: Department of Cell and Developmental Biology, Vanderbilt University, Nashville, TN 37232-0494, USA.
authors: Christopher V E
authors: CV
authors: Wright
authors: Department of Integrative Biosciences, Oregon Health & Science University, Portland, OR 97239-3098, USA.
authors: Michael V
authors: MV
authors: Danilchik
mesh_terms: D000818:Animals; D019521:Body Patterning; D004622:Embryo, Mammalian / Q000033:anatomy & histology / Q000196:embryology; D004625:Embryo, Nonmammalian / Q000033:anatomy & histology / Q000196:embryology; D005399:Fishes / Q000196:embryology; D018507:Gene Expression Regulation, Developmental; D017506:H(+)-K(+)-Exchanging ATPase / Q000235:genetics / Q000378:metabolism; D055458:Left-Right Determination Factors / Q000378:metabolism; D008322:Mammals / Q000196:embryology; D008648:Mesoderm / Q000378:metabolism; D055457:Nodal Protein / Q000378:metabolism; D020897:Organizers, Embryonic / Q000502:physiology; D012701:Serotonin / Q000378:metabolism; D015398:Signal Transduction / Q000235:genetics; D014714:Vertebrates / Q000033:anatomy & histology / Q000196:embryology*; D014981:Xenopus / Q000196:embryology
chemical_list: D055458:Left-Right Determination Factors; D055457:Nodal Protein; D012701:Serotonin; D017506:H(+)-K(+)-Exchanging ATPase
keywords: Cilia; Ion-flux model; Leftward flow; Left–right asymmetry; Symmetry breakage