The fine structure of the centrohelidian heliozoan Heterophrys marina.

The fine structure of Heterophrys marina (Centrohelidia, Heliozoa) is described with special reference to centroplast structure, morphogenesis and "behavior" of kinetocysts (= axopodial granules which perform saltatory movement), and formation of organic spicules in a new type of organelle located in the plasma membrane. A low calcium pretreatment and fixation was used to improve preservation of highly labile axopodia which near their distal end contain a single microtubule (MT) only. Two varieties of H. marina with a respective maximum of 6 and 12 MTs per axopodium, and 2 hitherto undescribed species, H. elati and H. multipoda, were found among 9 stocks collected in Europe and North America. In all species only the central 6 MTs of each axoneme originate from a scaffolding layer of electron dense material which surrounds the central granule. Evidence is presented which indicates that in Heterophrys self-linkage is not the only principle of MT pattern generation but that instead precisely localized MT nucleation and specific linkage of MTs within the cortex of the centroplast lead to the MT patterns observed. Prekinetocysts originate from vesicles found in the neighborhood of the dictyosomes. After maturation the kinetocysts become attached to the plasma membrane which seems to play an important role both in selection of particles transported in the axopodia and particle movement as well.