Body plan of sea urchin embryo: An ancestral type animal

Sea urchin embryos are though to possess a body plan characteristic of early deuterostomes. Sea urchins contain homologs of Otx, Lim, T-brain and Hox gene cluster, which are involved in head and segment formation in vertebrate development, although the sea urchin has not evolved a head or segments. We described here that sea urchin Otx is involved in various aspects of early development and that the Hox genes do not obey spatial colinearity in sea urchin embryo. The Otx and Hox genes seems to be used subsequently for head formation and determining the anteroposterior axis respectively during chordate evolution. We propose that the Precambrian was a period where these regulatory genes were utilized in many different combinations during animal development, leading to the evolution of a wide range of body plans, many of which were successful. We also discuss the role of chromatin boundaries and the mechanism of cell specification along animal vegetal axis, especially differentiation of the large micromere progeny, which are the prospective primary mesenchyme cells and play a role as an organizer in sea urchin embryos.