The position of the microtubule-organizing center relative to the nucleus is independent of the direction of cell migration in Dictyostelium discoideum

Dictyostelium amoebae can migrate in several different modes. We tested for correlations of the direction of cell locomotion with the relative positions of the nucleus and microtubule-organizing center (MTOC). Five cases were analyzed on electron micrographs with a microcomputer. Each mode of movement showed characteristic locations of the MTOC relative to the nucleus; however, they differed in the various cases. In randomly migrating interphase amoebae, the number of cells with the MTOC located behind the nucleus was twice as great as those with the MTOC located ahead of the nucleus. During chemotactic migration toward folic acid, cells with the MTOC behind the nucleus were more numerous, with a concomitant reduction of anterior MTOCs. When amoebae aggregated on agar plates, a posterior location of the MTOC was most strikingly favored, whereas in cells aggregating under submerged conditions, the MTOC was indifferently anterior or posterior to the nucleus. (It may be significant that EDTA-resistant cell-cell adhesion was fully expressed in the former cells, but weaker in the latter.) Finally, in the case of chemotactically migrating cells from dissociated pseudoplasmodia, which adhere by means of other molecules, the MTOC was consistently ahead of the nucleus. Thus the MTOC shows no necessary preferential position anterior or posterior to the nucleus; its position, rather, correlates with the type of migration and perhaps with the nature of cell-cell adhesion.     

Partial directed coherence: a new concept in neural structure determination

 This paper introduces a new frequency-domain approach to describe the relationships (direction of information flow) between multivariate time series based on the decomposition of multivariate partial coherences computed from multivariate autoregressive models. We discuss its application and compare its performance to other approaches to the problem of determining neural structure relations from the simultaneous measurement of neural electrophysiological signals. The new concept is shown to reflect a frequency-domain representation of the concept of Granger causality. Received: 25 April 2000 / Accepted in revised form: 13 November 2000