Correct wiring is crucial for the proper functioning of the nervous system. Molecular gradients provide critical signals to guide growth cones, which are the motile tips of developing axons, to their targets. However, in vitro, growth cones trace highly stochastic trajectories, and exactly how molecular gradients bias their movement is unclear. Here, we introduce a mathematical model based on persistence, bias, and noise to describe this behaviour, constrained directly by measurements of the detailed statistics of growth cone movements in both attractive and repulsive gradients in a microfluidic device. This model provides a mathematical explanation for why average axon turning angles in gradients in vitro saturate very rapidly with time at relatively small values. This work introduces the most accurate predictive model of growth cone trajectories to date, and deepens our understanding of axon guidance events both in vitro and in vivo.

BACKGROUND: Over the past 10 years, the use of saliva as a diagnostic fluid has gained attention and has become a translational research success story. Some of the current nanotechnologies have been demonstrated to have the analytical sensitivity required for the use of saliva as a diagnostic medium to detect and predict disease progression. However, these technologies have not yet been integrated into current clinical practice and work flow. CONTENT: As a diagnostic fluid, saliva offers advantages over serum because it can be collected noninvasively by individuals with modest training, and it offers a cost-effective approach for the screening of large populations. Gland-specific saliva can also be used for diagnosis of pathology specific to one of the major salivary glands. There is minimal risk of contracting infections during saliva collection, and saliva can be used in clinically challenging situations, such as obtaining samples from children or handicapped or anxious patients, in whom blood sampling could be a difficult act to perform. In this review we highlight the production of and secretion of saliva, the salivary proteome, transportation of biomolecules from blood capillaries to salivary glands, and the diagnostic potential of saliva for use in detection of cardiovascular disease and oral and breast cancers. We also highlight the barriers to application of saliva testing and its advancement in clinical settings. SUMMARY: Saliva has the potential to become a first-line diagnostic sample of choice owing to the advancements in detection technologies coupled with combinations of biomolecules with clinical relevance.

The mechanical properties of the extracellular matrix (ECM) can exert significant influence in determining cell fate. Human mesenchymal stem cells (MSCs) grown on substrates with varying stiffness have been shown to express various cell lineage markers, without the use of toxic DNA demethylation agents or complex cocktails of expensive growth factors. Here we investigated the myogenic and osteogenic potential of various polyacrylamide gel substrates that were coated with covalently bound tissue-specific ECM proteins (collagen I, collagen IV, laminin, or fibronectin). The gel-protein substrates were shown to support the growth and proliferation of MSCs in a stiffness-dependent manner. Higher stiffness substrates encouraged up to a 10-fold increase in cell number over lower stiffness gels. There appears to be definitive interplay between substrate stiffness and ECM protein with regard to the expression of both osteogenic and myogenic transcription factors by MSCs. Of the 16 gel-protein combinations investigated, osteogenic differentiation was found to occur significantly only on collagen I-coated gels with the highest modulus gel tested (80 kPa). Myogenic differentiation occurred on all gel-protein combinations that had stiffnesses >9 kPa but to varying extents as ascertained by MyoD1 expression. Peak MyoD1 expression was seen on gels with a modulus of 25 kPa coated in fibronectin, with similar levels of expression observed on 80-kPa collagen I-coated gels. The modulation of myogenic and osteogenic transcription factors by various ECM proteins demonstrates that substrate stiffness alone does not direct stem cell lineage specification. This has important implications in the development of tailored biomaterial systems that more closely mimic the microenvironment found in native tissues.

The dynamics of drop formation and pinch-off have been investigated for a series of low viscosity elastic fluids possessing similar shear viscosities, but differing substantially in elastic properties. On initial approach to the pinch region, the viscoelastic fluids all exhibit the same global necking behavior that is observed for a Newtonian fluid of equivalent shear viscosity. For these low viscosity dilute polymer solutions, inertial and capillary forces form the dominant balance in this potential flow regime, with the viscous force being negligible. The approach to the pinch point, which corresponds to the point of rupture for a Newtonian fluid, is extremely rapid in such solutions, with the sudden increase in curvature producing very large extension rates at this location. In this region the polymer molecules are significantly extended, causing a localized increase in the elastic stresses, which grow to balance the capillary pressure. This prevents the necked fluid from breaking off, as would occur in the equivalent Newtonian fluid. Alternatively, a cylindrical filament forms in which elastic stresses and capillary pressure balance, and the radius decreases exponentially with time. A $(0+1)$ -dimensional finitely extensible nonlinear elastic dumbbell theory incorporating inertial, capillary, and elastic stresses is able to capture the basic features of the experimental observations. Before the critical “pinch time” $t_p$ , an inertial-capillary balance leads to the expected $2∕3$ -power scaling of the minimum radius with time: $R_{\min }\sim (t_p-t{)}^{2∕3}$ . However, the diverging deformation rate results in large molecular deformations and rapid crossover to an elastocapillary balance for times $t>t_p$ . In this region, the filament radius decreases exponentially with time $R_{\min }\sim \exp \left[\right(t_p-t)∕{\lambda }_1],$ where ${\lambda }_1$ is the characteristic time constant of the polymer molecules. Measurements of the relaxation times of polyethylene oxide solutions of varying concentrations and molecular weights obtained from high speed imaging of the rate of change of filament radius are significantly higher than the relaxation times estimated from Rouse-Zimm theory, even though the solutions are within the dilute concentration region as determined using intrinsic viscosity measurements. The effective relaxation times exhibit the expected scaling with molecular weight but with an additional dependence on the concentration of the polymer in solution. This is consistent with the expectation that the polymer molecules are in fact highly extended during the approach to the pinch region (i.e., prior to the elastocapillary filament thinning regime) and subsequently as the filament is formed they are further extended by filament stretching at a constant rate until full extension of the polymer coil is achieved. In this highly extended state, intermolecular interactions become significant, producing relaxation times far above theoretical predictions for dilute polymer solutions under equilibrium conditions.