Differential lateral and basal tension drive folding of Drosophila wing discs through two distinct mechanisms

Epithelial folding transforms simple sheets of cells into complex three-dimensional tissues and organs during animal development. Epithelial folding has mainly been attributed to mechanical forces generated by an apically localized actomyosin network, however, contributions of forces generated at basal and lateral cell surfaces remain largely unknown. Here we show that a local decrease of basal tension and an increased lateral tension, but not apical constriction, drive the formation of two neighboring folds in developing Drosophila wing imaginal discs. Spatially defined reduction of extracellular matrix density results in local decrease of basal tension in the first fold; fluctuations in F-actin lead to increased lateral tension in the second fold. Simulations using a 3D vertex model show that the two distinct mechanisms can drive epithelial folding. Our combination of lateral and basal tension measurements with a mechanical tissue model reveals how simple modulations of surface and edge tension drive complex three-dimensional morphological changes.

An actin-like gene can determine cell polarity in bacteria

Achieving proper polarity is essential for cellular function. In bacteria, cell polarity has been observed by using both morphological and molecular markers; however, no general regulators of bacterial cell polarity have been identified. Here we investigate the effect on cell polarity of two cytoskeletal elements previously implicated in cell shape determination. We find that the actin-like MreB protein mediates global cell polarity in Caulobacter crescentus, although the intermediate filament-like CreS protein influences cell shape without affecting cell polarity. MreB is organized in an axial spiral that is dynamically rearranged during the cell cycle, and MreB dynamics may be critical for the determination of cell polarity. By examining depletion and overexpression strains, we demonstrate that MreB is required both for the polar localization of the chromosomal origin sequence and the dynamic localization of regulatory proteins to the correct cell pole. We propose that the molecular polarity inherent in an actin-like filament is translated into a mechanism for directing global cell polarity.

Success factors for the future survival of rural hospitals

Rural hospitals are among those at greatest risk in the changing world of healthcare delivery. There are, however, those that are continuing to thrive despite the odds. The common thread among these are factors found in select Midwest rural hospitals.

Concept of character revisited: an ego-psychological view with some thoughts on the "depressive" character

The concept of character in the psychoanalytic literature is reviewed with reference to differentiation of character and its theoretical derivatives from the concept of neurotic adaptation. Developments in psychoanalytic ego psychology and cognitive psychology are incorporated into a proposed framework for viewing character structure in light of socialization processes and the role of ego functions. This formulation of character more consonant with generalized life-styles is applied to an apparent “type” of character structure and organization at post-genital levels, labeled “depressive character.” Some preliminary implications for tactical issues in the therapy process are noted.