Metabolism of oocyte construction and the generation of Histospecificity in the cleaving egg. lessons from nereid annelids

The growing oocyte and the developing egg of nereid polychaetes are easily accessible to observation and experimental work, a precondition for our research. In preparation for a single semelparous act of reproduction, nereid females reutilize somatic biomass for the synchronized production of numerous oocytes. To keep oogenesis going somatic resources become recycled by the eleocytes and are supplied to the oocytes in form of vitellogenin and nucleotides (among other identified and yet unidentified substances). Both oocytes and eleocytes are free-floating coelomic cells. We postulate that availability of metabolites produced by the eleocytes might suffice to drive synchronous oocyte growth. The cortex of the fully differentiated oocyte contains numerous cortical granules which after fertilization empty by exocytosis thus causing a profound structural reorganization of the zygote cortex. Early development of nereids is extremely constant in time and spatial pattern and from the onset cleavages create diversity among the blastomeres. We have documented a correlation between the quality and amount of cytoplasm, the cell cycle duration and the histogenetic fate of such blastomeres. Experimental change of cytoplasmic proportions of early cleavage cells has serious consequences for axial development. Using a number of differentiation markers we were able to analyze the necessity of certain cleavage steps for the acquisition of the determined state