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Published March 1998 | public
Journal Article

Late Specification of Veg_1 Lineages to Endodermal Fate in the Sea Urchin Embryo

Abstract

Single blastomeres of the sixth-cleavage veg_1 and veg_2 tiers of Strongylocentrotus purpuratusembryos were labeled with DiI lineage tracer, and the disposition of the progeny was followed through the blastula and gastrula stages in order to determine their respective endodermal and ectodermal contributions. In the endoderm of postgastrula embryos, veg_1-derived cells constituted nearly all of the prospective hindgut and about half of the prospective midgut, while veg_2-derived cells made up the prospective foregut and half the midgut. Oral veg_1 clones consistently contributed more cells to endoderm than aboral veg_1 clones. Oral veg_1 clones extended along the archenteron up to the foregut region, while aboral veg_1 clones contributed only small numbers of hindgut cells but large patches of ectoderm cells that extended out to the prospective larval vertex. The oral/aboral asymmetry in veg_1 allocations was also demonstrated using chimeric embryos, the animal halves of which were labeled with a rhodamine-dextran. Lineages expressing the vegetal plate marker Endo16 were more precisely determined by combining lineage tracer injection with whole-mount in situ hybridization. Endo16 expression was found in all cells that are going to participate in gastrulation. Recruitment of new cells to the Endo16 domain occurs in advance of the actual invagination of those cells. During the blastula stages Endo16 expression expands radially until all cells in the veg_2 lineages express this gene. The first phase of gastrulation, including the normal buckling of the vegetal plate and primary invagination of the archenteron, involves only the Endo16-expressing cells of the veg_2 lineages. As the archenteron begins to elongate, marking the onset of the second phase of gastrulation, there is an asymmetric expansion of Endo16 into the veg_1-derived cells that will contribute to the hindgut and midgut in accordance with lineage tracing observations. The results indicate a relatively late specification of veg_1-derived cells, resulting in late recruitment to the periphery of the vegetal plate territory as gastrulation proceeds. Differential recruitment of veg_1-derived cells on the oral side of the embryo introduces an oral bias to gastrulation by disproportionately increasing the number of cells on the oral side that are competent to participate in gastrulation.

Additional Information

© 1998 Academic Press. Received 1 October 1997, Accepted 19 November 1997. We gratefully acknowledge the helpful reviews of the manuscript by Dr. Andrew Cameron and Dr. James Coffman of California Institute of Technology. We also thank A. Cameron for assisting with the lineage-trace microinjections and the processing of images. This research was supported by the National Institutes of Health Grant HD-05753.

Additional details

Created:
August 19, 2023
Modified:
October 18, 2023