Embryonic cellular organization: Differential restriction of fates as revealed by cell aggregates and lineage markers

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Abstract
Cleavage‐stage Lytechinus variegatus embryos were dissociated and the cells were aggregated in an experimental system designed to address questions of embryonic organizational capability. Using monoclonal antibodies against stage‐ and structure‐specific antigens, it was determined that germ‐layer specific molecules were expressed in the expected locations in aggregates and that the germ layers differentiated in the normal temporal sequence, but the dorsoventral axis of the embryo was not reestablished properly. In other experiments individual rhodamine‐labeled blastomeres were incorporated into unlabeled aggregates. Micromeres localized and differentiated normally. Mesomeres, however, which in normal embryos form only ectoderm, were found to change their specified fate and participate in gut formation.The sequence of aggregate organization revealed other properties of the embryo. Ectodermal and endodermal epithelia formed via two temporally distinct epithelialization events. Ectoderm separated from a mass of interior cells at about 12 hrs, and endoderm compacted from the interior cells at about 20 hr. A lathrytic agent that prevents gut formation in normal embryos also prevented gut formation in aggregates; however, it did not affect formation of the ectoderm. Hence, formation of the triploblastic structure in aggregates appears to be dependent upon developmentally regulated, distinct cell adhesion events.