CYTOSKELETAL ARCHITECTURE AND ORGANELLE TRANSPORT IN GIANT SYNCYTIA FORMED BY FUSION OF HEXACTINELLID SPONGE TISSUES

Dissociated tissue from the hexactinellid sponge Rhabdocalyptus dawson i adheres to coated substrates and aggregates by the fusion of tissue pieces to form a giant syncytium. Video microscopy shows that the piec es contact each other by way of lamellipodia or filopodia. Fusion, cor roborated by evidence of dye spread, occurs about 1 hour after plating and is characterized by two-way transport of individual organelles, i ncluding nuclei, at an average rate of 2.15 mu m . s(-1), and bulk str eaming of cytoplasm at an average velocity of 1.72 mu m . s(-1). In th e cellular sponge Haliclona, by contrast, dye does not spread through aggregates and no streaming can be seen. That transport in Rhabdocalyp tus is microtubule-based is indicated by the reversible inhibition of streaming caused by colcemid and nocodazole. Immunofluorescence and el ectron microscopy reveal an extensive network of microtubule bundles w ithin the aggregates. The cytoskeleton also includes microfilament bun dles that traverse aggregates and run around the periphery and giant, actin-dense rods that extend from the edges. Cytochalasin B reversibly disrupts the microfilamentous framework without blocking streaming. I n contrast to demosponges where the cytoskeleton is organized on the b asis of individual cells, in hexactinellids it provides a supporting f ramework and transport pathways within vast, multinucleate tissue mass es. If we take this preparation as a model for tissue organization in the intact sponge, these findings support the view that hexactinellids are syncytial organisms, probably the largest in the animal kingdom, and suggest that food products may be distributed through the sponge i ntracellularly rather than by wandering amoebocytes. The findings stre ngthen the case for establishing the Hexactinellida as a subphylum wit hin the Porifera.