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A complete fate map has been produced for the 32-cell stage of Xenopus laevis. Embryos with a regular cleavage pattern were selected and individual blastomeres were injected with the lineage label fluorescein-dextran-amine (FDA). The spatial location of the clones was deduced from three-dimensional (3D) reconstructions of later stages and the volume of each tissue colonized by labelled cells in each tissue was measured. The results from 107 cases were pooled to give a fate map which shows the fate of each blastomere in terms of tissue types, the composition of each tissue by blastomere, the location of each prospective region on the embryo and the fate of each blastomere in terms of spatial localization. Morphogenetic movements up to stage 10 (early gastrula) were assessed by carrying out a number of orthotopic grafts at blastula and gastrula stages using donor embryos uniformly labelled with FDA. Although there is a regular topographic projection from the 32-cell stage this varies a little between individuals because of variability of positions of cleavage planes and because of short-range cell mixing during gastrulation. The cell mixing means that the topographic projection fails for anteroposterior segments of the dorsal axial structures and it is not possible to include short segments of notochord or neural tube or individual somites on the pregastrulation fate map.
Fig. 1. (A) Nomenclature of blastomeres at the 32-cell stage. D, dorsal; V, ventral; AP, animal pole; VP, vegetal pole. (B) Orthotopic grafts at stage 10 viewed from vegetal pole. (C) Orthotopic grafts viewed in median section. AP, animal pole; VP, vegetal pole; DMZ, dorsal marginal zone; DLMZ, dorsolateral marginal zone; VLMZ, ventrolateral marginal zone; VMZ, ventral marginal zone.
Fig. 2. Examples of actual cases. All photographs show bright fluorescence of FDA-labelled cells against a faint autofluorescent background. (A) A l blastomere, showing label in brain, retina and sucker, with contralateral contributions. (B) A4 blastomere, midtrunk level showing label confined to epidermis. (C) Cl blastomere, anterior trunk, showing label in notochord and endoderm. (D) C3 blastomere, showing label in pronephros, ventral somite and trunk mesenchyme (probably sclerotome). (E) C4 blastomere, posterior level, showing label in lateral plate and originally ventral,somite. (F) D3 blastomere, showing sharp boundaries between endoderm and mesoderm and along first cleavage plane. (G) C3 blastomere, showing how dorsal convergence of ventral somite yields separate patches of cells. Bar, 60/an. (H) B3 blastomere, showing short-range cell mixing in somite. Magnification as G.
(I) A l blastomere, showing short-range cell mixing in neural tube. Magnification as G
Figure 5. 3D reconstructions of typical cases representing each of the 16 blastomere injections. [A blastomeres]
Figure 5. 3D reconstructions of typical cases representing each of the 16 blastomere injections. [B blastomeres]
Figure 5. 3D reconstructions of typical cases representing each of the 16 blastomere injections.[C blastomeres]
Figure 5. 3D reconstructions of typical cases representing each of the 16 blastomere injections. [D blastomeres]
Fig. 7. 3D reconstructions of typical cases resulting from orthotopic grafts of animal pole regions.
Fig. 8. 3D reconstructions of typical cases resulting from orthotopic grafts of marginal zone regions.