The onset of phagocytosis and identity in the embryo of Lytechinus variegatus.

The stage at which phagocytosis can first be characterized in the embryos of the sea urchin Lytechinus variegatus was investigated by microinjecting the yeast Saccharomyces cerevisiae into its blastocoele. Secondary mesenchyme cells were first observed phagocytosing during the mid-gastrula stage. Subsequently, as the incubation time increased, the number of yeast per phagocyte rose. Using vital fluorescence dyes, stained free yeast were seen in the blastocoele during late-gastrula stage, indicating cell death and suggesting specific factors, such as proteases, in the extracellular environment. The starting point of phagocytic activity reflects a biological capacity for distinguishing between self and nonself. Thus, the phagocytosis of yeast by mesenchymal cells beginning in the mid-gastrula stage in L. variegatus may indicate the moment of acquisition of 'identity' (self) in this organism. Comparative aspects of embryo and adult phagocytes in L. variegatus are also discussed.