Role of Target Geometry in Phagocytosis Champion, Julie A.; Mitragotri, Samir
Proceedings of the National Academy of Sciences,
03/2006, Volume:
103, Issue:
13
Journal Article
Peer reviewed
Open access
Phagocytosis is a principal component of the body's innate immunity in which macrophages internalize targets in an actin-dependent manner. Targets vary widely in shape and size and include particles ...such as pathogens and senescent cells. Despite considerable progress in understanding this complicated process, the role of target geometry in phagocytosis has remained elusive. Previous studies on phagocytosis have been performed using spherical targets, thereby overlooking the role of particle shape. Using polystyrene particles of various sizes and shapes, we studied phagocytosis by alveolar macrophages. We report a surprising finding that particle shape, not size, plays a dominant role in phagocytosis. All shapes were capable of initiating phagocytosis in at least one orientation. However, the local particle shape, measured by tangent angles, at the point of initial contact dictates whether macrophages initiate phagocytosis or simply spread on particles. The local shape determines the complexity of the actin structure that must be created to initiate phagocytosis and allow the membrane to move over the particle. Failure to create the required actin structure results in simple spreading and not internalization. Particle size primarily impacts the completion of phagocytosis in cases where particle volume exceeds the cell volume.
The mitotic spindle must function in cell types that vary greatly in size, and its dimensions scale with the rapid, reductive cell divisions that accompany early stages of development. The mechanism ...responsible for this scaling is unclear, because uncoupling cell size from a developmental or cellular context has proven experimentally challenging. We combined microfluidic technology with Xenopus egg extracts to characterize spindle assembly within discrete, geometrically defined volumes of cytoplasm. Reductions in cytoplasmic volume, rather than developmental cues or changes in cell shape, were sufficient to recapitulate spindle scaling observed in Xenopus embryos. Thus, mechanisms extrinsic to the spindle, specifically a limiting pool of cytoplasmic component(s), play a major role in determining spindle size.
The relationship between morphometrics and phylogenetic analysis has long been controversial. Here we propose an approach that is based on mapping morphometric traits onto phylogenies derived from ...other data and thus avoids the pitfalls encountered by previous studies. This method treats shape as a single, multidimensional character. We propose a test for the presence of a phylogenetic signal in morphometric data, which simulates the null hypothesis of the complete absence of phylogenetic structure by permutation of the shape data among the terminal taxa. We also propose 2 measures of the fit of morphometric data to the phylogeny that are direct extensions of the consistency index and retention index used in traditional cladistics. We apply these methods to a small study of the evolution of wing shape in the Drosophila melanogaster subgroup, for which a very strongly supported phylogeny is available. This case study reveals a significant phylogenetic signal and a relatively low degree of homoplasy. Despite the low homoplasy, the shortest tree computed from landmark data on wing shape is inconsistent with the well-supported phylogenetic tree from molecular data, underscoring that morphometric data may not provide reliable information for inferring phylogeny.
We present a lens-free optical tomographic microscope, which enables imaging a large volume of approximately 15 mm³ on a chip, with a spatial resolution of < 1 μm x < 1 μm x < 3 μm in x, y and z ...dimensions, respectively. In this lens-free tomography modality, the sample is placed directly on a digital sensor array with, e.g., less-than or equal to 4 mm distance to its active area. A partially coherent light source placed approximately 70 mm away from the sensor is employed to record lens-free in-line holograms of the sample from different viewing angles. At each illumination angle, multiple subpixel shifted holograms are also recorded, which are digitally processed using a pixel superresolution technique to create a single high-resolution hologram of each angular projection of the object. These superresolved holograms are digitally reconstructed for an angular range of ± 50°, which are then back-projected to compute tomograms of the sample. In order to minimize the artifacts due to limited angular range of tilted illumination, a dual-axis tomography scheme is adopted, where the light source is rotated along two orthogonal axes. Tomographic imaging performance is quantified using microbeads of different dimensions, as well as by imaging wild-type Caenorhabditis elegans. Probing a large volume with a decent 3D spatial resolution, this lens-free optical tomography platform on a chip could provide a powerful tool for high-throughput imaging applications in, e.g., cell and developmental biology.
The integrated information theory (IIT) starts from phenomenology and makes use of thought experiments to claim that consciousness is integrated information. Specifically: (i) the quantity of ...consciousness corresponds to the amount of integrated information generated by a complex of elements; (ii) the quality of experience is specified by the set of informational relationships generated within that complex. Integrated information (Φ) is defined as the amount of information generated by a complex of elements, above and beyond the information generated by its parts. Qualia space (Q) is a space where each axis represents a possible state of the complex, each point is a probability distribution of its states, and arrows between points represent the informational relationships among its elements generated by causal mechanisms (connections). Together, the set of informational relationships within a complex constitute a shape in Q that completely and univocally specifies a particular experience. Several observations concerning the neural substrate of consciousness fall naturally into place within the IIT framework. Among them are the association of consciousness with certain neural systems rather than with others; the fact that neural processes underlying consciousness can influence or be influenced by neural processes that remain unconscious; the reduction of consciousness during dreamless sleep and generalized seizures; and the distinct role of different cortical architectures in affecting the quality of experience. Equating consciousness with integrated information carries several implications for our view of nature.
Theoretical models have long pointed to the dentate gyrus as a possible source of neuronal pattern separation. In agreement with predictions from these models, we show that minimal changes in the ...shape of the environment in which rats are exploring can substantially alter correlated activity patterns among place-modulated granule cells in the dentate gyrus. When the environments are made more different, new cell populations are recruited in CA3 but not in the dentate gyrus. These results imply a dual mechanism for pattern separation in which signals from the entorhinal cortex can be decorrelated both by changes in coincidence patterns in the dentate gyrus and by recruitment of nonoverlapping cell assemblies in CA3.
The unicellular green alga Chlamydomonas swims with two flagella that can synchronize their beat. Synchronized beating is required to swim both fast and straight. A long-standing hypothesis proposes ...that synchronization of flagella results from hydrodynamic coupling, but the details are not understood. Here, we present realistic hydrodynamic computations and high-speed tracking experiments of swimming cells that show how a perturbation from the synchronized state causes rotational motion of the cell body. This rotation feeds back on the flagellar dynamics via hydrodynamic friction forces and rapidly restores the synchronized state in our theory. We calculate that this “cell-body rocking” provides the dominant contribution to synchronization in swimming cells, whereas direct hydrodynamic interactions between the flagella contribute negligibly. We experimentally confirmed the two-way coupling between flagellar beating and cell-body rocking predicted by our theory.
Modelling natural and artificial hands with synergies Bicchi, Antonio; Gabiccini, Marco; Santello, Marco
Philosophical transactions of the Royal Society of London. Series B. Biological sciences,
11/2011, Volume:
366, Issue:
1581
Journal Article
Peer reviewed
Open access
We report on recent work in modelling the process of grasping and active touch by natural and artificial hands. Starting from observations made in human hands about the correlation of degrees of ...freedom in patterns of more frequent use (postural synergies), we consider the implications of a geometrical model accounting for such data, which is applicable to the pre-grasping phase occurring when shaping the hand before actual contact with the grasped object. To extend applicability of the synergy model to study force distribution in the actual grasp, we introduce a modified model including the mechanical compliance of the hand's musculotendinous system. Numerical results obtained by this model indicate that the same principal synergies observed from pre-grasp postural data are also fundamental in achieving proper grasp force distribution. To illustrate the concept of synergies in the dual domain of haptic sensing, we provide a review of models of how the complexity and heterogeneity of sensory information from touch can be harnessed in simplified, tractable abstractions. These abstractions are amenable to fast processing to enable quick reflexes as well as elaboration of high-level percepts. Applications of the synergy model to the design and control of artificial hands and tactile sensors are illustrated.
Attention to Information in Working Memory Oberauer, Klaus; Hein, Laura
Current directions in psychological science : a journal of the American Psychological Society,
06/2012, Volume:
21, Issue:
3
Journal Article
Peer reviewed
Working memory retains information and makes it available for processing. People often need to hold several chunks of information available while concentrating on only one of them. This process ...requires selective attention to the contents of working memory. In this article, we summarize evidence for both a broad focus of attention with a capacity of approximately four chunks and a narrow focus of attention that selects a single chunk at a time.
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