Siliceous products, deposited at the cell surface of amoeboid protists, include a wide variety of species-specific structures; i.e., spicules, scales, solid plates, granules, meshworks frustules, and other elaborate geometric forms. A common secretory mechanism has been reported in testate amoebae, heliozoa and heliozoon-like amoebae, and radiolaria. Silica deposition vesicles (SDVs), either situated in the cell cytoplasm (as in testate amoebae and heliozoa and relatives) or within an expanded portion of the peripheral cytoplasm known as a cytokalymma (in radiolaria), are the site of silicification. In some testate amoebae, moreover, Golgi-derived vesicles fuse with the membrane surrounding silica deposition sites. These vesicles possibly contribute additional silica-secreting membrane into the surface of the SDV while increasing the membrane surface area. Silica products of testate amoebae and heliozoa are deposited on the cell surface by exocytosis. The cytokalymma of radiolaria, while containing a silica-secreting vacuolar space, is decidedly different in form and activity from the intracellular secretory spaces of testate amoebae and heliozoa. The cytokalymma is a dynamic structure exhibiting cytoplasmic flowing activity, and in a mold-like manner determines the remarkable species-specific shape of the skeleton. Consequently, the deposited silicate product of radiolaria is an endoskeleton and is not released on the surface by exocytosis. Further research is needed to determine if Golgi-derived vesicles, designated Golgi-fibrillar vesicles (GFV) in some testate amoebae, are also the source of SDV membranes in other silicate secreting sarcodines.
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