title: Origin of the vertebrate body plan via mechanically biased conservation of regular geometrical patterns in the structure of the blastula.
abstract: We present a plausible account of the origin of the archetypal vertebrate bauplan. We offer a theoretical reconstruction of the geometrically regular structure of the blastula resulting from the sequential subdivision of the egg, followed by mechanical deformations of the blastula in subsequent stages of gastrulation. We suggest that the formation of the vertebrate bauplan during development, as well as fixation of its variants over the course of evolution, have been constrained and guided by global mechanical biases. Arguably, the role of such biases in directing morphology-though all but neglected in previous accounts of both development and macroevolution-is critical to any substantive explanation for the origin of the archetypal vertebrate bauplan. We surmise that the blastula inherently preserves the underlying geometry of the cuboidal array of eight cells produced by the first three cleavages that ultimately define the medial-lateral, dorsal-ventral, and anterior-posterior axes of the future body plan. Through graphical depictions, we demonstrate the formation of principal structures of the vertebrate body via mechanical deformation of predictable geometrical patterns during gastrulation. The descriptive rigor of our model is supported through comparisons with previous characterizations of the embryonic and adult vertebrate bauplane. Though speculative, the model addresses the poignant absence in the literature of any plausible account of the origin of vertebrate morphology. A robust solution to the problem of morphogenesis-currently an elusive goal-will only emerge from consideration of both top-down (e.g., the mechanical constraints and geometric properties considered here) and bottom-up (e.g., molecular and mechano-chemical) influences.
authors.affiliation: Department of Psychological Sciences, University of San Diego, Serra Hall 158, 5998 Alcalá Park, San Diego, CA 92110, USA. Electronic address: dedelman@sandiego.edu.
authors.forename: David B
authors.initials: DB
authors.lastname: Edelman
authors.affiliation: 303 Clapp Laboratory, Mount Holyoke College, 50 College Street, South Hadley, MA 01075, USA. Electronic address: mmcmenam@mtholyoke.edu.
authors.forename: Mark
authors.initials: M
authors.lastname: McMenamin
authors.affiliation: Evergreen State College, 2700 Evergreen Pkwy NW, Olympia, WA 98505, USA. Electronic address: shepet12@evergreen.edu.
authors.forename: Peter
authors.initials: P
authors.lastname: Sheesley
authors.affiliation: Chief Scientific Officer and Chairman, Chem-Tainer Industries, Inc., 361Neptune Avenue, West Babylon, NY 11704, USA. Electronic address: spivar@chemtainer.com.
authors.forename: Stuart
authors.initials: S
authors.lastname: Pivar
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D001755:Blastocyst; D036703:Blastula / Q000196:embryology*; D047108:Embryonic Development; D006801:Humans; D055595:Mechanical Phenomena; D014714:Vertebrates / Q000196:embryology*
keywords: Blastula; Evolution; Gastrula; Geometry; Morphogenesis; Vertebrate bauplan