Symmetry Breaks in Early Development of Multicellular Organisms: Instabilities and Morphomechanics
Introduction Virtually all Metazoa embryos undergo during early development several steps o f reduc
tion o f symmetry (symmetry breaks), especially if taking as a criteria o f a symmetry order so called “color” (qualitative) characteristics, rather than only geometrical ones. Thus, an im mature oocyte, at least prior to the start o f vitellogenesis, can be qualified, regardless of its geometrical shape, as a body of a highest possible symmetry (that o f a sphere: «> / «> m) because any one of its axes can become the main polar (animo-vegetal) axis o f a mature egg. The axis position becomes ultimately determined as the extrusion o f polar bodies (more precisely, the second polar body1). From this moment on the symmetry o f egg and early embryo is reduced to ©o · m. The next symmetry break is associated with the establishment of dorso-ventrality, when the symmetry order reduces to l m (Fig. 1). In not all Metazoa is this latter symmetry break well expressed. Some called Radiata (to these belong Coelenterata, adult Echinodermata and several other groups of invertebrates), look as retaining, at least in some periods o f their life cycle, a perfect radial symmetry, although of a diminished order (n • m , rather than «> · m ). More careful investigations show however that some elements of dorso-ventrality are acquired by these organisms as well, although in a more or less hidden form.2 Others, called Bilateria, obtain 1 · m symmetry in early development and do not lose it during the entire life cycle. A number o f further morphogenetic events can be qualified as
♦Lev V. Beloussov-Laboratory of Developmental Biophysics, Department of Embryology, Faculty of Biology, Moscow State University, Moscow, Russia. Email: [email protected]
Behavioral and M orphological Asymmetries in Vertebrates, edited by Yegor B. Malashichev and A Wallace Deckel. ©2006 Landes Bioscience.