YouTube is an important source of news and entertainment worldwide, but the scale makes it challenging to study the ideas and topics being discussed on the platform. This paper presents new methods to discover and classify YouTube channels which enable the analysis of communities and categories on the platform using orders of magnitude more channels than have been used in previous studies. Instead of using channel and video data as features for classification as other researchers have, these methods use a self-supervised learning approach that leverages the public subscription pages of commenters. We test the classification method on the task of predicting the political lean of YouTube news channels and find that it outperforms the previous best model on the task. Further experiments also show that there are important advantages to using commenter subscriptions to discover channels. The subscription data, along with an iterative approach, is applied to discover, to our current understanding, the most comprehensive set of English language socio-political YouTube channels yet to be analyzed. We experiment with predicting more fine grained political tags for channels using a previously annotated dataset and find that our model performs better than the average individual human reviewer for most of the top tags. This fine grained political tag model is then applied to the newly discovered English language socio-political channels to create a new dataset to analyze the amount of traffic going to different political content. The data shows that some tags, such as "Partisan Right" and "Conspiracy", are significantly under represented when looking only at the most popular socio-political channels. Through the use of our methods, we are able to get a much more accurate picture of the size of these communities on YouTube.

Part-of-speech information is a pre-requisite in many NLP algorithms. However, Twitter text is difficult to part-of-speech tag: it is noisy, with linguistic errors and idiosyncratic style. We present a detailed error analysis of existing taggers, motivating a series of tagger augmentations which are demonstrated to improve performance. We identify and evaluate techniques for improving English part-of-speech tagging performance in this genre. Further, we present a novel approach to system combination for the case where available taggers use different tagsets, based on voteconstrained bootstrapping with unlabeled data. Coupled with assigning prior probabilities to some tokens and handling of unknown words and slang, we reach 88.7% tagging accuracy (90.5% on development data). This is a new high in PTB-compatible tweet part-of-speech tagging, reducing token error by 26.8% and sentence error by 12.2%. The model, training data and tools are made available.

This paper is an overview of the SwatCS system submitted to SemEval-2013 Task 2A: Contextual Polarity Disambiguation. The sentiment of individual phrases within a tweet are labeled using a combination of classifiers trained on a range of lexical features. The classifiers are combined by estimating the accuracy of the classifiers on each tweet. Performance is measured when using only the provided training data, and separately when including external data.

Tweets are the most up-to-date and inclusive stream of in- formation and commentary on current events, but they are also fragmented and noisy, motivating the need for systems that can extract, aggregate and categorize important events. Previous work on extracting structured representations of events has focused largely on newswire text; Twitter's unique characteristics present new challenges and opportunities for open-domain event extraction. This paper describes TwiCal-- the first open-domain event-extraction and categorization system for Twitter. We demonstrate that accurately extracting an open-domain calendar of significant events from Twitter is indeed feasible. In addition, we present a novel approach for discovering important event categories and classifying extracted events based on latent variable models. By leveraging large volumes of unlabeled data, our approach achieves a 14% increase in maximum F1 over a supervised baseline. A continuously updating demonstration of our system can be viewed at http://statuscalendar.com; Our NLP tools are available at http://github.com/aritter/ twitter_nlp.

People tweet more than 100 Million times daily, yielding a noisy, informal, but sometimes informative corpus of 140-character messages that mirrors the zeitgeist in an unprecedented manner. The performance of standard NLP tools is severely degraded on tweets. This paper addresses this issue by re-building the NLP pipeline beginning with part-of-speech tagging, through chunking, to named-entity recognition. Our novel T-ner system doubles F1 score compared with the Stanford NER system. T-ner leverages the redundancy inherent in tweets to achieve this performance, using LabeledLDA to exploit Freebase dictionaries as a source of distant supervision. LabeledLDA outperforms co-training, increasing F1 by 25% over ten common entity types. Our NLP tools are available at: http://github.com/aritter/twitter_nlp

"The first comprehensive description of the thymus as seen by electron microscopy, this paper appeared at a time when ignorance and confusion were suddenly being replaced by knowledge of the functions of the lymphoid system in immunity. "My interest was aroused in the mid-1950s, while teaching histology to first-year medical students at Washington University. I was arrested by the assertion, attributed to Arnold Rich, that ignorance of the function of small lymphocytes, the most numerous cells in the body, was a major scandal in biology. Therefore, I set out to study the fragile tissues of lymph nodes by electron microscopy, a feat recently made possible by improvements in microtomes and the introduction of epoxy resins for imbedding. I found that the delicate collagenous fibers in lymph nodes were surrounded by the enveloping processes of reticular cells, and that the lymphoid cells lay in a sort of vascular space, continuous with the lymphatic sinuses, but separated from connective tissue by reticular cells. Furthermore, such isolation of lymphoid cells proved to be a widespread phenomenon in lymphoid tissues. "Meanwhile, the late 1950s saw an explosive growth of knowledge concerning the roles of lymphoid cells in immunity, catalyzed by Sir Macfarlane Burnet's clonal selection theory, which he unveiled during his Flexner Lectureship at Vanderbilt University in 1958. I met him there because he and Lady Burnet stayed in my father's house during those weeks in Nashville. I was struck with the idea that the sequestered environments I was seeing in lymphoid tissues might be the environments in which Burnet's quasi-evolutionary clonal selection takes place. "At the same time, the function of the thymus was beginning to be understood through the work of J.F.A.P. Miller, who demonstrated the necessity for its presence during postnatal development of lymphoid tissues, and Donald Metcalf, who produced evidence that a thymic factor stimulates lymphopoiesis. Marshall and White overcame the thymus's usual failure to produce antibodies by injecting antigen directly into the thymus, and postulated a blood-thymic barrier to penetration of antigens into the thymus. My paper presented the first systematic study of the thymus with the electron microscope, although Hoshino had described some of its epithelial cells. I confirmed its epithelial nature, and demonstrated a barrier of epithelial cells interposed between thymic lymphocytes and the rest of the body. There was abundant morphological evidence for secretion by thymic epithelial cells, the putative source of still incompletely defined thymic hormones. "This paper, coming as it did at a pivotal point in the history of the field, brought together a widely scattered literature and emphasized to immunologists the potential value of a cellular point of view."

In suckling rats absorptive cells in the distal two-thirds of the small intestine ingest colloids by pinocytosis. In the middle third of the intestine pinocytosis ceases 17 to 21 days after birth and can be halted prematurely by glucocorticoids. The adrenal cortex is relatively inactive in newborn rats and begins to secrete glucocorticoids two weeks after birth; this secretion has been suspected to cause the termination of pinocytosis. Glucocorticoids do not stop pinocytosis in absorptive cells already present on the villi but induce the renewing population of cells in the crypts to replace the cells on the villi with new ones that do not engage in pinocytosis. Therefore, glucocorticoids act by inducing a new direction of cellular differentiation. BUDR (5-bromodeoxyuridine) is reported to inhibit cellular differentiation reversibly in vitro . When injected into 8 to 9-day-old rats, simultaneously with cortisol, it inhibited the change in cellular differentiation induced by cortisol; that is, pinocytosis continued unabated. BUDR did not appear to be toxic and did not interfere with cellular replacement in the small intestine, as judged autoradio-graphically. Thus it appears that BUDR inhibits the initiation of cellular differentiation by cortisol but does not interfere with cellular proliferation or the pre-established program of cellular differentiation that leads to pinocytosis. The mechanism of action of BUDR is still unknown; it is discussed in light of the possibility that BUDR may after cellular differentiation without acting directly on the genome.

The thymus was examined in suckling mice during normal development and the involution and regeneration produced by injection of cortisol, in experiments designed to test the hypothesis that medullary epithelial cells secrete a ymphopoietic hormone responsible for controlling the magnitude of thymic lymphopoiesis. Cellular events were observed by light and electron microscopy. Lymphopoiesis was assessed, after injection of thymidine-3H, by counting the proportion of lymphocytes labeled in radioautographs of thymus. Cortical lymphopoiesis was distributed heterogeneously, being concentrated in the subcapsular region, but medullary lymphopoiesis was statistically homogeneous in distribution and similar in magnitude to the average level of cortical lymphopoiesis in suckling mice. Therefore counts of the labeling index in the medulla were used to estimate the size of the proliferating population of lymphocytes. Epithelial secretory activity was estimated by measuring the incorporation of 36sulfate by the thymus, using gel filtration chromatography to isolate soluble macromolecular 35sulfate—presumed on radioautographic evidence to represent the mucoid epithelial secretory product. Incorporated 35sulfate accumulated rapidly for 4 hr, reached a peak at 12 hr, and had fallen to half that level by 24 hr after a single injection—as would be expected of a secretory product. During normal postnatal development the size of the proliferating population of lymphocytes and the magnitude of 35sulfate incorporation increased in parallel. During acute involution induced by cortisol both parameters diminished greatly but rose to high levels during subsequent regeneration. Accordingly, lymphopoiesis and sulfate incorporation —as defined and measured in these experiments—correlated linearly over a wide range of variation, providing circumstantial evidence to support the hypothesis that medullary epithelial cells secrete a sulfated mucoid lymphopoietic hormone. This conclusion is discussed in terms of the roles of thymus and adrenal cortex in development of the lymphoid system and maturation of immunological competence.

Chapter 2 Cytological Evidences of Secretion in the Thymus Sam L. Clark Jr, Sam L. Clark Jr Department of Anatomy, Washington University School of Medicine, St. Louis, MissouriSearch for more papers by this author Sam L. Clark Jr, Sam L. Clark Jr Department of Anatomy, Washington University School of Medicine, St. Louis, MissouriSearch for more papers by this author Book Editor(s):G. E. W. Wolstenholme O.B.E., M.A., F.R.C.P., F.I.Biol., G. E. W. Wolstenholme O.B.E., M.A., F.R.C.P., F.I.Biol.Search for more papers by this authorRuth Porter M.R.C.P., Ruth Porter M.R.C.P.Search for more papers by this author First published: 01 January 1966 https://doi.org/10.1002/9780470719466.ch2Citations: 11Book Series:Novartis Foundation Symposia AboutPDFPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShareShare a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Ciba Foundation Symposium - The Thymus: Experimental and Clinical Studies RelatedInformation

American Journal of AnatomyVolume 110, Issue 3 p. 217-257 Article The reticulum of lymph nodes in mice studied with the electron microscope† Sam L. Clark Jr., Sam L. Clark Jr. Department of Anatomy, Washington University School of Medicine, St. Louis, Missouri This work was performed during the tenure of a Senior Fellowship of the National Institutes of Health, United States Public Health Service.Search for more papers by this author Sam L. Clark Jr., Sam L. Clark Jr. Department of Anatomy, Washington University School of Medicine, St. Louis, Missouri This work was performed during the tenure of a Senior Fellowship of the National Institutes of Health, United States Public Health Service.Search for more papers by this author First published: May 1962 https://doi.org/10.1002/aja.1001100303Citations: 108 † This work was supported in part by grants RG-3784 and RG-7176(C1) from the National Institutes of Health, United States Public Health Service. AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume110, Issue3May 1962Pages 217-257 RelatedInformation

American Journal of AnatomyVolume 109, Issue 1 p. 57-83 Article The localization of alkaline phosphatase in tissues of mice, using the electron microscope† Sam L. Clark Jr., Sam L. Clark Jr. Department of Anatomy, Washington University School of Medicine, Saint Louis, Missouri This work was performed during the tenure of a Senior Fellowship of the National Institutes of Health, United States Public Health Service.Search for more papers by this author Sam L. Clark Jr., Sam L. Clark Jr. Department of Anatomy, Washington University School of Medicine, Saint Louis, Missouri This work was performed during the tenure of a Senior Fellowship of the National Institutes of Health, United States Public Health Service.Search for more papers by this author First published: July 1961 https://doi.org/10.1002/aja.1001090106Citations: 75 † This work was supported in part by grants RG-3784 and RG-7176(C1) from the National Institutes of Health, United States Public Health Service. AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume109, Issue1July 1961Pages 57-83 RelatedInformation