HISTOSPECIFIC ACETYLCHOLINESTERASE DEVELOPMENT IN QUARTER ASCIDIAN EMBRYOS DERIVED FROM EACH BLASTOMERE PAIR OF THE EIGHT-CELL STAGE

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Abstract
Recent cell lineage studies of ascidian embryos have shown that muscle cells of the larval tail are derived not only from the B4.1-cell pair of 8-cell embryos, as was formerly believed, but also from the b4.2- and A4.1-cell pairs. Therefore, we re-examined the developmental autonomy of blastomere pairs in 8-cell ascidian embryos. The four blastomere-pairs (a4.2, b4.2, A4.1, and B4.1) were isolated from the 8-cell embryos of Ciona intestinalis and Halocynthia roretzi and allowed to develop into quarter embryos. More than 80% of the B4.1 quarter embryos of both species produced histochemically detectable, putative muscle-specific acetylcholinesterase (AChE). About 10% of the Ciona b4.2 quarter embryos and 1% of the Halocynthia b4.2 quarter embryos showed AChE activity. About 2% of the Halocynthia A4.1 quarter embryos developed AChE activity, but none of the Ciona A4.1 quarter embryos showed AChE activity. Although the frequency of the b4.2 or A4.1 quarter embryos with AChE activity was relatively low, these results indicate that not only isolated B4.1 blastomeres but also isolated b4.2 or A4.1 blastomeres could produce AChE independently from the interaction with progeny cells of the other pairs. In addition, about 3-4% of the a4.2 quarter embryos of both species produced AChE. This activity, found in cells thought not to contribute to the muscle cell lineage, may be due to the expression of AChE activity in the larval brain of Ciona and the larval brain and pharynx of Halocynthia.