Establishment of segment polarity in the ectoderm of the leech Helobdella

The segmented ectoderm and mesoderm of the leech arise via a stereotyped ce ll lineage from embryonic stem cells called teloblasts, Each teloblast give s rise to a column of primary blast cell daughters, and the blast cells gen erate descendant clones that serve as the segmental repeats of their partic ular teloblast lineage. We have examined the mechanism by which the leech p rimary blast cell clones acquire segment polarity - i.e. a fixed sequence o f positional values ordered along the anteroposterior axis of the segmental repeat. In the O and P teloblast lineages, the earliest divisions of the p rimary blast cell segregate anterior and posterior cell fates along the ant eroposterior axis. Using a laser microbeam, we ablated single cells from bo th o and p blast cell clones at stages when the clone was two to four cells in length, The developmental fate of the remaining cells was characterized with rhodamine-dextran lineage tracer. Twelve different progeny cells were ablated, and in every case the ablation eliminated the normal descendants of the ablated cell while having little or no detectable effect on the deve lopmental fate of the remaining cells. This included experiments in which w e specifically ablated those blast cell progeny that are known to express t he engrailed gene, or their lineal precursors. These findings confirm and e xtend a previous study by showing that the establishment of segment polarit y in the leech ectoderm is largely independent of cell interactions conveye d along the anteroposterior axis, Both intercellular signaling and engraile d expression play an important role in the segment polarity specification o f the Drosophila embryo, and our findings suggest that there may be little or no conservation of this developmental mechanism between those two organi sms.