Determination of Alkaline Phosphatase Expression in Endodermal Cell Lineages of an Ascidian Embryo.

Ciona intestinalis embryos develop a strong histochemical localization of alkaline phosphatase activity in their known endodermal tissues. Such tissues arise solely from the four vegetal blastomeres at the 8-cell stage and six vegetal blastomeres at the 16-cell stage; these vegetal cells inherit an endodermal lineage cytoplasm. Pairs of blastomeres from the bilaterally symmetrical 8- and 16-cell stages were isolated and reared as partial embroys. Only those partial embryos derived from endoderm-containing lineages developed a histochemically localized alkaline phosphatase activity. From the results of such restricted developmental autonomy (self-differentiation), one can deduce that this enzymic expression of endodermal fate could be specified by events of cytoplasmic segregation that occur during the early cleavages. This conclusion offers additional support to the theory that specification of cell fate in ascidian embryos involves an early differential segregation of histodetermining egg cytoplasmic materials.