© Springer International Publishing Switzerland 2017 209
F. Pelegri et al. (eds.), Vertebrate Development, Advances in Experimental
Medicine and Biology 953, DOI 10.1007/978-3-319-46095-6_6
Chapter 6
Vertebrate Axial Patterning: From Egg
to Asymmetry
Douglas W. Houston
Abstract The emergence of the bilateral embryonic body axis from a symmetrical
egg has been a long-standing question in developmental biology. Historical and
modern experiments point to an initial symmetry-breaking event leading to local-
ized Wnt and Nodal growth factor signaling and subsequent induction and forma-
tion of a self-regulating dorsal “organizer.” This organizer forms at the site of
notochord cell internalization and expresses primarily Bone Morphogenetic Protein
(BMP) growth factor antagonists that establish a spatiotemporal gradient of BMP
signaling across the embryo, directing initial cell differentiation and morphogene-
sis. Although the basics of this model have been known for some time, many of the
molecular and cellular details have only recently been elucidated and the extent that
these events remain conserved throughout vertebrate evolution remains unclear.
This chapter summarizes historical perspectives as well as recent molecular and
genetic advances regarding: (1) the mechanisms that regulate symmetry-breaking in
the vertebrate egg and early embryo, (2) the pathways that are activated by these
events, in particular the Wnt pathway, and the role of these pathways in the forma-
tion and function of the organizer, and (3) how these pathways also mediate antero-
posterior patterning and axial morphogenesis. Emphasis is placed on comparative
aspects of the egg-to-embryo transition across vertebrates and their evolution. The
future prospects for work regarding self-organization and gene regulatory networks
in the context of early axis formation are also discussed.
Keywords Vertebrate embryology • Axis formation • Cortical rotation • Spemann
organizer • Dorsoventral patterning • Anteroposterior patterning • Embryonic induc-
tion • Nieuwkoop center • Anterior visceral endoderm • Gastrulation
D.W. Houston (*)
Department of Biology, The University of Iowa, 257 BB, Iowa City, IA 52242, USA
e-mail: [email protected]