Filamentation of a relativistic proton bunch in plasma.

We show in experiments that a long, underdense, relativistic proton bunch propagating in plasma undergoes the oblique instability, which we observe as filamentation. We determine a threshold value for the ratio between the bunch transverse size and plasma skin depth for the instability to occur. At the threshold, the outcome of the experiment alternates between filamentation and self-modulation instability (evidenced by longitudinal modulation into microbunches). Time-resolved images of the bunch density distribution reveal that filamentation grows to an observable level late along the bunch, confirming the spatiotemporal nature of the instability. We provide a rough estimate of the amplitude of the magnetic field generated in the plasma by the instability and show that the associated magnetic energy increases with plasma density.

Growth factor responsiveness of human articular chondrocytes: distinct profiles in primary chondrocytes, subcultured chondrocytes, and fibroblasts.

The objectives of this study were to establish a growth factor response profile for adult human articular chondrocytes, to determine whether this is unique for chondrocytes or influenced by the differentiation status of the cells, and to characterize growth factor interactions. It is shown that transforming growth factor-beta (TGF-beta) is the most potent mitogen among a variety of factors tested. All three isoforms of TGF-beta caused similar dose-dependent increases in chondrocyte proliferation. Other members of the TGF-beta family, including bone morphogenetic protein 2B (BMP2B), activin, and inhibin, did not detectably increase chondrocyte proliferation. Platelet-derived growth factor-AA (PDGF-AA), basic fibroblast growth factor (bFGF), and insulin-like growth factor 1 (IGF-1) also stimulated proliferation but were less effective than TGF-beta. In contrast to findings with other cell types, the effects of TGF-beta on chondrocyte proliferation were not dependent on the endogenous production of PDGF. The cytokines Interleukin 1 (IL-1) and tumor necrosis factor-alpha (TNF-alpha) gave no stimulation, but IL-1 inhibited chondrocyte proliferation induced by TGF-beta or serum. This response profile was characteristic for primary chondrocytes from human adults and distinct from subcultured (dedifferentiated) chondrocytes or skin fibroblasts. The latter preferentially responded to PDGF, and IL-1 caused greater increases in proliferation than TGF-beta. In summary, these results describe growth factor responses that are characteristic for chondrocytes and provide a basis for the analysis of changes in chondrocyte growth proliferation that occur in aging and tissue injury.