Nonparametric estimation of the bivariate recurrence time distribution

This article considers statistical models in which two different types of events, such as the diagnosis of a disease and the remission of the disease, occur alternately over time and are observed subject to right censoring. We propose nonparametric estimators for the joint distribution of bivariate recurrence times and the marginal distribution of the first recurrence time. In general, the marginal distribution of the second recurrence time cannot be estimated due to an identifiability problem, but a conditional distribution of the second recurrence time can be estimated non-parametrically. In the literature, statistical methods have been developed to estimate the joint distribution of bivariate recurrence times based on data on the first pair of censored bivariate recurrence times. These methods are inefficient in the model considered here because recurrence times of higher orders are not used. Asymptotic properties of the proposed estimators are established. Numerical studies demonstrate the estimators perform well with practical sample sizes. We apply the proposed method to the South Verona, Italy, psychiatric case register (PCR) data set for illustration of the methods and theory.     

Animal vocal sequences: not the Markov chains we thought they were

Many animals produce vocal sequences that appear complex. Most researchers assume that these sequences are well characterized as Markov chains (i.e. that the probability of a particular vocal element can be calculated from the history of only a finite number of preceding elements). However, this assumption has never been explicitly tested. Furthermore, it is unclear how language could evolve in a single step from a Markovian origin, as is frequently assumed, as no intermediate forms have been found between animal communication and human language. Here, we assess whether animal taxa produce vocal sequences that are better described by Markov chains, or by non-Markovian dynamics such as the ‘renewal process’ (RP), characterized by a strong tendency to repeat elements. We examined vocal sequences of seven taxa: Bengalese finches Lonchura striata domestica, Carolina chickadees Poecile carolinensis, free-tailed bats Tadarida brasiliensis, rock hyraxes Procavia capensis, pilot whales Globicephala macrorhynchus, killer whales Orcinus orca and orangutans Pongo spp. The vocal systems of most of these species are more consistent with a non-Markovian RP than with the Markovian models traditionally assumed. Our data suggest that non-Markovian vocal sequences may be more common than Markov sequences, which must be taken into account when evaluating alternative hypotheses for the evolution of signalling complexity, and perhaps human language origins.     

On Some Application of Non-Homogeneous Semi-Markov Processes

In this paper a non-homogeneous semi-Markov process is applied to a study of the behaviour of a cell after irradiation. The definition and main properties of the non-homogeneous semi-Markov process are reminded. Moreover one assumes that the cell may stay in one of four states namely, normal state, damaged state 1, damaged state 2, state of death, which are defined. Using the results of the theory of non-homogeneous semi-Markov processes and Markov renewal theory the characteristics of interest pertaining to the cell behaviour are studied.     

A moving–resting process with an embedded Brownian motion for animal movements

Animal movements are of great importance in studying home ranges, migration routes, resource selection, and social interactions. The Global Positioning System provides relatively continuous animal tracking over time and long distances. Nevertheless, the continuous trajectory of an animal’s movement is usually only observed at discrete time points. Brownian bridge models have been used to model movement of an animal between two observed locations within a reasonably short time interval. Assuming that animals are in perpetual motion, these models ignore inactivity such as resting or sleeping. Using the latest developments in applied probability, we propose a moving–resting process model where an animal is assumed to alternate between a moving state, during which it moves in a Brownian motion, and a resting state, during which it does not move. Theoretical properties of the process are studied as a first step towards more realistic models for animal movements. Analytic expressions are derived for the distribution of one increment and two consecutive increments, and are validated with simulations. The induced bridge model conditioning on the starting and end points is used to compute an animal’s probability of occurrence in an observation area during the time of observation, which has wide applications in wildlife behavior research.     

Deviation of spontaneous firing in frog medullar auditory units from the renewal point process

Statistical characteristics of spontaneous activity (distribution of interspike intervals, hazard function, autocorrelation function, autocorrelation function for the process with mixed intervals, and interdependence between adjoining intervals) were analyzed for 123 neurons of the frog medullar dorsal nucleus (homolog of the mammalian cochlear nucleus). In the majority of cells, this activity was distinct from the Poisson process, and firing periodicity was noticed in some cases. In addition, deviations of the spontaneous activity from the renewal process were usually observed. Weak yet reliable positive interspike-interval correlation was typical of most neurons; however, a negative correlation between short adjoining intervals was recorded for some units. These data suggest the effects of memory in the activity of single neurons of the auditory system.     

存量规划中大型公共空间更新的公众参与机制研究——以美国东海岸防灾项目为例

面对存量规划背景下城市更新中复杂利益主体,在规划管理过程引入公众参与已逐渐成为中国广泛使用的规划策略。而面对大型的城市公共空间更新,在中国非营利组织体系并未建立完善的现况下,为使自下而上的空间需求与自上而下的规划策略有效对接,以美国纽约东海岸灾后重建计划的实施过程为例,通过比较研究,分析其中公众参与的保障机制,强调顶层法律对建立一套精准、具体的公众参与技术标准的重要性。同时探讨更新规划过程中,公众参与的途径及其中市民阶层的角色参与。讨论政府职能在整体公众参与的保障体系中所具备的多向把控、整体触媒的系统价值。研究为适用于中国国情的、基于公众参与的整体性城市公共空间更新提供新的视角和策略参考。     

Epithelial cell proliferation in the intestine of the winter flounder,Pseudopleuronectes americanus

Summary To study epithelial cell proliferation in the North American flounder (Pseudopleuronectes americanus), fed and fasted fish received intravenous injections of ³H-thymidine and were killed 11/2 to 2 h later. Radioautographs of proximal, middle, and distal intestinal segments revealed proliferating epithelial cells at all levels of intestinal folds including the crest although labelled nuclei were most abundant in the epithelial cells on the lower half of folds and between folds. Mature appearing goblet cells with labelled nuclei were observed at all levels of the folds. The mean labelling index was greater in the epithelium of fed than fasted flounder. In fed flounder the mean labelling index was greatest in the proximal segment and least in the distal segment; no substantive differences in mean labelling indices were observed in the various segments of intestine from fasted fish. Electron microscopy revealed no major structural differences among epithelial cells along the base of folds compared to cells near the crest of folds. These findings indicate that 1) epithelial cell proliferation occurs at all levels of the folds of flounder intestine and is not compartmentalized to the base of the folds and interfold epithelium as reported in other teleosts, and 2) epithelial cell proliferation in the flounder intestine varies with feeding status.Supported be research grants AM 17537 and RR 05764 from the National Institutes of Health, Bethesda, Maryland and grant DEB7826821 AO1 from the National Science Foundation, Washington, D.C.The authors are grateful to Dr. Michael Field for stimulating discussions and suggestions and for providing facilities for collecting material from fish     

Histological assessment of cellular half-life in tissues in vivo

The assessment of cellular half-life is of fundamental importance for cell biology and biomedicine. Here, we show that cellular half-life in tissues can be histologically measured under steady state conditions in vivo by analyzing the loss of 5-bromo-2′-deoxyuridine (BrdU)-labeled cells over time after withdrawal of long-term BrdU labeling. To achieve efficient continuous cell labeling, we implanted BrdU-containing subcutaneous slow-release pellets into 12-month-old male Fischer 344 rats, delivering BrdU at a dose of 75 mg/kg per day over 1 (n = 20) or 3 weeks (n = 20). Four to five rats each were killed directly after the labeling or 1, 3, and 7 weeks post-labeling. Cellular half-life after withdrawal of BrdU was analyzed by nonlinear regression analysis of the labeling index, using a model of one-phase exponential decay. We initially validated our technique in the duodenum, where we determined a half-life of 2.4 days for crypt cells. Next, we applied this method to other tissues, and found a half-life of 2.2 weeks for cardiac endothelial cells, and of 5–6 days for pancreatic duct cells. In conclusion, we believe that this novel approach is an important step forward in the histological assessment of cellular half-life.     

一个超前和有胆识的科学推论——吴素萱先生关于“细胞核的更新现象”及其科学意义

１９５６年,吴素萱先生在《植物学报》第５卷第１期上发表了题为《细胞核的更新现象》的论文［１］。该文用大蒜鳞片作材料,在其走出休眠后,观察到细胞核释放出常带有核仁的嗜酸性核物质,被亮绿染成绿色。其后这种物质逐步变成嗜碱性,经孚尔根反应染成紫红色,并逐渐...     

Self-fertilization is the main sexual reproduction mechanism in native wine yeast populations

Saccharomyces cerevisiae is a model eukaryotic organism for classical genetics and genomics, and yet its ecology is still largely unknown. In this work, a population genetic analysis was performed on five yeast populations isolated from wine-making areas with different enological practices using simple sequence repeats and restriction fragment length polymorphism of mitochondrial DNA as molecular markers on 292 strains. In accordance with other studies, genome size estimation suggests that native S. cerevisiae strains are mainly homothallic and diploids. Analysis of mtDNA data showed that yeast populations from nonindustrial areas have 40% higher genetic diversity than populations isolated from industrial areas, demonstrating that industrial enological practices are likely to affect native yeast populations negatively by reducing its biodiversity. On the other hand, genetic differentiation analysis based on their microsatellite showed no correlation between genetic and geographic distance and a nonsignificant value when a Mantel test was applied. Finally, in the five populations studied, positive inbreeding ( F _is) values from 0.4 to 0.75, a low but significant level of linkage disequilibrium and a high number of multilocus genotypes were detected. These results strongly advocate that sexual reproduction is frequent enough to erase clonal signature in natural populations and that self-fertilization is the main mating system.     

Cdk1 is required for the self-renewal of mouse embryonic stem cells

Cyclin-dependent kinase 1 (Cdk1) is indispensible for the early development of the embryo. However, its role in maintaining the undifferentiated state of the embryonic stem (ES) cells remains unknown. In this study, we dissected the function of Cdk1 in mouse ES cells by RNA-interference and gene expression analyses. Cdk1 expression is tightly correlated with the undifferentiated state of the ES cells. Upon differentiation, Cdk1 expression reduced drastically. Cdk1 knock-down by RNA interference resulted in the loss of proliferation and colony formation potential of the ES cells. Consequentially, expression of self-renewal genes was reduced while differentiation markers such as Cdx2 were induced. Our results suggest a role for Cdk1 in maintaining the unique undifferentiated and self-renewing state of the mouse ES cells.     

浑善达克沙地黄柳种子发芽和幼苗生长特性的研究

在实验室通过模拟野外自然条件,研究了土壤水分、沙埋厚度和温度对黄柳种子发芽与幼苗生长的影响。结果表明:(1)黄柳种子对水分条件很敏感,发芽的适宜土壤含水量为3%~20%,低于3%或高于20%发芽都会受到抑制;(2)种子萌发需要沙土覆盖,适宜覆沙厚度为0.1~0.4 cm,此条件下第5天种子发芽率达80%以上;但当沙埋厚度超过0.6 cm时,种子出苗就会受到抑制;当沙埋厚度超过1.2 cm时,将不能出苗;(3)种子发芽的适宜温度为15~35℃,当温度高于45℃时种子将不能萌发并最终死亡;(4)幼苗的耐热性较强,40℃处理下幼苗出现热害症状,并且热害症状发展缓慢,60℃处理下90 min后幼苗干枯死亡;(5)种子的寿命为120 d左右。     

The effect of a liquid elemental diet on cell proliferation in the colon of rats

Summary A liquid elemental diet (Vivonex) was given to rats for 6 days while control animals received a normal diet. At the end of the experiment each animal received one intraperitoneal injection of tritiated thvmidine at 8a.m. Animals from each group were killed hourly during the first 24h after the injection and the proliferative activity was studied by autoradiography of the mucosa of the colon using the labeled mitoses-wave method.The epithelial cell proliferation was significantly decreased in the colon of the Vivonex-fed animals.