ANTEROPOSTERIOR SEGREGATION OF SUPERFICIAL AND DEEP CELLS DURING GASTRULATION IN PLEURODELES WALTL AND RANA-PIPIENS EMBRYOS

Segregation of dorsal superficial and deep cells was analyzed in early Pleurodeles waltl (urodele) and Rana pipiens (anuran) gastrulae in or der to gain new comparative insights into anterior and posterior speci fication of mesodermally fated cells in species other than Xenopus. We used a novel technique of orthotopic transplantation of double-labele d grafts into defined regions. This technique involves labeling all ce lls in grafts with a rhodamine-lysinated dextran (RLDx) cell lineage t racer (by injecting uncleaved eggs) and subsequently labeling superfic ial cells with I-125. Small defined grafts of the dorsal marginal zone (DMZ) were placed orthotopically into unlabeled host embryos. RLDx wa s also used to label individual superficial cells in the early gastrul a dorsal midline region as well as large blastomeres in the presumptiv e DMZ at the 32-cell stage. Labeled progeny were studied in gastrulae and neurulae. The distribution of dorsal mesodermal cells depends on s egregation between superficial and deep cells which occurs along the a nteroposterior axis during involution in both species studied here. De ep cells are the first to involute. Then they divide into two subpopul ations during gastrulation. The first anterior subpopulation migrates with only a few superficial cells. The second, more posterior, subpopu lation involutes later and codistributes with most of the superficial cell population. Our results indicate that at the beginning of gastrul ation, much greater mixing between superficial and deep cells occurs i n Pleurodeles than in Rana. These results may have bearing on studies of the mechanism of anteroposterior specification of amphibian mesoder m. Differences in the timing and topology of expression of cell:cell a nd cell:substrate adhesion molecules between these amphibian species i n early development could account for observed differences in morphoge netic cell movements. (C) 1996 Wiley-Liss, Inc.