The Timing of Mesoderm Formation in Xenopus

In 1970, Nakamura and Takasaki showed that explants from the equatorial region of the midblastula (the piece removed in Nieuwkoop's experiments) are able to form mesoderm in culture. Explants from the same regions of 32- and 64-cell embryos failed to form mesoderm, becoming ciliated ectodermal cells. These experiments suggested (1) that prospective mesoderm cells are not determined immediately but achieve their prospective fates by the progressive interaction of animal and vegetal cells, and (2) that the equatorial precursors already have their mesodermal fate determined before gastrulation.

These suggestions have been confirmed by transplantation experiments. At about the 32-cell stage, DMZ cells begin acquiring the ability to become dorsal mesoderm on their own (Figure 1; Gimlich, 1986; Jones and Woodland, 1987). Takasaki and Konishi (1989) demonstrated that DMZ blastomeres (B1 and C1) of the 32-cell stage from Xenopus laevis transplanted into the ventral marginal zone of Xenopus borealis produced descendants that differentiated into chordamesoderm and somite tissues and induced the host tissues to form a secondary neural tube with a head and tail. However, none of these embryos had complete anterior structures (full cement gland and nose), indicating that not all the information from the dorsalmost vegetal D1 blastomeres is present in these cells at this early stage.

Figure 1 Rescue of irradiated embryos by the transplantation of dorsal marginal cells of normal 32-cell embryos. Without such a rescue, irradiated embryos fail to gastrulate, forming "belly pieces." If the dorsal marginal zone cells of a normal embryo are transplanted into the irradiated embryo, they will induce the formation of mesoderm and the body axis, thereby producing a normal tadpole. (After Gimlich, 1986.)

The frequency with which the DMZ transplants can produce the complete axis in irradiated host embryos starts off very low and increases with age. Conversely, the mesoderm-inducing activity of the dorsal vegetal cells declines over this period (Gimlich, 1985; Boterenbrood and Nieuwkoop, 1973). It appears, then, that the cells of the dorsal vegetal quadrant in some way activate (or produce) factors within the DMZ cells, and that the complete "inductive mesoderm" phenotype develops gradually as the vegetal cells influence the marginal cells above them.

Sargent and colleagues (1986) and Gurdon and his co-workers (1985) have also provided molecular confirmation of Nieuwkoop's observations. If Nieuwkoop's morphological studies were correct, a-actin gene activity should be induced in animal hemisphere cells when such cells (which would normally give rise to ectodermal tissues) are placed directly upon vegetal cells. Gurdon and his co-workers dissected away the marginal cells of midblastula embryos and recombined the animal pole cells with the vegetal cell mass (Figure 2). They found that the animal pole cells began transcribing mesoderm-specific a-actin message. The vegetal cells had induced a mesoderm-specific gene to be turned on in the presumptive ectodermal tissue. At the same time, these interactions appear to cause the loss of ectoderm-specific gene activity (Sargent et al., 1986).

Figure 2 Animal-vegetal recombination experiments. (A) The marginal cells are removed from an 8-cell blastula and the animal and vegetal cells allowed to come into contact. (B) Using an RNase protection assay for the detection of actin, Muscle-specific (alpha) actin was found in the marginal (equatorial) region and in the recombined animal and vegetal cells, but not in vegetal or animal cells alone. (After Gurdon et al., 1985.)

Literature Cited

Boterenbrood, E. C. and Nieuwkoop, P. D. 1973. The formation of the mesoderm in urodelean amphibians. V. Its regional induction by the endoderm. Wilhelm Roux Arch. Entwicklungsmech. Org. 173: 319-332.

Gimlich, R. L. 1985. Cytoplasmic localization and chordamesoderm induction in the frog embryo. J. Embryol. Exp. Morphol. 89 89-111.

Gimlich, R. L. 1986. Acquisition of developmental autonomy in the equatorial region of the Xenopus embryo. Dev. Biol. 116: 340-352.

Gurdon, J. B., Fairman, S., Mohun, T. J. and Brennan, S. 1985. The activation of muscle-specific actin genes in Xenopus development by an induction between animal and vegetal cells of a blastula. Cell 41: 913-922.

Jones, E. A. and Woodland, H. R. 1987. The development of animal cap cells in Xenopus: a measure of the start of animal cap competence to form mesoderm. Development 101: 557-564.

Nakamura, O. and Takasaki, H. 1970. Further studies on the differentiation capacity of the dorsal marginal zone in the morula of Triturus pyrrhogaster. Proc. Japan Acad. 46: 700-705.

Sargent, T. D., Jamrich, M. and Dawid, I. 1986. Cell interactions and the control of gene activity during early development of Xenopus laevis. Dev. Biol. 114: 238-246.

Takasaki, H. and Konishi, H. 1989. Dorsal blastomeres in the equatorial region of the 32-cell Xenopus embryo autonomously produce progeny committed to the organizer. Dev. Growth Differ. 31: 147-156.

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