Biomechanical stimuli play important roles in the formation of articular cartilage during early foetal life, and optimal mechanical load is a crucial regulatory factor of adult chondrocyte metabolism and function. In this study, we undertook to analyse mechanotransduction pathways during in vitro chondrogenesis. Chondroprogenitor cells isolated from limb buds of 4-day-old chicken embryos were cultivated as high density cell cultures for 6 days. Mechanical stimulation was carried out by a self-designed bioreactor that exerted uniaxial intermittent cyclic load transmitted by the culture medium as hydrostatic pressure and fluid shear to differentiating cells. The loading scheme (0.05 Hz, 600 Pa; for 30 min) was applied on culturing days 2 and 3, when final commitment and differentiation of chondroprogenitor cells occurred in this model. The applied mechanical load significantly augmented cartilage matrix production and elevated mRNA expression of several cartilage matrix constituents, including collagen type II and aggrecan core protein, as well as matrixproducing hyaluronan synthases through enhanced expression, phosphorylation and nuclear signals of the main chondrogenic transcription factor Sox9. Along with increased cAMP levels, a significantly enhanced protein kinase A (PKA) activity was also detected and CREB, the archetypal downstream transcription factor of PKA signalling, exhibited elevated phosphorylation levels and stronger nuclear signals in response to mechanical stimuli. All the above effects were diminished by the PKA-inhibitor H89. Inhibition of the PKA-independent cAMP-mediators Epac1 and Epac2 with HJC0197 resulted in enhanced cartilage formation, which was additive to that of the mechanical stimulation, implying that the chondrogenesispromoting effect of mechanical load was independent of Epac. At the same time, PP2A activity was reduced following mechanical load and treatments with the PP2A-inhibitor okadaic acid were able to mimic the effects of the intervention. Our results indicate that proper mechanical stimuli augment in vitro cartilage formation via promoting both Juhász and Matta et al. 3 differentiation and matrix production of chondrogenic cells, and the opposing regulation of the PKA/CREB-Sox9 and the PP2A signalling pathways is crucial in this phenomenon.

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Surface relief gratings formation in amorphous selenium thin films in two recording configurations with light intensity modulation were studied in situ by real-time atomic force microscopy and diffraction efficiency measurements. We report observation of mass transport effect in films induced by band-gap irradiation when the light polarization of the recording beams has a component along the light intensity gradient ("p-p" scheme of recording) that allows obtaining giant stable gratings in this versatile chalcogenide material. On the contrary, only a pure scalar weak grating caused by photoinduced volume shrinkage is obtained in the "s-s" recording configuration, even for long-term irradiation. Chalcogenide glasses (ChG) are known to exhibit a rich variety of photoinduced changes when illuminated with near bandgap light including changes in mechanical properties, termed as photoplastic effects.1 Among them, the effect of light induced surface deformation and direct fabrication of surface relief gratings (SRGs) in ChG is intensively studied over the last decade both experimentally 2-10 and theoretically.11 Two main types of SRG induced by holographic recording, due to the excitation intensity modulation, with near band-gap light in ChG can be distinguished according to their formation mechanism and their properties:10 (1) small scalar SRG induced by either volume expansion or shrinkage due to different response of the material in the bright and dark zones of the interference pattern formed and (2) giant vectorial SRG induced by lateral mass transport in the case where the light polarization of the recording beams has a component along the light intensity gradient.Experiments have shown that only few compositions of ChG demonstrate both types of SRG. For example, vectorial SRG have been observed only for Se-rich films in the binary As-Se system (As 20 Se 80 ) 10 and for the compositions close to As 40 S 60 in As-S glasses 3 while scalar SRGs are common to all glass compositions of As-S and As-Se glasses. While for As-S glass the above observation (lack of vectorial SRG) could be accounted for by considering the photo-induced polymerization effect in S-rich compositions, 12 in the case of As-Se glasses the effect is not yet well understood. Although elemental amorphous Se (a-Se) is a model glass-former in chalcogenide science, no systematic studies of SRG formation have been carried out up to now, while investigations of other photoinduced effects abound. 13 A fact that perplexes studies of SRG formation relates to the photoinduced crystallization of a-Se (Ref. 14) under band-gap illumination, thus preventing the formation of advanced surface relief stable in time.Few papers report the formation of surface relief induced by holographic recording for a-Se films. 2,5,15 In an early study, Haro-Poniatowski et al.,15 employing phase conjugation, haveshown that small (20 nm in height) photoinduced surface relief gratings appears in 4 lm-thick films with the same spatial period. Slightly higher reliefs ($ 40 nm) formed a...

We demonstrate, using computer simulations based on deterministic kinetic equations and Monte Carlo technique, that during intermixing in an ideal AB system with an initially wide A=B interfaceif the diffusion coefficient D strongly depends on concentration-the interface can become sharp on nanoscale. The sharp interface shifts proportionally with time (in contrast to the square root law). Furthermore, it is also shown that at the beginning of the intermixing in a finite bilayer or in multilayers, the diminution of the concentration gradient takes place by filling up one of the initially pure layers (layer B if D is large there) and by the shift of the sharpening interface. DOI: 10.1103/PhysRevLett.89.165901 PACS numbers: 66.30.Pa, 68.35.Fx Very recently, studying the dissolution of thin (3 and 8 monolayers thick) Ni films into Cu(111) substrate, it was shown (both by experiments and by computer simulations, based on deterministic kinetic equations) that the interface remains sharp and shifts proportionally with the time t (in contrast to a shift with t p ) even in ideal systems having complete mutual solubility [1]. This result is inherently related to the strong nonlinearity of the problem: the strong concentration dependence of the diffusion coefficient D shifts the validity limit of the continuum approach (see also [2]), from which a parabolic law would be expected, out of the nanometer range. It was also shown in [1] that the parabolic law is obeyed at longer times and thus this ''irregular'' behavior can be observed only on nanoscale. Furthermore, it was obtained from computer simulations in [2] that, at the beginning of the intermixing in a finite bilayer or in multilayers with initially sharp A=B interfaces, the diminution of the concentration gradient takes place by filling up one of the initially pure layers (layer B if D is large there) and by the shift of the sharp interface. Although the above simulations were carried out for a discrete lattice, this phenomenon was also observed experimentally in the amorphous Si-Ge multilayers [3] by Auger depth profiling technique [4], illustrating that the above effects are rather related to the nonlinearity than to the discrete character of the medium, where the diffusion takes place.In this Letter, using computer simulations based on deterministic kinetic equations (discrete, atomic approach) [5] and Monte Carlo technique (which contains the effects of fluctuations as well), we demonstrate that on nanoscale (short diffusion distances, short time) and again in the case of strongly concentration dependent D, an initially wide A=B interface can become sharp even in an ideal system. While such a process is obvious in an alloy with a large miscibility gap (the metastable solid solution in the smeared interface region decomposes and a sharp interface should be formed), it is surprising at first sight in systems with complete mutual solubility, because according to the macroscopic Fick I law the direction of the atomic flux is always opposite to the direction of t...

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