Fibrillar-Level Strain Gradients Across the Calcified Bone-Cartilage Interface

The bone-cartilage interface (BCI) and underlying calcified plate is a universal feature in diarthrodial joints. The BCI is an important mechanically-graded interface subjected to shear and compressive strains, and changes at the BCI have been linked to osteoarthritis progression. Here we report the existence of a physiological internal strain gradient (pre-strain) across the BCI at the ultrastructural scale of the extracellular matrix constituents, specifically the collagen fibril. We use X-ray scattering that probes changes in the axial periodicity of fibril-level D-stagger of tropocollagen molecules in the matrix fibrils, as a measure of microscopic pre-strain. We find that mineralized collagen nanofibrils in the calcified BCI are in tension pre-strain relative to the underlying trabecular bone. This behaviour contrasts with the previously accepted notion that fibrillar pre-strain (or D-stagger) in collagenous tissues always reduces with mineralization due to reduced hydration and associated swelling pressure. Within the calcified tissue, a finer-scale gradient in pre-strain over ~50μm is likely linked to the tidemark. The increased fibrillar pre-strain at the BCI is linked to prior research reporting large tissue-level residual strains under compression. The findings may have biomechanical adaptative significance: higher in-built molecular level resilience/damage resistance to physiological compression, and the disruption of the molecular-level pre-strains during remodelling of the BCI may be a potential factor in osteoarthritis-based degeneration.

About the Influences of Some Major Biotic Drivers on the Population Dynamics in Host-parasite Systems: Interpretative Lessons from an Extended Nicholson-Bailey Model

Population dynamics within host-parasite systems in insects is governed by a series of factors, both endogenous and exogenous. Among them, five factors may be considered as major drivers: the respective inherent rates of increase of the host and of the parasite, the level of resource available to the host, the respective immigration rates of the host and of the parasite. While only the first two (the inherent rates of increase of host and parasite) are considered in the original Nicholson and Bailey model, an extended version of the model includes also the other three parameters, thus providing a broader (although still schematic) approach to the host-parasite population dynamics. A brief analysis of the respective influences of each of these five driving parameters on the main features of host-parasite dynamics is derived accordingly, based upon this extended model. Finally, specific attention is paid to the major concerns due to the cyclic outbreaks of both the host and the parasite, regarding in particular the amplitude, the periodicity and the conditions of onset of the cyclicity. Both the practical aspects of the cyclic regime and its possible adaptative significance are discussed. As a whole, this approach aims to provide some general clues for the interpretation of various features of the host-parasite dynamics, as reported from field observations.

Submicroscopical Features of Leaves of Xyris Species

The leaf ultrastructure of five Xyris species were examined using scanning electron microscope (SEM), transmission electron microscope (TEM) and histochemical methods. All studied leaves show some features in epidermis and mesophyll, which were of considerable adaptative significance to drought stress. Such features included the occurrence of a pectic layer on the stomatal guard cells and the presence of a network of pectic compounds in the cuticle. Pectic compunds were also in abundance in lamellated walls of the mesophyll cells and on the inner surface of the sclerified cell walls of the vascular bundle sheaths. There were also specialized chlorenchymatous "peg cells" in the mesophyll and drops of phenolic compounds inside the epidermal cells.