STEM CELL GROWTH AND DIFFERENTIATION IN HYDRA ATTENUATA

The differentiation of nerve cells and nematocytes from interstitial stem cells in Hydra has been investigated under conditions of changing stem cell density. Interstitial stem cells were cultured in a feeder layer system consisting of aggregates of nitrogen mustard-inactivated tissue. The aggregates were seeded with varying numbers of stem cells from 10 to 400 per aggregate; between 4 and 7 days later the rates of nerve and nematocyte differentiation were measured. Nerve differentiation was scored by labelling the stem cell population with [3H]-thymidine and counting nests of 4 proliferating nematoblasts. In both cases the numbers of differentiating cells were normalized to the size of the stem cell population. The results indicate that the rate of nematocyte differentiation increases as the concentration of stem cells increases in aggregates; under the same conditions the rate of nerve differentiation remains essentially constant. To calculate the numbers of stem cells entering each pathway per generation, a computer was programmed to simulate the growth and differentiation of interstitial stem cells. Standard curves were prepared from the simulations relating the rates of nerve and nematocyte differentiation to the fraction of stem cells committed to each pathway per generation. The rates of nerve and nematocyte commitment were then estimated from the experimentally observed rates of differentiation using the standard curves. The results indicate that nerve commitment remains constant at about 0.13 stem cells per generation over a wide range of stem cell concentration. Nematocyte commitment, by comparison, increases from 0.15 to 0.21 stem cells per generation as stem cell concentration increases in aggregates. The fact that the ratio of nerve to nematocyte commitment changes under our conditions suggests that stem cell commitment is not a stochastic process but subject to control by environmental stimuli

RIOT -- A Simple Graphical Chip Assembly Tool

Riot is a simple interactive graphical tool designed to facilitate the assembly of cells into integrated systems. Riot supplies the user with primitive operations of connection -- abutment, routing and stretching -- in an interactive graphic environment. The designer retains full control of the design, including the assignment of positions to instances of cells and the choice of connection mechanism. The computer takes care of the tedious and exacting implementation detail, guaranteeing that connections are made correctly. The powerful connection primitives give the user of Riot the ability to quickly assemble a custom chip from a collection of low-level cells. This document provides a discussion of the motivation for Riot and a description of the Riot chip assembly system, its capabilities and its use

Nerve commitment in Hydra. I. Role of morphogenetic signals

The kinetics of nerve commitment during head regeneration in Hydra were investigated using a newly developed assay for committed cells. Committed nerve precursors were assayed by their ability to continue nerve differentiation following explanation of small pieces of tissue. Committed nerve precursors appear at the site of regeneration within 6 hr after cutting and increase rapidly. The increase is localized to the site of regeneration and does not occur at proximal sites in the body column of the regenerate. The increase is delayed about 8–12 hr when regeneration occurs at sites lower in the body column. The results show, furthermore, that redistribution of committed precursors does not play a major role in the pattern of nerve differentiation during regeneration. Since the increase in committed nerves coincides with the increase in morphogenetic potential of the regenerating tissue, the results strengthen the idea that morphogenetic signals are involved directly in the control of nerve commitment in Hydra

Distribution of interstitial stem cells in Hydra

The distribution of interstitial stem cells along the Hydra body column was determined using a simplified cloning assay. The assay measures stem cells as clone-forming units (CFU) in aggregates of nitrogen mustard inactivated Hydra tissue. The concentration of stem cells in the gastric region was uniform at about 0.02 CFU/epithelial cell. In both the hypostome and basal disk the concentration was 20-fold lower. A decrease in the ratio of stem cells to committed nerve and nematocyte precursors was correlated with the decrease in stem cell concentration in both hypostome and basal disk. The ratio of stem cells to committed precursors is a sensitive indicator of the rate of self-renewal in the stem cell population. From the ratio it can be estimated that <10% of stem cells self-renew in the hypostome and basal disk compared to 60% in the gastric region. Thus, the results provide an explanation for the observed depletion of stem cells in these regions. The results also suggest that differentiation and self-renewal compete for the same stem cell population

Histories of hating

This roundtable discussion presents a dialogue between digital culture scholars on the seemingly increased presence of hating and hate speech online. Revolving primarily around the recent #GamerGate campaign of intensely misogynistic discourse aimed at women in video games, the discussion suggests that the current moment for hate online needs to be situated historically. From the perspective of intersecting cultural histories of hate speech, discrimination, and networked communication, we interrogate the ontological specificity of online hating before going on to explore potential responses to the harmful consequences of hateful speech. Finally, a research agenda for furthering the historical understandings of contemporary online hating is suggested in order to address the urgent need for scholarly interventions into the exclusionary cultures of networked media

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Communicating health decisions: an analysis of messages posted to online prostate cancer forums

Background  Experiential websites such as message forums and blogs allow Prostate Cancer (PCa) patients to communicate their health decisions to peers. The issues surrounding this form of indirect involvement in public health are little understood. Objective  This paper explores the types of decision-making processes that people are exposed to on PCa online message boards. The kinds of treatment choices patients are making and the reports of their decision-making processes to peers through an online environment are examined in the context of the Heuristic Systematic Model. Method  Messages about treatment decision making were collected from four PCa websites. In total, 137 messages were selected from blogs and online forums and their decision-making processes coded. Results  Men looking online for information about treatment options for PCa are exposed to a range of decision-making processes. Just under half (49.6%) of the messages reported non-systematic decision processes, with deferral to the doctor and proof of cancer removal being the most common. For systematic processing (36.5%), messages most commonly considered treatment outcomes and side-effects. Processes did not vary between the blogs and online forums. Discussion and conclusion  Compared to previous studies far fewer messages reported non-systematic decision processes and only a small number of messages reflected lay beliefs or misbeliefs about PCa treatment. Implications for men and their clinicians of seeking health information online are discussed

Estimating good discrete partitions from observed data: symbolic false nearest neighbors

A symbolic analysis of observed time series data requires making a discrete partition of a continuous state space containing observations of the dynamics. A particular kind of partition, called ``generating'', preserves all dynamical information of a deterministic map in the symbolic representation, but such partitions are not obvious beyond one dimension, and existing methods to find them require significant knowledge of the dynamical evolution operator or the spectrum of unstable periodic orbits. We introduce a statistic and algorithm to refine empirical partitions for symbolic state reconstruction. This method optimizes an essential property of a generating partition: avoiding topological degeneracies. It requires only the observed time series and is sensible even in the presence of noise when no truly generating partition is possible. Because of its resemblance to a geometrical statistic frequently used for reconstructing valid time-delay embeddings, we call the algorithm ``symbolic false nearest neighbors''