摘要：

During early embryogenesis of Caenorhabditis elegans the serial stem cell-like cleavages of the germ line cells P 0-P 3 generate a number of somatic founder cells with different developmental potentials. Observations on partial embryos show that in the first two of these unequal divisions in the germ line the somatic daughter cell comes to lie anterior to the new germ line cell. In the following two, however, the somatic daughter cell comes to lie posterior to the new germ line cell, suggesting a reversal of polarity in the germ line. By the use of a laser microbeam, egg fragments can be extruded from young embryos; the fragments often cleave like partial twins. Depending on whether the fragment is derived from the posterior region of the uncleaved zygote P 0 or its daughter P 1, the mirror image duplications that are generated are joined at their larger soma-like cells or at their smaller germ line-like cells, respectively. This result is best explained as a reversal of polarity taking place in the germ line cell P 2. This notion is strengthened by the finding that partial embryos derived from the posterior region of the P 2 cell in late interphase do not undergo stem cell-like (i.e., unequal) cleavages in contrast to those derived from P 0 or P 1. Finally, an apparent early cell-cell interaction is described which is inconsistent with the classical notion of "mosaic" nematode development: removal of the germline cell P 2 results in an altered developmental pattern of its somatic sister cell EMS. A working model is presented linking reversal of polarity and cell-cell interaction and offers an explanation for the unique behavior of the EMS cell in normal development.

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