CONTENTS Main Page Dynamic Development The Foundations of Developmental Biology Gametogenesis From Sperm and Egg to Embryo Genetic Regulation of Development Organizing the Multicellular Embryo Generating Cell Diversity Dynamic Development at a Glance Learning Resources Research Resources The Developmental Biology Journal Club Developmental Biology Tutorial

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 microtubular cytoskeleton appears to be involved in the rotation and in extensive reorganization of the animal cytoplasm (Houliston and Elinson, 1991; see also Larabell et al., 1996). Because the vegetal yolk mass and the vegetal cortex are displaced in opposite directions, vegetal cortex is translocated to the more equatorial position opposite the sperm entry site; i.e. to the dorsal vegetal location. 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) UV-irradiated fertilized eggs can be rescued by tilting them against gravity, which apparently mimics the normal rotation (Scharf and Gerhart, 1980; Vincent and Gerhart, 1987). They can also be rescued at the 32-cell stage by exposing them to lithium ions, which may alter an intracellular signaling pathway that is involved in dorsalization (Kao and Elinson, 1988). (Lithium blocks the phosphotidylinositol cycle, which is involved in intracellular signaling.) In addition to ventralization by UV treatment of fertilized eggs, UV treatment of vegetal hemispheres of full-grown oocytes renders embryos derived from the oocytes incapable of forming dorsal structures (Holwill et al., 1987). However, unlike UV-treated eggs, these eggs cannot be rescued by tilting (Elinson and Pasceri, 1989). They can, however, be rescued by exposure to lithium at the 32-cell stage. These observations suggest that the targets of UV irradiation in oocytes are dorsal determinants, rather than the microtubule motor of rotation. Dorsalizing factors may be translocated into the dorsal vegetal cortex during rotation, with their effects mediated by subsequent signaling events. Do dorsalizing factors exist? Holowacz and Elinson (1993) assayed for the presence of dorsalizing factors in the dorsal vegetal cortex by injecting it into ventral vegetal blastomeres (Holowacz and Elinson, 1993, Fig. 1). Forty-four percent of recipient embryos formed a secondary dorsal axis (Holowacz and Elinson, 1993, Table 1, Fig. 2), whereas none who received ventral cortical cytoplasm developed secondary axes. As discussed previously, UV treatment of full-grown oocytes ventralizes the embryos derived from them. This implies that the dorsalizing factors may be located in the vegetal cortex in the oocytes, where they presumably persist until after fertilization when they becomes localized to the dorsal-vegetal cortical region. To test for the presence of the dorsal determinants in unfertilized eggs, cytoplasmic transfers were conducted. As shown in Table 1, 36% of recipients formed a secondary axis. No other region of the egg was capable of producing this effect. 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. Learning Objectives How is formation of the gray crescent related to bilateralization of the embryo? What role does the cytoskeleton play in this process? How were the roles of the dorsal vegetal cells in development demonstrated ? What is the Nieuwkoop center? How can UV-irradiated fertilized eggs be rescued? How can embryos derived from UV treatment of vegetal hemispheres of full-grown oocytes be rescued? What is the evidence that dorsalizing factors exist? Digging Deeper: Recent Literature 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 The dorsalizing process in Xenopus. References 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. Browder, L.W., C. A. Erickson and W.R. Jeffery. 1991. Developmental Biology. Third Edition. Saunders College Publishing. Philadelphia. Elinson, R.P. and P. Pasceri. 1989. Two UV-sensitive targets in dorsoanterior specification of frog embryos. Development 106: 511-518. Gilbert, S.F. 1997. Developmental Biology. Fifth Edition. Sinauer. Sunderland, MA. Holawacz, T. and R.P. Elinson. 1993. Cortical cytoplasm, which induces dorsal axis formation in Xenopus, is inactivated by UV irradiation of the oocyte. Development 119: 277-285. Holwill, S., J. Heasman, C.R. Crawley, and C.C. Wylie. 1987. Axis and germ line deficiencies caused by UV-irradiation of Xenopus oocytes cultured in vitro. Development 100: 735-743. Houliston, E. and R.P. Elinson. 1991. Patterns of microtubule polymerization relating to cortical rotation in Xenopus laevis eggs. Development 112: 107-117. Kalthoff, K. 1996. Analysis of Biological Development. McGraw-Hill. New York. 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. Shostak, S. 1991. Embryology. An Introduction to Developmental Biology. HarperCollins. New York. 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. Sokol, S., Christian, J.L., Moon, R.T. and D.A. Melton. 1991. Injected wnt RNA induces a complete body axis in Xenopus embryos. Cell 67: 741-752. Sudarwati, S. and P.D. Nieuwkoop. 1971. Mesoderm formation in the anuran Xenopus laevis (Daudin). W. Roux' Arch. 166: 189-204. Vincent, J.-P. and J.C. Gerhart, 1987. Subcortical rotation in Xenopus eggs: an early step in embryonic axis specification. Dev. Biol. 123: 526-539. Vincent, J.-P., G.F. Oster and J.C. Gerhart. 1986. Kinematics of grey crescent formation in Xenopus eggs: the displacement of subcortical cytoplasm relative to the egg surface. Dev. Biol. 113: 484-500 Wolpert, L., Beddington, R., Brockes, J., Jessell, T., Lawrence, P. and Meyerowitz, E. 1998. Principles of Development. Current Biology. London.

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This material may be reproduced for educational purposes only provided credit is given to the original source. Leon Browder & Laurie Iten (Ed.) Dynamic Development Last revised Wednesday, June 24, 1998