The mitochondrial compartment of budding yeast (Saccharomyces cerevisiae) is a highly dynamic net-like structure of tubules that constantly undergo fusion and fission. The outer membrane protein Fis1p plays a crucial role in mitochondrial fission. Here we report on the temporal and spatial dynamics of this organelle in wild-type cells and in fis1Δ mutants. Mitochondria of fis1Δ mutants adapt their mitochondrial network to a change in carbon source. We find that the frequencies of apparent matrix separation and fusion events decrease in both wild-type cells and in mutants lacking Fis1p upon glucose repression. Matrix separation could be caused by matrix constriction and does not necessarily require fission of the inner or outer membrane. Double-labelling experiments demonstrated that some of these matrix separations in fis1 mutants are due to genuine tubule fissions, whereas others do not involve fission of the outer membrane. The rates of matrix separation in fis1Δ mutants almost approach those of the wildtype,demonstrating that, although apparently involved in outer membrane fission,Fis1p is not crucial for the separation of the mitochondrial matrix. In mutants lacking the GTPase Dnm1p no complete tubule fissions were recorded,although dnm1Δ mutants display matrix separations as well. The data suggest that different molecular machineries are responsible for the separation of the matrix and the fission of the outer membrane in budding yeast.

SummaryHigh-resolution light microscopy of glycerol-mounted biological specimens is performed almost exclusively with oil immersion lenses. The reason is that the index of refraction of the oil and the cover slip of ~1.51 is close to that of ~1.45 of the glycerol mountant, so that refractive index mismatchinduced spherical aberrations are tolerable to some extent. Here we report the application of novel cover glass-corrected glycerol immersion lenses of high numerical aperture (NA) and the avoidance of these aberrations. The new lenses feature a semi-aperture angle of 68.5°, which is slightly larger than that of the diffraction-limited 1.4 NA oil immersion lenses. The glycerol lenses are corrected for a quartz cover glass of 220 µm thickness and for a 80% glycerol-water immersion solution. Featuring an aberration correction collar, the lens can adapt to glycerol concentrations ranging between 72% and 88%, to slight variations of the temperature, and to the cover glass thickness. As the refractive index mismatch-induced aberrations are particularly important to quantitative confocal fluorescence microscopy, we investigated the axial sectioning ability and the axial chromatic aberrations in such a microscope as well as the image brightness as a function of the penetration depth. Whereas there is a significant decrease in image brightness associated with oil immersion, this decrease is absent with the glycerol immersion system. In addition, we show directly the compression of the optic axis in the case of oil immersion and its absence in the glycerol system. The unique advantages of these new lenses in high-resolution microscopy with two coherently used opposing lenses, such as 4 Pi-microscopy, are discussed.

The praesoma of the acanthocephalan parasite Paratenuisentis ambiguus was studied at the light and the electron microscope level, with special reference to the lateral sense organs and the musculature, in order to substantiate the basal pattern of the Acanthocephala and to analyse the phylogeny of the taxon. The study includes the first ultrastructural description of a lateral sense organ in the Acanthocephala. Two sensory support cell ducts extend from the binucleate pericaryon of the sensory support cell to the lateral sense organs. On their way to the lateral sense organs the ducts penetrate the receptacle and join the anterior ventral nerves. Each lateral sense organ consists of a conical termination of one of the sensory support cell ducts, in which the neuronal fibres and dendritic terminations of the equilateral anterior ventral nerve are embedded. An analysis of the available data of praesomal sense organs in Acanthocephala suggests that lateral and apical sense organs are absent in the basal pattern of the Acanthocephala. It is likely that two lateral sense organs, a binucleate sensory support cell with two ducts and two anterior ventral nerves evolved within the stem-line of some Palaeacanthocephala, all Eoacanthocephala and all Archiacanthocephala, whereas two apical sense organs, a quadrinucleate sensory support cell with four ducts and two apical sensory nerves presumably represent an autapomorphic character of the Archiacanthocephala. Furthermore, it can be derived from data in the literature and the present study that the praesomal hooks are totally covered by epidermis in the basal pattern of the Acanthocephala, whereas the ontogenetic loss of the epidermal covering can be regarded as an autapomorphy of the Archiacanthocephala.

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