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- noun zoology Any
animalthat exhibits a spiral cleavagepattern (typically molluscs)
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Remarkably, 'spiralian' taxa have evolved disparate adult body plans, yet retain a close correspondence between early embryonic cell lineage
The schematic cell lineage table at bottom shows the typical relationships of the quadrants, and indicates the parallel generation of micromeres and micromere sublineages in the four quadrants of a generalized spiralian embryo. doi: 10.1371 / journal. pone.0005506.g001
Our present understanding of spiralian cell fate specification owes much to studies of the mud snail
Most spiralian embryos exhibit tier-specific patterns of cell division at early stages; the significance of this mitotic asynchrony is unknown, but presumably reflects differential inheritance of factors regulating cell division.
Thus, while the third cleavage in a frog, sea urchin or jellyfish embryo separates four animal ( 'northern') cells from vegetal ( 'southern') sister cells, third cleavage in a spiralian embryo separates four 'northwestern' cells from
In general, complex patterns of early lineage-specific differentiation seem to have evolved at the bases of several higher-level spiralian clades
Unlike other animal embryos, spiralian early cleavage planes are never perpendicular to the AV axis.
One might expect that due to metabolic noise, cell division timing should become progressively less predictable as development proceeds; such time-dependent decorrelation has indeed been noted in an early study of spiralian cell lineage
(A) Morphogenetic transformation of the rotationally symmetric spiralian embryo to a dorsoventrally polarized juvenile animal, as inferred from fate-mapping and other descriptive studies.
For the most part, each micromere clone in a spiralian embryo remains largely cohesive throughout embryogenesis, and adopts a highly predictable fate
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