Cytoskeletal Architecture and Organelle Transport in Giant Syncytia Formed by Fusion of Hexactinellid Sponge Tissues.

Dissociated tissue from the hexactinellid sponge Rhabdocalyptus dawsoni adheres to coated substrates and aggregates by the fusion of tissue pieces to form a giant syncytium. Video microscopy shows that the pieces contact each other by way of lamellipodia or filopodia. Fusion, corroborated by evidence of dye spread, occurs about 1 hour after plating and is characterized by two-way transport of individual organelles, including nuclei, at an average rate of 2.15 {mu}m.s-1, and bulk streaming of cytoplasm at an average velocity of 1.72 {mu}m . s-1. In the cellular sponge Haliclona, by contrast, dye does not spread through aggregates and no streaming can be seen. That transport in Rhabdocalyptus is microtubule-based is indicated by the reversible inhibition of streaming caused by colcemid and nocodazole. Immunofluorescence and electron microscopy reveal an extensive network of microtubule bundles within the aggregates. The cytoskeleton also includes microfilament bundles that traverse aggregates and run around the periphery and giant, actindense rods that extend from the edges. Cytochalasin B reversibly disrupts the microfilamentous framework without blocking streaming. In contrast to demosponges where the cytoskeleton is organized on the basis of individual cells, in hexactinellids it provides a supporting framework and transport pathways within vast, multinucleate tissue masses. 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 intracellularly rather than by wandering amoebocytes. The findings strengthen the case for establishing the Hexactinellida as a subphylum within the Porifera.