The Formation of Siliceous Scales by Raphidiophrys Ambigua (Protista, Centroheliozoa)

Ultrastructural aspects of the formation of siliceous scales by the protozoon Raphidiophrys ambigua are described. Cells in culture become scale-free in silicon-impoverished medium. Scales may be seen adhering to the cell surface within four to six hours of the addition of silicon to the medium. Silica scale formation occurs within deposition vesicles (SDVs) near the periphery of the cell. Many scales are formed at the same time; each may be in a different phase of scale formation. The source of the SDV membrane is not known. Silicification proceeds centrifugally from two pattern centres. The first parts of the forming scale are two opposed sterna, which establish the longitudinal axis of the scale. One sternum develops from each pattern centre. The scale develops outwards by formation of fairly evenly spaced lateral ribs from each central sternum. Distally, the ribs branch to form a reticulate pattern. A thin continuous sheet of silicon is added to the periphery of the scale, curving inwards to form a rim. The development of this pattern may be described in terms of a small number of morphogenetic processes.