The entire mesodermal mantle behaves as Spemann's organizer in dorsoanterior enhanced XXXX embryos.

@entity2270 @entity1091 @entity1238
The body plan of @entity1238 can be respecified by briefly exposing early cleavage stage embryos to @entity1091 . Such embryos develop exaggerated dorsoanterior structures such as a radial eye and cement gland (K.R. Kao, Y. Masui, and R.P. Elinson, 1986, Nature (London) 322, 371-373). In this paper, we demonstrate that the enhanced dorsoanterior phenotype results from an overcommitment of mesoderm to dorsoanterior mesoderm. Histological and immunohistochemical observations reveal that the embryos have a greatly enlarged notochord with very little muscle tissue. In addition, they develop a radial, beating heart, suggesting that @entity1091 also specifies anterior mesoderm and pharyngeal endoderm. Randomly oriented diametrically opposed marginal zone grafts from @entity1091 -treated embryos, when transplanted into ultraviolet (uv)-irradiated @entity2270 hosts, rescue dorsal axial structures. These transplantation experiments demonstrate that the entire marginal zone of the early gastrula consists of presumptive dorsal mesoderm. Vital dye marking experiments also indicate that the entire marginal zone maps to the prominent proboscis that is composed of chordamesoderm and represents the long axis of the embryo. These results suggest that @entity1091 respecifies the mesoderm of @entity1238 embryos so that it differentiates into the Spemann organizer. We suggest that the origin of the dorsoanterior enhanced phenotypes generated by @entity1091 and the dorsoanterior deficient phenotypes generated by uv irradiation are due to relative quantities of organizer. Our evidence demonstrates the existence of a continuum of body plan phenotypes based on this premise.
