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We present a new efficient leaf sequencing algorithm for the generation of intensity maps by a nonnegative combination of segments. Intensity maps describe the intensity modulation of beams in radiotherapy. We only study the static case (step and shoot). We exactly optimize the total number of monitor units and heuristically optimize the number of segments. We present a short exact proof for a formula giving the smallest total number of monitor units and describe a class of algorithms yielding this minimal value. A special member of this class provides a solution with a very small number of segments.

Abstract. The apex of hair cells of the chicken auditory organ contains three different kinds of assemblies of actin filaments in close spatial proximity. These are (a) paracrystals of actin filaments with identical polarity in stereocilia, (b) a dense gellike meshwork of actin filaments forming the cutieular plate, and (c) a bundle of parallel actin filaments with mixed polarities that constitute the circumferential filament belt attached to the cytoplasmic aspect of the zonula adhaerens (ZA). Each different supramolecular assembly of actin filaments contains a specific actin filament crosslinking protein which is unique to that particular assembly. Thus fimbrin appears to be responsible for paracrystallin packing of actin filaments in stereocilia;an isoform of speetrin resides in the cuticular plate where it forms the whisker-like crossbridges, and a actinin is the actin crosslinking protein of the circumferential ZA bundle. Tropomyosin, which stabilizes actin filaments, is present in all the actin filament assemblies except for the stereocilia. Another striking finding was that myosin appears to be absent from the ZA ring and cuticular plate of hair cells although present in the ZA ring of supporting cells. The abundance of myosin in the ZA ring of the surrounding supporting cells means that it may be important in forming a supporting tensile cellular framework in which the hair cells are inserted.Oth E of the most fascinating features of cells is that at e same moment in time a cell can carry out a diverse array of activities, most of which involve the cytoskeleton. For example, a cell can maintain static surface projections or microvilli, nearby it can phagocytose particles, and at another location it can move by ameboid motion. Thus, an important problem in cell biology is how the cell controls different assemblies of actin filaments. We have chosen the hair cell of the cochlea as a model system that may help us to understand how different actin assemblies are formed and maintained in the same cell at the same time.Located at the apical end of hair cells in the chick inner ear are three different assemblies of actin filaments. One is a paracrystalline bundle of actin filaments present in each sterexxfilium. These filaments are parallel, have identical polarities, and are maximally crossbridged together (Tilney et al., 1983). The second, the cuticular plate, is a complex actin gel located immediately beneath the stereocilia. Here the actin filaments are randomly oriented relative to each other, yet connected together by tiny whiskerlike strands of proteinaceous material (DeRosier and Tilney, 1989). The third is a circumferential belt of actin filaments encircling the apicolateral margins of the cell at the zonula adhaerens (ZA) 1. The filaments that make up this belt, although paral-1. Abbreviations used in this paper: F-actin, filamentous actin; ZA, zonula adhaerens.lel, are of mixed polarities (Hirokawa and Tflney, 1982). Since these three different assemblies of actin filaments all occur in the same cell...

Microtexturing of implant surfaces is of major relevance in the endeavor to improve biorelevant implant designs. In order to elucidate the role of biomaterial’s topography on cell physiology, obtaining quantitative correlations between cellular behavior and distinct microarchitectural properties is in great demand. Until now, the microscopically observed reorganization of the cytoskeleton on structured biomaterials has been difficult to convert into data. We used geometrically microtextured silicon-titanium arrays as a model system. Samples were prepared by deep reactive-ion etching of silicon wafers, resulting in rectangular grooves (width and height: 2 µm) and cubic pillars (pillar dimensions: 2 × 2 × 5 and 5 × 5 × 5 µm); finally sputter-coated with 100 nm titanium. We focused on the morphometric analysis of MG-63 osteoblasts, including a quantification of the actin cytoskeleton. By means of our novel software FilaQuant, especially developed for automatic actin filament recognition, we were first able to quantify the alterations of the actin network dependent on the microtexture of a material surface. The cells’ actin fibers were significantly reduced in length on the pillared surfaces versus the grooved array (4–5 fold) and completely reorganized on the micropillars, but without altering the orientation of cells. Our morpho-functional approach opens new possibilities for the data correlation of cell-material interactions.