![]() Spemann’s work revealed that when two blastomeres are separated such that only one of the two cells contains the crescent, only the blastomere containing the gray crescent develops normally (Figure 2B).įigure 2 Asymmetry in the amphibian egg. However, should this cleavage plane be aberrant (either in the rare natural event or in an experiment), the gray crescent material passes into only one of the two blastomeres. If these cells are then separated, two complete larvae develop (Figure 2A). The first cleavage plane normally splits this gray crescent equally between the two blastomeres (see Figure 11.2D). As we saw earlier in Chapter 11 of the textbook (see Figure 11.2), there are dramatic movements in the cytoplasm following the fertilization of amphibian eggs, and in some amphibians these movements expose a gray, crescent-shaped area of cytoplasm in the region directly opposite the point of sperm entry. Fortunately, the salamander egg was a good organism to test that hypothesis. Why did these two experiments have such different results? One possibility was that when the egg was divided perpendicular to the first cleavage plane, some cytoplasmic substance was not equally distributed into the two halves. This tissue mass was a ball of epidermal cells (ectoderm) containing blood cells and mesenchyme (mesoderm) and gut cells (endoderm), but it contained no dorsal structures such as nervous system, notochord, or somites. The other side produced an unorganized tissue mass of ventral cells, which Spemann called the Bauchstück (“belly piece”). The nuclei continued to divide on both sides of the constriction, but only one side-the future dorsal side of the embryo-gave rise to a normal larva. However, when Spemann performed a similar experiment with the constriction still longitudinal but perpendicular to the plane of the first cleavage (i.e., separating the future dorsal and ventral regions rather than the right and left sides), he obtained a different result altogether. (C) After 14 days, each side had developed into a normal embryo. (B) At the 16-cell stage, a single nucleus entered the as-yet undivided half, and the ligature was further constricted to complete the separation of the two halves. The cleavage on that side of the embryo reached the 8-cell stage, while the other side remained undivided. (A) When the fertilized egg of the newt Triturus taeniatus was constricted by a ligature, the nucleus was restricted to one half of the embryo. North Holland, Amsterdam, The Netherlands.Figure 1 Spemann’s demonstration of nuclear equivalence in newt cleavage. (1997) Head induction by simultaneous repression of Bmp and wnt signalling in Xenopus. Glinka, A., Wu, W., Onichtchouk, D., Blumenstock, C., and Niehrs, C. (1997) Structurally related receptors and antagonists compete for secreted wnt ligands. (1991) Overexpression of a homeodomain protein confers axis-forming activity to uncommitted Xenopus embryonic cells. (1989) Interaction between peptide growth factors and homeobox genes in the establishment of antero-posterior polarity in frog embryos. (1994) The Einsteck-method: position and structure of projections formed by implants of a ventral character. (1996) The vertebrate organizer: structure and molecules. This process is experimental and the keywords may be updated as the learning algorithm improves. These keywords were added by machine and not by the authors. However, the Einsteck can be also carried out with heterologous material, cell pellets, and injected animal caps ( 3, 4) and is more versatile than the organizer transplant. While the Einsteck is much easier to perform than the organizer transplant, the position of the implant can be less well controlled, which needs to be considered when interpreting the results ( 2). This results in the formation of an induced structure which, depending on the embryonic stage of the upper blastopore lip, is a head, trunk, or a tail. Organizer activity is revealed either by transplantation of the upper dorsal blastopore lip into the ventral side of a host gastrula or by inserting the tissue into the gastrula blastocoel (Einsteck method). Its two major inducing activities are neural induction and dorsalization of ventral mesoderm differentiation. The amphibian organizer corresponding to the upper dorsal blastopore lip of the early gastrula, is one of the classical systems in which embryonic patterning has been studied ( 1). ![]()
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