Hae-Moon Chung

Destruction of components of the neural induction system of the amphibian egg with ultraviolet irradiation.

Abstract Ultraviolet irradiation of the vegetal hemisphere of the fertilized amphibian (Xenopus laevis) egg prior to first cleavage results in the embryo developing an incomplete set of neural structures. The effects of irradiation on various morphogenetic processes, including cell division, formation of the dorsal lip, invagination at gastrulation, and neural induction by the primary organizer, were examined. A decrease in the capacity for invagination during gastrulation and a diminution in the neural inducing capacity of the primary organizer were found to account for defective neurulation in irradiated embryos. Consequently, irradiation of the uncleaved egg leads to interference with the events of both gastrulation and neurulation.

Establishment of the dorsal/ventral polarity of the amphibian embryo: Use of ultraviolet irradiation and egg rotation as probes

Abstract Ultraviolet irradiation and egg rotations were employed as probes for the study of the establishment of the dorsal/ventral polarity of the amphibian embryo. Ultraviolet irradiation was discovered to alter the natural position of the doral lip and to modify the pigmentation pattern of the early embryo. Rotation of the uncleaved egg was found to succeed in relocating the dorsal lip to a new site in the embryo. Also, rotation of the egg was capable of preventing the characteristic defects associated with irradiation of the uncleaved egg. A combination of these probes was employed, and the results were interpreted in terms of models for the role of the egg surface and the internal cytoplasm in the establishment of the dorsal/ventral polarity of the egg.

The association of primary embryonic organizer activity with the future dorsal side of amphibian eggs and early embryos

Abstract Contrary to previous reports, the capacity to induce a secondary set of axial structures (primary embryonic organizer activity) is not a property of the egg (zygote) cortex. Rather, as embryogenesis proceeds through cleavage division to blastulation, cells on the future dorsal side of the embryo progressively acquire primary organizer activity. That is, dorsal cells appear during embryogenesis to develop the ability to promote morphogenesis of a secondary set of axial structures. When implanted into the blastocoel of normal hosts, or cultured in the presence of competent ectoderm, dorsal cells from blastula- and early gastrula-stage embryos induced secondary axes.

Experimental studies on two mutant genes, r and x, in the Mexican axolotl (Ambystoma mexicanum)

Abstract Mutant genes r and x, discovered in two unrelated axolotl stocks, are simple recessives determining autonomous cell lethal traits. These traits become recognizable by their characteristic gill and limb patterns which appear in each at about the same period of development. The life spans of the two mutants are approximately the same. Larvae homozygous for both mutants are easily recognized by their smaller size, reduced gill development, and unusually small eyes. None of the three mutant phenotypes (r/r, x/x, rr/xx) is benefited by parabiosis with a normal larva. Transplants of the forelimb area from all three usually were soon invaded by tissues of host origin, resulting in limbs ranging from those almost normal to those reduced to functionless stumps. Those from r/r donors produced the highest percentage of useful limbs. Transplants of the gill-forming area produced gills of the mutant type which, in all cases, regressed. Distention and rupture of gill vessels led to death of some animals. In others the gills became reduced to mere stubs or even disappeared. The failure of replacement of pharyngeal structures of mutant graft origin resulted in the death of all grafted animals from vascular accidents or by prevention of normal feeding or respiration.