Comparative ultrastructure of Aurantiactinomyxon and Raabeia, actinosporean stages of Myxozoan life cycles

The ultrastructure of two actinosporean stages, Aurantiactinomyxon sp. (A), and Raabeia (R) forming part of the life cycle of Myxobolus cultusYokoyama, Ogawa & Wakabayashi, 1995 has been studied. In both A and R, binucleate early stages were observed; their proliferation was not detected. In A, intracellular early stages were also found. Earliest stage of pansporocyst formation in R was a uninucleate cell enveloped by an outer one which revealed surface villosities. The latter may be instrumental in facilitating transport of nutrients to sporoblast cells. Cell junctions exist in A and R between pansporocyst cells (gap junctions), attaching vasogenic cells to capsulogenic cells (gap junctions) and between valvogenic cells (septate junctions). The very simplest myxozoan cells represent the “polar bodies” resulting from the meiotic process; in its course, synaptonemal complexes were recorded in R. The structural features, even those little known ones recorded in A and/or R (fibres spaced at 11 nm running on the surface of the immature polar filament, meridional ridges on the upper part of the capsular primordium, the apex of polar capsule consisting of inner dense cone covered by a layer of microtubules, malformations in the development of the capsule, close association of the sporoplasm vegetative nuclei and infective cells, iodinophilous vacuole-like agglomerations of glycogen in the sporoplasm) show the unity of cell structures in myxozoa expressed in the course of the actino-myxosporean alternation. The unique features observed were viral inclusions in A capsulogenic cells, special modifications of RER cisternae and vesicles with stacks of small discs. Infective cells in the sporoplasm can be compared with the inner cells of myxosporean proliferative cycles, both constituting stem cells endowed with extraordinary morphogenetic potential.