ULTRASTRUCTURE OF SPORE DIFFERENTIATION IN DICTYOSTELIUM - PRESPORE VACUOLE

Electron microscopy of spore differentiation in Dictyostelium discoideum and other cellular slime molds reveals the presence of unusual vacuoles in the prespore cells. These vacuoles, termed prespore vacuoles, are lined with a two-layered, inner coat of material that will at a later stage form the initial layers of the spore coat. The size and number of the prespore vacuoles increases during early culmination, and they then move to the periphery of the cells, where they fuse to the plasma membrane. The area of contact between the plasma membrane and the vacuoles disintegrates, and the contents of the vacuoles are released to the outside, while the membranes of the vacuoles become continuous with and incorporated into the plasma membrane. The inner two-layered coating of the prespore vacuoles now faces the outside of the cell and, as more prespore vacuoles arrive at the cell periphery, this coat becomes continuous over the whole cell surface and forms the intitial spore coat. As a last step of spore differentiation, the coat thickens by expansion of the inner one of the two layers. The mature spore has a three-layered coat, a cytoplasm densely packed with granules, a nucleus with a nucleolus, crenated mitochondria, polyvesicular bodies, crystals, and small, electron transparent vacuoles. The significance of the three major functions of the prespore vacuole in secretion, spore coat formation, and plasma membrane formation is discussed. Barium permanganate is shown to be a useful fixative for preservation of the prespore cells. © 1969 Academic Press, Inc.