Cytoplasmic localization and chordamesoderm induction in the frog embryo.

The experiments described here were designed to reveal the distribution in the frog early embryo of components which are sufficient for specification of the dorsal structures of the embryonic body axis. The approach was to allow cleavage planes to divide the embryo into various well-defined regions and to transplant cells from each region into recipient embryos which would otherwise fail to form axial structures. Partial or complete body axis development could then be scored by the use of external criteria or histological methods. Recipients were embryos which had been irradiated before first cleavage with ultraviolet light on the vegetal surface. Irradiated embryos display a well-characterized set of deficiencies in the dorsal structures of the body axis, but their development can be 'rescued' toward normalcy in several ways. In particular, transplantation of certain small groups of blastomeres from the normal 32- to 64-cell embryo into irradiated recipients was sufficient to cause partial or complete axis development. Cell groups which could cause rescue were located in the vegetal and equatorial levels of one quadrant of the normal embryo--the quadrant centered on the future dorsal midline. Clonal marking analysis showed that the vegetal-most cells of this quadrant contribute primarily to endodermal structures in normal development. In rescued recipient embryos, these cells also contributed only to the endoderm; the dorsal mesoderm and central nervous system were formed exclusively by host cells which originated near the transplant. Rescue could also result from transplantation of equatorial cells from the dorsal quadrant of the normal embryo. As in normal development, these cells formed primarily the chordamesoderm of the rescued embryo. Host cells were induced to contribute the somitic mesoderm, central nervous system, and other structures which would have been missing but for the presence of the transplanted cells. The frequency and degree of rescue caused by equatorial and vegetal transplants is variable. This was explained by the discovery that the location of components needed for rescue varies among individual embryos without regard to the positions of cleavage planes. This was true even when donor embryos were selected on the basis of a precisely regular pattern of cleavage. In such selected embryos, particular blastomeres make a predictable contribution of progeny to the body axis. Thus it may be that the positions of components which can cause axis formation vary without exact regard to the fate map of prospective areas. The implications of this for the study of cytoplasmic localization in the early embryo are discussed.(ABSTRACT TRUNCATED AT 400 WORDS)