Competence of blastomeres for the expression of molecular tissue markers is acquired by diverse mechanisms in the embryo of Platynereis (Annelida).

This paper is devoted to the role of cell divisions for the establishment of histospecificity in the embryo of the spiralian, Platynereis dumerilii (Annelida). We have incubated successive cleavage stages in cytochalasin B (CCB) and observed whether the cells thereafter were able to acquire the competence for expressing histospecific antigens of larval gland cells (labelled by the monoclonal antibody OI64) and of neural components of the ventral nerve cord (labelled by mAb OI7 or by testing acety1cholinesterase activity), respectively. Incubation in CCB results in permanent cleavage arrest, but does not necessarily interfere with biochemical differentiation of such markers. Synthesis of the differentiation marker specific for larval gland cells does not require any cleavages but this capacity becomes restricted to the 1a and 1b cell lines if cleavages are allowed to occur. In contrast, the progenitors of neural cells need at least 6.5 h of normal development before they acquire the competence to synthesize two neural differentiation markers, which can be demonstrated after at least two more days of development. Thus, prespecifying and diversifying cleavages are a prerequisite for neurogenesis and production of the investigated neural markers. Competence for the expression of histospecific antigens may also depend on cell-cell interactions. If 20-24 h old embryos are treated with puromycin, pioneering fibres fail to grow out from a pair of posterior nerve cell progenitors as they would have done normally 24-48 h after fertilization. Concomitantly, a number of potential nerve cells which now do not come into contact with pioneering fibres do not express the neural antigen. This suggests that a local inductive stimulus from the pioneering fibres normally imprints cell fate onto ventral plate cells and turns them into neuroblasts.