MESENDODERM CELL AND ARCHENTERON FORMATION IN ISOLATED BLASTOMERES FROM THE SHRIMP SICYONIA-INGENTIS

The fate map of 2- and 4-cell-stage Sicyonia ingentis embryos was dete rmined by microinjection of lysyl-rhodamine-dextran into single blasto meres. Microinjected embryos were cultured to the limb bud stage, when the body plan of the nauplius larva was evident, The animal blastomer e, AB, gave rise to anterior ectoderm, while the vegetal blastomere, C D, gave rise to posterior structures, including the invagination site during gastrulation. The A blastomere gave rise to mirror-image patter ns of dorsal-lateral ectoderm, while the B blastomere gave rise to ant erior, ventral ectoderm. The C blastomere gave rise to posterior, dors al-lateral ectoderm, complementary to the A pattern, as well as some n aupliar mesoderm. The D blastomere gave rise to mesendoderm, naupliar mesoderm, and some posterior ectoderm. To study the specification of t he early blastomeres, they were microsurgically separated and cultured in isolation. Two mesendoderm cells formed in 1/2 , 1/4, 1/8, and 1/1 6 blastomeres in embryos dissociated at the 2-, 4-, 8-, and 16-cell st ages, respectively. CD and D blastomeres could be distinguished by the ir larger size and gave rise to the mesendoderm cells. Archenteron for mation and elongation of the embryo occurred in CD but not in AB isola tes. Isolated blastomeres were recombined in various ways to determine whether their state of commitment could be altered in different cellu lar environments. Duplicated mesendoderm cells and archenterons formed in CD + CD recombinations, while AB + AB recombinations formed blastu lae but did not produce mesendoderm cells and did not invaginate. The normal number of mesendoderm cells and a single archenteron formed in D + AB recombinations, while C + AB recombinations remained as blastul ae and did not form mesendoderm cells. The results suggest that the me sendoderm cells are autonomously specified, possibly by cytoplasmic lo calization at the vegetal pole. The mesendoderm may also function as a signaling region to organize other developmental events. (C) 1994 Aca demic Press, Inc.