Ciba Found Symp 1989 Jan 01;144:187-201; discussion 201-7, 208-11.

Induction and the organization of the body plan in Xenopus development.

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Various experiments are surveyed in this paper that may throw light on how the degree of spatial organization of the Xenopus embryo increases during development. The events of the 100 minutes or so that follow fertilization may do little more than orientate and give proportions, within the egg's yolky vegetal region, to the system that originates patterned inductive signals for organization of the mesodermal and ectodermal regions during blastula stages (10(2)-10(4) cells). By onset of gastrulation, an outline plan for mediolateral and anteroposterior body organization has developed within induced tissue around the equator of the embryo, which seems to control subsequent development in two ways. It sets the spatial and temporal pattern of mechanical activities whereby the mesoderm rudiment drives the crucial shape changes that lay it and the neural rudiment out correctly. It is also the probable starting point for positionally specific gene transcription that begins immediately after gastrulation. Experiments with known inducing factors that explore the possible bases for the early 'pre-organization' in mesoderm are informative, but leave us far from a complete understanding. Evidence that inhibitory or modulating, as well as activating, signals are involved is surveyed.

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