A high-voltage electron microscope was used to study the structure of the centroplast and axonemes in the centrohelidian Heliozoan Raphidiophrys ambigua. In Raphidiophrys the microtubules (Mts) of each axoneme are arranged so that they describe the apices of an array of hexagonally shaped prisms and all axonemes originate from a centroplast located near the centre of the cell. The centroplast was found to consist of 3 components: (1) a tripartite central disk, (2) a sphere of electron-opaque material, approximately 0.75 micron in diameter, which immediately surrounds the central disk, and (3) a slightly less electron-opaque centroplast shell, approximately 2–3 micron in diameter which consists primarily of hitherto undescribed components associated with the proximal 1–2 micron of each axonemal base. The new axonemal components described in this study include: (1) a 20-nm rod positioned in the centre of each hexagonal prism of Mts comprising the axonemal pattern, and (2) an electron-opaque linkage material which connects each Mt of the axonemal pattern to its 4 nearest neighbours. Both of these structures are associated with the base of each axoneme within the centroplast shell, both form crossbridges with axonemal Mts, and both terminate at the periphery of the centroplast shell once the complete axonemal Mt pattern is established. It is suggested that these structures are involved in forming and maintaining the Mt pattern seen in the axopodia of Raphidiophrys.

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