Mechanical forces, such as mastication, influence morphological characteristics of the cranium. With varying degrees of prognathism found within and between populations, the ability to accommodate masticatory stress may vary, and this will have profound effects on final craniofacial form. The purpose of this research is a two-fold examination of mid-facial prognathism in modern African males. First, an osteometric and morphological examination of specific areas of the cranium involved in the masticatory apparatus was performed, and its relationship with prognathism assessed. Second, finite element analysis (FEA) was used to interpret the distribution of stress during mastication and the contribution of prognathism to this stress distribution. Two diametrically opposed facial forms (prognathic and orthognathic) were modelled to observe variation in displacement, pressure, and Von Mises stress patterns using linear elastic homogenous isotropic material properties. Boundary conditions simulating muscle contraction of the masseter, medial pterygoid, and temporalis were attributed to the models. A vertical compressive bite-force was applied at the left central incisor and the first molar, respectively. With the use of FEA, differences in the pattern and magnitude of Von Mises stress were noted under simulated mastication. The prognathic model consistently experienced more stress for a molar and incisal bite-force than the orthognathic model. More specifically, the prognathic model accommodated for larger areas of Von Mises stress in the regions of the zygomatic arch, nasal aperture, margins of the orbits, and in the inter-orbital area. As individual muscle forces were modeled, the temporalis and medial pterygoid caused the greatest difference in the stress at the articular eminence between the working and balancing sides. These muscles and their forces should be further investigated to understand their role in temporomandibular joint disorders. Several cranial dimensions were shown to increase or decrease with prognathism. The relationship between the gnathic index and facial parameters were statistically significant for nine cranial and seven dental dimensions. The orthognathic group showed a larger inter-orbital dimension with a subsequent decrease in stress in that area. The upper facial index, maxillary molar crown area and the dental arcade shape demonstrated statistically significant shape changes associated with the degree of prognathism. Morphological analysis did not show a significant distribution in browridge expression and robusticity as a means to accommodate masticatory stress. Stress distribution patterns were correlated with osteometric data and showed a significant difference in inter-orbital breadth between the two groups. Mechanical action of mastication may influence prognathic more than orthognatic facial forms. An orthognatic facial form is biomechanically more efficient under masticatory stress. Mechanical loading during mastication greatly influences the morphological patterns of the facial skeleton. Further investigation into patterns of stress is necessary when changes to the masticatory apparatus arises from clinical involvement, trauma, or as a means to avoid or predict failure in the underlying skeletal architectu / Thesis (PhD)--University of Pretoria, 2014. / Anatomy / unrestricted

Numerous intra and extra cellular factors take role in differentiation of cell towards a given lineage. These factors have crucial role in cell-cell and cell-environment interactions. In this study, the aim is to investigate the effect of mechanical forces on the adipogenic differentiation of preadipocytes and mesenchymal stem cells in an in vitro model. Human preadipocytes and mesenchymal stem cells were embedded in 2 % agarose discs. According to the stress-relaxation test results it was observed that initial mechanical properties of agarose-mesenchymal stem cell (MSC) discs did not change compared to acellular agarose whereas those of preadipocytes decreased significantly. The discs with cells were exposed to compression under different weights (1.4 &plusmn / 0.2 g, 7.5 &plusmn / 0.2 g, and 14.6 &plusmn / 0.3 g.) continuously in differentiation medium for 21 days. The control discs were treated with differentiation medium without any compressive weight on top of them. After 21 days, total ribonucleic acids (RNA) have been isolated. Adipogenic differentiation was investigated via reverse transcription coupled quantitative polymerase chain reaction (PCR). The expression of peroxisome proliferators-activated receptors (PPAR-gamma), CCAAT-enhancer binding protein (C/EBP-Beta), leptin, adiponectin, adipophilin and human stearoyl-CoA desaturase (hSCD) have been assessed as adipogenic markers. Differentiation to adipocytes has been further investigated by histochemical Sudan IV staining and immunochemistry and compared to control group. Decrease in the expression of adipogenic factors, size and number of lipid droplets were observed for both MSCs and preadipocytes subjected to compression in agarose discs. The decreases were correlated with the level of mechanical stress. The highest depletion of gene expression was observed in leptin and C/EBP&amp / #61538 / . From our results, it was shown for the first time that mechanical stress impaired the adipogenic differentiation of MSCs and preadipocytes in agarose discs. However, the differentiation pathways should be further investigated.

Cardiac valves are dynamic, sophisticated structures which interact closely with the surrounding hemodynamic environment. Altered mechanical stresses, including pressure, shear and bending stresses, are believed to cause changes in valve biology, but the cellular and molecular events involved in these processes are not well characterized. Therefore, the overall goal of this project is to determine the effects of pressure and shear stress on porcine aortic valve leaflets biology. Results from the pressure study showed that elevated constant pressure (140 and 170 mmHg) causes significant increases in collagen synthesis. The increases were 37.5% and 90% for 140 and 170 mmHg, respectively. No significant differences in DNA and sGAG synthesis were observed under constant pressure. In the cyclic pressure study, the effects of both pressure magnitude and pulse frequency were studied. With the frequency fixed at 1.167 Hz, collagen and sGAG synthesis increased proportionally with mean pressure level. At a fixed pressure level (80-120 mmHg), collagen and sGAG synthesis were slightly increased by 25% and 14% at 0.5 Hz, respectively. DNA synthesis was significantly increased by 72% at 2 Hz. An experiment combining high magnitude (150-190 mmHg) and high frequency (2 Hz) demonstrated significant increases in collagen and sGAG synthesis (collagen: 74%, sGAG: 56%), but no significant changes in cell proliferation. Shear levels ranging from 1 to 80 dyne/cm2 were studied. Scanning electron microscopy results indicated that 48 hrs exposure to shear stress did not alter the circumferential alignment of endothelial cells. Collagen synthesis was significantly enhanced at 9 and 25 dyne/cm2, but not different from static controls under other shear conditions. Leaflets denuded of the endothelium were exposed to identical shear stress and showed very different responses. Collagen synthesis was not affected at any shear levels, but sGAG content was increased at shear of 9, 25 and 40 dyne/cm2. Further studies showed that the increases in collagen synthesis under pressure or shear stress was concurrent with a decline in the expression and activities of cathepsins L and S. This converse relationship between collagen synthesis and cathepsin activity indicated that cathepsins might be involved in valvular ECM remodeling.

The surface charges of cellulosic fibers contribute to several papermaking operations that influence the manufacture and final properties of paper. This thesis investigates the effect of the surface charges on wet-end chemistry, e.g. through the interaction of cationic polyelectrolytes with the fiber surface charges, and on the network strength of pulp suspensions. The polyelectrolyte titration method was used to investigate the interaction of the fiber charges with cationic polyelectrolytes. Techniques were developed to fluorescent label the adsorbing cationic polyelectrolyte in order to visualize the adsorption behavior. Fluorescent confocal laser scanning microscopy (CLSM) was used to determine the extent to which the cationic polyelectrolyte adsorbs into the porous fiber wall. It was shown that the polyelectrolyte charge density limits the adsorption to the surface under electrolyte-free conditions. Adsorption into the fiber wall only occurs for two conditions: 1) if the molecular mass is sufficiently low or 2) the electrolyte concentration is high enough to screen the charges along the polyelectrolyte backbone but not the interactions between the polyelectrolyte and the fiber charges. Aside from the polyelectrolyte properties, the fiber charge density contributes to the adsorption behavior of cationic polyelectrolytes. The fiber charge profile was altered by bulk and surface carboxymethylation. The electrolyte concentration at which a deviation from 1:1 stoichiometry occurs was shown to be dependent on the amount of surface charges, such that the deviation in stoichiometry occurs at a higher electrolyte concentration for pulps having a higher surface charge. A hypothesis was developed to test the conditions at which the deviation in adsorption stoichiometry occurs, which was defined as the critical electrolyte concentration (CEC). It was found that the CEC corresponded to the electrolyte concentration at which the distance between the fiber charges was on the order of the Debye length. Electron spectroscopy for chemical analysis (ESCA) was used as an independent calibration procedure to validate for which a 1:1 stoichiometry occurs. The analysis with ESCA agreed well with the polyelectrolyte titration method for measurement of fiber surface charges. When measured under appropriate conditions, i.e. electrolyte concentration and molecular properties, the fiber surface charge can accurately be measured by the polyelectrolyte titration method. The charge profiles of various pulp types and treatments were also examined. Having been established as a valid technique, the polyelectrolyte titration method was again used to measure the surface charge while conductometric titration was used to measure the total charge content. The amount of bulk and surface charges vary depending on the pulping method and type of wood, although the ratio between the bulk and surface charge (i.e. the charge ratio) is similar for chemical pulps. The mechanical pulp has a higher charge ratio because it contains more fines material than chemical pulp. Bleaching of the chemical pulp decreases the amount of bulk and surface charges, although the charge ratio remains essentially constant. However, methods such as beating or carboxymethyl cellulose (CMC) grafting are available to increase the charge ratio. The effect of the charge profile on fiber-fiber interactions was studied on both a microscopic and macroscopic level. Colloidal probe microscopy (CPM) was used to investigate the microscopic interactions between two cellulose surfaces. Cellulose surfaces, prepared by spin-coating a dissolving pulp onto silica, were used to model the fiber surface, which is too rough for surface force measurements. The charge density of the model surface was increased by CMC grafting. Results showed that increasing the surface charge density created large electrosteric repulsions, due to CMC the chains protruding out from the surface. These interactions on the microscopic scale affect the fiber network strength, which was measured with a parallel plate rheometer. When the repulsion is increased between the fibers, caused by the increase in the surface charge, fiber flocs break apart more easily due to a reduced friction between the fiber surfaces. The forces acting on the fiber network can also be mechanical in origin. The fiber length and flexibility were altered in order to study the influence of mechanical surface linking and elastic fiber bending on the fiber network strength. Using the storage modulus (G’0) as a measure of fiber network strength, longer fibers were found to create a stronger network due to an increased amount of fiber contacts. Flexible fibers have a lower network strength than stiff fibers because the fibers come to rest in a less strained position such that the the influence of elastic fiber bending on the fiber network strength is predominant. / QC 20100831