Initiating the Embryonic Body Plan: Polarization of the Xenopus Embryo
Frogs have rights (and lefts), too.
(Please note that the links in the text are to images in the Amphibian Embryology Tutorial prepared by Dr. Jeff Hardin at The University of Wisconsin. They will be valuable resources to facilitate your understanding of this material.)
The unfertilized frog egg is radially symmetrical about an animal-vegetal axis. The bilateral symmetry that characterizes the embryo and adult is a consequence of cytoplasmic reorganization that occurs during the first cell cycle . This reorganization (rotation of symmetrization; Ancel and Vintemberger, 1948) occurs by a 30° rotation of the egg cortex in relation to the vegetal mass. In the vegetal hemisphere, the clear cortical cytoplasm is shifted upward away from the sperm entry site and toward the presumptive dorsal side of the embryo, whereas in the animal hemisphere, the pigmented cortex shifts downward toward the sperm entry site.
(See Browder et al., 1991, Fig. 6.4; Gilbert, Fig. 4.33; Kalthoff, Fig. 9.8; Shostak, Fig. 9.10; Wolpert et al., Fig. 3.3)
This rotation produces the gray crescent (the precursor of the dorsal lip) as the result of displacing part of the clear vegetal cortex so that it overlies the lightly pigmented marginal zone cytoplasm (Vincent et al., 1986). Thus, the grey crescent (and, hence, the dorsal lip) forms 180° opposite the sperm entry point. Another way of stating this is that the dorsal side forms where vegetal cortex meets animal cytoplasm, whereas the ventral side forms where animal cortex meets vegetal cytoplasm (Slack, 1994).
(See Browder et al., 1991, Fig. 6.2)
The roles of the dorsal vegetal cells in development were demonstrated by transplantation experiments. Fertilized eggs were irradiated in the vegetal hemisphere with ultraviolet light, which causes ventralization of the embryos. At the 64-cell stage, two vegetal blastomeres were removed and replaced by donor blastomeres from either the dorsal or ventral side of a non-irradiated donor embryo. Only the dorsal blastomeres were capable of rescuing the irradiated embryos, which formed a normal complement of dorsal structures.
(See Browder et al., 1991, Fig. 6.8; Gilbert, Fig. 6.20; Kalthoff, Fig. 9.16; Wolpert et al., Fig. 3.5)
Did the donor cells themselves develop as dorsal structures, or did they induce host cells to do so? To answer this question, the investigators utilized donor cells that had been injected with a fluorescent tracer. The fluorescent donor cells were confined to the gut and did not become dorsal cells themselves. The dorsal structures were derived from the marginal zone cells of the host. Thus, the dorsal vegetal cells have the capacity to release a signal to induce the overlying animal hemisphere blastomeres to develop as dorsal mesoderm. The dorsal vegetal cells form the so-called Nieuwkoop center, which is named after the investigator who first demonstrated the capacity of this portion of the embryo to induce dorsal mesoderm (Sudarwati and Nieuwkoop, 1971).
(See Browder et al., 1991, Fig. 6.9)
The evidence is strong that dorsal determinants exist. Obviously, their identification is an important goal. An important clue to their identities came from experiments demonstrating that injections of Wnt mRNA will cause duplication of the Xenopus embryonic axis (McMahon and Moon, 1989; Smith and Harland, 1991; Sokol et al., 1991). These observations have led to extensive investigations into the dorsalizing process in Xenopus.
Kageura, H. 1997. Activation of dorsal development by contact between the cortical dorsal determinant and the equatorial core cytoplasm in eggs of Xenopus laevis. Development 124: 1543-1551.
Logan, C.Y. and McClay, D.R. 1997. The allocation of early blastomeres to the ectoderm and endoderm is variable in the sea urchin embryo. Develop. Biol. 124: 2213-2223.
Links to Related Material
Ancel, P. and P. Vintemberger. 1948. Recherches sur le déterminisme
des la symetrie bilaterale dans l'oeuf des amphibiens. Bull. Biol. Fr. Belg.
31 (Suppl.): 1-182.
Gilbert, S.F. 1997. Developmental Biology. Fifth Edition. Sinauer.
Kao, K.R. and R.P. Elinson. 1988. The entire mesodermal mantle behaves as Spemann's organizer in dorsoanterior enhanced Xenopus laevis embryos. Dev. Biol. 127: 64-77.
Larabell, C.A., Rowning, B.A., Wells, J., Wu, M. and Gerhart, J.C. 1996. Confocal microscopy analysis of living Xenopus eggs and the mechanism of cortical rotation. Development 122: 1281-1289.
McMahon, A.P. and Moon, R.T. 1989. Ectopic expression of the proto-oncogene int-1 in Xenopus embryos leads to duplication of the embryonic axis. Cell 58: 1075-1084.
Scharf, S.R. and J.C. Gerhart. Determination of the dorso-ventral axis
in eggs of Xenopus laevis: complete rescue of UV-impaired eggs by
oblique orientation before first cleavage. Dev. Biol. 79: 181-198.
Slack, J. 1994. Inducing factors in Xenopus early embryos. Curr. Biol. 4: 116-126.
Smith, W.C. and Harland, R.M. 1991. Injected Xwnt-8 RNA acts early in
Xenopus embryos to promote formation of a vegetal dorsalizing center.
Cell 67: 753-765.
Wolpert, L., Beddington, R., Brockes, J., Jessell, T., Lawrence, P. and Meyerowitz, E. 1998. Principles of Development. Current Biology. London.
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Leon Browder & Laurie Iten (Ed.) Dynamic Development
Last revised Wednesday, June 24, 1998