Actin and tubulin in dissociated sponge cells. Evidence for peculiar actin-containing microextensions

The distribution of F-actin and tubulin in dissociate sponge cells of the calcareous sponge Clathrina cerebrum has been shown by staining with fluorescein-labelled phalloidin (F-PHD) and by indirect immunofluorescence employing tubulin antibodies. A peculiar feature of sponge cells is the presence of long thin extensions provided with a filamentous core containing F-actin. These projections are termed scleropodia in view of their remarkable stiffness most likely attributable to a longitudinal skeleton of microfilaments deeply projecting within the cell body. Cells also produce lamellipodia showing ruffles stained by F-PHD. Lamellipodia often contain radial actin-containing segments. The latter are likely to result from the disassembly of the scleropodial axis after microextension retraction. Tubulin is concentrated in the cell body and is absent from microextensions. It forms a fluorescent sheath around the nucleus probably corresponding to a meshwork of microtubules which cannot be individually resolved.