Dynamics of the control of body pattern in the development of Xenopus laevis. I. Timing and pattern in the development of dorsoanterior and posterior blastomere pairs, isolated at the 4-cell stage.

Xenopus embryos have been selected in which the second cleavage is occurring in a frontal plane, i.e. one tending to lie at right angles to the prospective plane of bilateral symmetry for the body pattern. Some of these have been used to deduce a map of the disposition of materials for the normal mesodermal pattern (the normal 'fate map') by injecting blastomeres to found fluorescently marked clones from 4- to 32-cell stages. Other such 4-cell embryos have been separated into two isolates across this second cleavage; in fate-map terms, prospective dorsoanterior and posterior isolates. These have been allowed to develop to control axial larval stages, with examination of the time schedule of their gastrulation movements in relation to cofertilized whole controls. The patterns of mesoderm produced have been examined and interpreted in the light of quantitative knowledge about the normal pattern, and our current understanding of the map. A meaningful fate map exists for the egg material even at this early, essentially acellular stage, and it differs appreciably from what might have been expected in view of that traditionally shown for early gastrula stages. The patterns developed in the isolates show that at least in many eggs, widespread information that positively specifies material as to its body position is available from at most 1 h after the events that give rise to bilateral symmetry upon fertilization. This information usually leads to a mosaic development of the appropriate mesodermal part-pattern in dorsoanterior isolates, and frequently allows development that approximates to this in the reciprocal posterior part. Regulation, i.e. the replacement of removed information to specify a development more complete than the normal contribution in isolates, is not observed. The results suggest a revision of former claims for regulative ability in at least this amphibian embryo. They also imply that systems for ascribing position value (positional information) to early embryonic tissue can be diverse in dynamics, even among embryos whose body plans are obviously homologous as are those of vertebrates.