Cellular Differentiation in Submerged Monolayers of Dictyostelium discoideum: Possible Functions of Cytoplasmic Ca

Cytoplasmic calcium ion (Ca2+ ) has generally been proposed to be a key factor of numerous cellular processes. Among several agents which might be expected to alter cytoplasmic Ca2+ -concentration ([Ca2+ ]i ), unexpectedly Ca2+ -antagonist TMB-8 was found to raise considerably [Ca2+ ]i , and inhibited not only the formation of prespore cells, but also their maintenance in the monolayer cultures of Dictyostelium discoideum. This seems to indicate that higher [Ca2+ ]i is unfavorable to the prespore differentiation. In this study, we adopted the monolayer culture technique to monitor cell differentiation. However, in high density monolayers there arised a number of unique cells which was highly vacuolated and morphologically intermediate between the stalk and spore cells. These vacuolated cells having both cellulosic wall and spore coat were also induced by differentiation inducing factor (DIF). Thus the monolayer culture system used might be not necessarily qualified to monitor the terminal differentiation of Dictyostelium cells. Nevertheless, the data presented here have strongly suggested that DIF have two physiologically valued roles: 1) Induction of the membrane fusion of vesicles and/or vacuoles (vacuolization), and 2) Induction of the fusion between the cell membrane and vacuole (or vesicle) membrane (exocytosis).