Lineage-specific segregation of maternal mRNA

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The cell lineages of the second (2d) and fourth (4d) micromeres of the D-cell line of the oligochaete annelid Tubifex embryo were examined by means of intracellular microinjection of horseradish peroxidase (HRP) and a lipophilic tracer Dil. 2d produces four bilateral pairs of teloblasts (N,O,P and Q) , and 4d gives rise to a bilateral pair of M-teloblasts. The germ bands derived form teloblasts N-Q contribute to epidermal cells, ventral nerve cord, peripheral neurons and dorsal and ventral setal sacs. The germ bands derived from M-teloblasts contribute to nearly all of mesodernal tissues, viz., circular and longitudinal muscles, nephridia, primordial germ cells and coelomic walls including blood vessels ; a few cells located at the center of each segmental ganglion are also derived from these embryonic regions. Alkaline phosphatase activity was specifically expressed in mesodermal cells at the 7th and 8th segments. Cell ablation experiments in combination with Dillabeling suggested that segmental identity is determined at the birth of m-blast cells. These results confirm some of Penners' (1924) inference, but have uncovered several important points that should be corrected. To examine whether 2d and 4d inherit specific maternal messages, subtraction experiments were performed between intact embryos and those from which 2d and 4d had been ablated. To date, any significant molecules have not been found.