Huang S et al. (1998), Blastomeres show differential fate changes in 8...

XB-ART-15086

Dev Growth Differ 1998 Apr 01;402:189-98. doi: 10.1046/j.1440-169x.1998.00008.x.

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Blastomeres show differential fate changes in 8-cell Xenopus laevis embryos that are rotated 90 degrees before first cleavage.

Huang S , Johnson KE , Wang HZ .

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To study the mechanisms of dorsal axis specification, the alteration in dorsal cell fate of cleavage stage blastomeres in axis-respecified Xenopus laevis embryos was investigated. Fertilized eggs were rotated 90 degrees with the sperm entry point up or down with respect to the gravitational field. At the 8-cell stage, blastomeres were injected with the lineage tracers, Texas Red- or FITC-Dextran Amines. The distribution of the labeled progeny was mapped at the tail-bud stages (stages 35-38) and compared with the fate map of an 8-cell embryo raised in a normal orientation. As in the normal embryos, each blastomere in the rotated embryos has a characteristic and predictable cell fate. After 90 degrees rotation the blastomeres in the 8-cell stage embryo roughly switched their position by 90 degrees, but the fate of the blastomeres did not simply show a 90 degrees switch appropriate for their new location. Four types of fate change were observed: (i) the normal fate of the blastomere is conserved with little change; (ii) the normal fate is completely changed and a new fate is adopted according to the blastomere's new position: (iii) the normal fate is completely changed, but the new fate is not appropriate for its new position; and (4) the blastomere partially changed its fate and the new fate is a combination of its original fate and a fate appropriate to its new location. According to the changed fates, the blastomeres that adopt dorsal fates were identified in rotated embryos. This identification of dorsal blastomeres provides basic important information for further study of dorsal signaling in Xenopus embryos.

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	Fig. 1. Schematic diagrams showing the goo rotation of fertilized eggs and the nomenclature of the blastomeres in 8-cell embryos. Unrotated normal embryo (left), goo rotated/SEP up (middle) and goo rotated/SEP down (right). The pigmented, animal hemisphere is shaded.
	Fig. 2. Eggs with SEP (arrow) at or near the equator were selected for the study. (A) The egg was rotated so that the SEP is up. (B) After 90° rotation, the first cleavage (1st) divides the embryo into left and right halves. The second cleavage (2nd) divides the original animal (pigmented) and vegetal (unpigmented) halves. (C) The third cleavage (3rd) goes horizontally and passes near the an1mal (·)and vegetal poles. (B) Dorsal view of a 4-cell stage embryo. (C) Dorsal animal pole view of an 8-cell stage embryo.
	Fig. 3. Confocal micrographs demonstrating the distribution of blastomere progeny in normal, unrotated tadpoles. (A) 01 (red) and 02 (green); and (B) normal V1 (red) and V2 (green). Two examples of blastomeres, 01 and 02, are given to show the consistency of the distribution of progeny. The distribution pattern of the progeny in truncal somites is also characteristic; the dorsal blastomeres usually produce a dCv pattern (A) and the ventral blastomeres usually produce a DcV pattern (B). A DcV pattern is shown in section in (C). Lineage dyes were injected into blastomeres in the left and right sides of the embryo; V1, red and V2, green. br, brain; c, central somite; em, cement gland; d, dorsal somite; h, heart; I, liver; n, notochord; ne, pronephros; r, retina; v, ventral somite; pr, proctodeum. Bars, 1 mm (A, B); 100 J.Jm (C).
	Fig. 4. Confocal micrographs demonstrating the distribution of blastomere progeny in the 90° rotated embryos. (A) up-D1 (red) and up-D2 (green); (B) up-V1 (red) and up-V2 (green); (C) dn-D1 (red) and dn-D2 (green); (D) dn-V1 (red) and dn-V2 (green). (E) Distribution of the progeny in truncal somites also shows either a DcV (red, up-D1) or a dCv (green, up-D2) pattern in the rotated embryos. Bars, 1 mm (A-D); 100 ~m (E)
	Fig. 5. Drawings showing the location of the blastomeres that adopt dorsal fate (01 and 02) in the normal, goo rotated/SEP up and goo rotated/SEP down 8-cell embryos. An, the original animal hemisphere (shaded): Vg, the original vegetal hemisphere (A) 01 and 02 blastomere positioning based on actual fate maps; the dorsal side is on the lateral side of the rotated embryo and formed by the original vegetal blastomeres. (B) 01 and 02 positioning in rotated embryos, assuming that cytoplasmic components do not move much. (C) 01 and 02 positioning in rotated embryos assuming that cytoplasmic components slide to gravitational equilibrium and shift position as a coherent mass.