Nonfibrillar Matrix Research Articles

A fibril reinforced nonhomogeneous poroelastic model for articular cartilage: inhomogeneous response in unconfined compression

The depth dependence of material properties of articular cartilage, known as the zonal differences, is incorporated into a nonlinear fibril-reinforced poroelastic model developed previously in order to explore the significance of material heterogeneity in the mechanical behavior of cartilage. The material variations proposed are based on extensive observations. The collagen fibrils are modeled as a distinct constituent which reinforces the other two constituents representing proteoglycans and water. The Young's modulus and Poisson's ratio of the drained nonfibrillar matrix are so determined that the aggregate compressive modulus for confined geometry fits the experimental data. Three nonlinear factors are considered, i.e. the effect of finite deformation, the dependence of permeability on dilatation and the fibril stiffening with its tensile strain. Solutions are extracted using a finite element procedure to simulate unconfined compression tests. The features of the model are then demonstrated with an emphasis on the results obtainable only with a nonhomogeneous model, showing reasonable agreement with experiments. The model suggests mechanical behaviors significantly different from those revealed by homogeneous models: not only the depth variations of the strains which are expected by qualitative analyses, but also, for instance, the relaxation-time dependence of the axial strain which is normally not expected in a relaxation test. Therefore, such a nonhomogeneous model is necessary for better understanding of the mechanical behavior of cartilage.

Molecular structure and functional morphology of echinoderm collagen fibrils.

The collagenous tissues of echinoderms, which have the unique capacity to rapidly and reversibly alter their mechanical properties, resemble the collagenous tissues of other phyla in consisting of collagen fibrils in a nonfibrillar matrix. Knowledge of the composition and structure of their collagen fibrils and interfibrillar matrix is thus important for an understanding of the physiology of these tissues. In this report it is shown that the collagen molecules from the fibrils of the spine ligament of a sea-urchin and the deep dermis of a sea-cucumber are the same length as those from vertebrate fibrils and that they assemble into fibrils with the same repeat period and gap/overlap ratio as do those of vertebrate fibrils. The distributions of charged residues in echinoderm and vertebrate molecules are somewhat different, giving rise to segment-long-spacing crystallites and fibrils with different banding patterns. Compared to the vertebrate pattern, the banding pattern of echinoderm fibrils is characterized by greatly increased stain intensity in the c3 band and greatly reduced stain intensity in the a3 and b2 bands. The fibrils are spindle-shaped, possessing no constant-diameter region throughout their length. The shape of the fibrils is mechanically advantageous for their reinforcing role in a discontinuous fiber-composite material.

Synthesis of Extracellular Matrix by Macrophage-Modulated Retinal Pigment Epithelium

In proliferative vitreoretinopathy, macrophages and retinal pigment epithelial cells are associated with microfibrillar matrix proteins in the vitreous cavity, but the contribution of this extracellular matrix to the pathophysiology is not known. We used radiolabeling techniques on cultured human retinal pigment epithelial cells to correlate the secretion of extracellular matrix proteins with macrophage-induced modulation of cell proliferation and morphologic features. Retinal pigment epithelial cells incubated in a macrophage-conditioned medium assumed fibrocytelike morphologic characteristics, grew faster, and exhibited a decreased cellular release of fibrillar and nonfibrillar matrix components. However, due to a simultaneous greater increase in cell numbers in these modulated cultures, the total production of fibrillar and nonfibrillar matrix components by the culture population was increased.

Collagen and proteoglycan in a sea urchin ligament with mutable mechanical properties.

The "problematic ligament" of sea urchins is a connective tissue which crosses the ball-and-socket joint between spine and body wall. The problem of this ligament is that it is composed of parallel collagen fibrils, yet normally undergoes rapid and dramatic alterations in mechanical properties and in length. Previous work has suggested that the collagen fibrils of the ligament are able to slide past one another during length changes but are inhibited from sliding when the ligament is in "catch". In this model of the ligament both the collagen fibrils and the interfibrillar matrix are mechanically important. We have found that the collagen fibrils of the spine ligament of the pencil urchin Eucidaris tribuloides are discontinuous and end by tapering within the body of the ligament. Intact fibrils that have been isolated from the ligament vary by more than an order of magnitude in length and in radius but have a constant length/radius (aspect) ratio of about 5,300. This is the first determination of the aspect ratio of collagen fibrils from any source. The constant aspect ratio of the fibrils is consistent with their functioning as the discontinuous fiber phase in a fiber-reinforced composite material, while the high value of the aspect ratio indicates that the nonfibrillar matrix, which must act to transfer stress between fibrils, can produce a stiff and strong ligament even if it is several orders of magnitude weaker and more compliant than the fibrils. Moreover, the tensile properties of the ligament may be determined by the properties of the matrix. A prominent component of the interfibrillar matrix is a proteoglycan which associates with specific bands at the surface of the collagen fibrils through noncovalent binding of its core protein. The glycosaminoglycan moiety of this proteoglycan is partly comprised of chondroitin sulfate/dermatan sulfate polymers. These results are consistent with the "sliding fibril" hypothesis and suggest that the proteoglycan may be an important component of the stress-transfer matrix.

Differentiation of interstitial cells and stromal proteins in the secondary septum of early postnatal rat: effect of maternal chronic exposure to whole cigarette smoke.

The intention of this investigation was to ascertain the effect of maternal exposure to cigarette smoke on the early postnatal morphogenesis of pulmonary interstitium in offspring. Female rats were chronically exposed to whole cigarette smoke. Offspring of these and control animals were sacrificed at postnatal day 15, and their tissues were prepared for quantitative and qualitative analyses. Results indicate a diminished quantitative representation of parenchymal tissue (P less than 0.01) and a slower pace of secondary septal growth (P less than 0.07) in the experimental lung. Furthermore, a greater cellular volume density (P less than 0.0002) and, inversely, a lesser quantitative representation of extracellular matrix (P less than 0.0002) was ascertained for the experimental septal interstitium. There was proportionately less of elastin substances (P less than 0.009), collagen together with basal laminae (P less than 0.0008), and nonfibrillar, amorphous matrix (P less than 0.02) in the experimental extracellular stroma. Fibrillar collagen and nonfibrillar matrix were represented quantitatively 6.3 times more in the experimental extracellular interstitium than elastin, whereas that ratio for the control tissue was only 4.2. Most experimental interstitial cells (80%) contained numerous lipid globules, which, in contrast, were only occasionally present in control cells (7.3%). Experimental cells, consequently, possessed a larger cross-sectional diameter and a smaller nucleus-to-cytoplasm volume ratio than control cells. These divergent developmental patterns are possibly suggestive of a delayed differentiation of interstitial cells and a modified production to degradation balance of stromal proteins in offspring of animals chronically exposed to whole cigarette smoke.

The Photo-Oxidation of Polypropylene Monofilaments

The photo-oxidative degradation of polypropylene monofilaments with xenon-arc irradiation results initially in a drop in elongation-to-break, followed by a decrease in tensile strength on prolonged irradiation. A microscopic examination of fibers that were first peeled to reveal the interior of the filament and then uv-irradiated showed that the interior is partially fibrillar, and that the fibrillar zones are more resistant to photo-oxidative scission than the surrounding nonfibrillar matrix. The formation of cracks in the outer sheath of the fiber and through the nonfibrillar regions correlates with the drop in elongation-to-break. Fibrils are not severed until after long uv exposure times, corresponding to the observed drop in tensile strength. Fiber melting temperatures estimated from DSC endotherms drop steadily during photo-oxidation. This drop and the observed decrease in x-ray crystallinity may be associated with the photo-oxidative cleavage of tie molecules and chain-extended crystalline zones.

The Growth of Plant Cell walls

Publisher Summary This chapter discusses the growth of plant cell walls. In the course of cell growth, that may involves an increase in cell surface by a factor somewhere between 10 and 105, the wall grows accordingly, retaining in this process a remarkable unity, constancy, and coherence of structure. Growth involves increase in thickness as well as in area; these may increase simultaneously or successively. The tough elastic membrane characteristic of the mature cell is thus commonly its most conspicuous part, and in the aggregate such membranes form the skeletal framework of the plant. The fundamental feature of wall structure is its dual nature. Electron microscopy reveals the presence of microfibrils of indefinite length. Morphological studies of wall structure and growth refer almost wholly to the micellar or microfibrillar component of the wall which has these supramolecular features of structure. The chapter discusses the microfibrillar framework and its changes in growth, and also the nonfibrillar matrix of the wall and its role in wall extensibility.