Isolated Dorsal Animal Blastomeres of Xenopus laevis Are Capable to Form Mesodermal Derivatives, while the Ventral Animal Blastomeres Differentiate into Ciliated Epidermis Only

Abstract

Isolated two dorsal animal blastomeres of 8-cell-stage Xenopus embryos differentiated in about 24% of the cases into mesodermal structures, while the two ventral animal ones formed exclusively atypical epidermis. Of special interest is the fact that most of the dorsal animal blastomeres without mesodermal structures, differentiated into atypical epidermis with large parts of cement gland. Cement glands could not be detected in the derivatives of the ventral animal blastomeres. In earlier concepts it has been suggested that the animal hemisphere of the amphibian egg is an uncommitted area, which receives its instructions for further determination and differentiation from the vegetal part of the embryo. However, the results, shown in this paper, support the view that the developmental determinants are distributed in distinct gradients already in the early cleavage stages. Although the highest concentrations of these putative determinants are located in the dorsal vegetal area (Melton, 1995) and the dorsal marginal zone, lower concentrations of these substances are thought to be present in the animal part, especially in the dorsal animal hemisphere of the egg and the early embryo. Our results indicate that dorsal animal blastomeres of early cleavage stage embryos possess the capacity to form mesodermal derivatives after their separation from the vegetal hemisphere.

Establishment of the Dorso-ventral Axis in Xenopus Embryos Is Presaged by Early Asymmetries in β-Catenin That Are Modulated by the Wnt Signaling Pathway

“…These observations have led some in the field to consider that the Nieuwkoop Center activity is a discrete physical entity and/or that it is present only in the dorsal vegetal region of embryos. However, this is not consistent with blastomere transplantation experiments showing an axis-inducing activity extending into the animal hemisphere (see introduction), nor does it readily explain the mesoderm-forming ability of isolated or rotated eight-cell animal pole cells (Cardellini, 1988;Grunz, 1994;Li et al, 1996). To account for these data, we have previously suggested (Kimelman et al, 1992;Moon and Christian, 1992) that the Nieuwkoop Center comprises a large region of the dorsal side of the embryo, spreading from the dorsal animal to the dorsal vegetal cells, with maximum activity in the dorsal vegetal cells.…”

Pluripotent cells (stem cells) and their determination and differentiation in early vertebrate embryogenesis†

“…Nieuwkoop concluded from these experiments that the mesoderm within the embryo, which differentiates from the marginal zone (the region of the fertilized egg and early embryo between endoderm and ectoderm), is also induced by the vegetal endoderm. More recent experiments have cast doubt on this interpretation (Gallagher et al 1991; Grunz 1994; Li et al 1996). The four animal blastomeres, which were manually isolated from regularly cleaved X. laevis 8‐cell stages as a quartet, differentiated to dorsal and dorsolateral mesodermal (notochord, somites) and (secondarily induced) neural tissues in over 50% of cases.…”