Mounted ill. Bibliography: leaves [221-240]

published_or_final_version / Physiology / Master / Master of Philosophy

published_or_final_version / Surgery / Doctoral / Doctor of Philosophy

Throughout much of the blood circulatory system, supply vessels (arteries and arterioles) are situated adjacent to corresponding draining vessels (veins and venules), which flow in the opposite direction. In this dissertation, mathematical models are developed to describe diffusive exchange of heat, oxygen and inert gases between such paired countercurrent blood vessels and surrounding tissue. In preliminary analyses, exchange between a single vessel and surrounding tissue is considered. The concept of equilibration length is developed. Then a well-known solution for two-dimensional diffusion between two vessels situated in an infinite domain is presented. This provides a basis for developing semianalytic solutions for two vessels situated in a cylindrical tissue region with Dirichlet or zero-flux conditions at the outer boundary. A general approach is then developed for obtaining semianalytic solutions for domains with non-circular cross-sections, and applied to the case of a rectangular domain. The governing equations for paired blood vessels are then solved to obtain the axial variation of temperature or concentration for a variety of cases, including Dirichlet and zero-flux boundary conditions, with and without deposition or consumption of heat or gas. For the Dirichlet case, the equilibration length is compared to that for a single vessel, showing that equilibrium is achieved more rapidly when a single vessel is replaced by two vessels with the same diameter as the single vessel. For the zero-flux case, particular solutions to the full three-dimensional diffusion equation in the tissue are obtained from the two-dimensional solutions. The total transport (convective and diffusive) in the axial direction is evaluated, with and without consumption/deposition, and the results are interpreted in terms of an enhanced diffusivity. Finally, the complementary roles of convection and diffusion in mass and heat transport in the axial direction are considered. It is shown that as vessel diameter decreases, countercurrent exchange eventually results in a reduction of convective transport. Axial diffusion becomes significant at approximately the same range of diameters. This finding is interpreted in terms of the efficiency by which a branching network can transport heat and mass to its extremities.

No description available.

Roundabout (Robo) receptors were first identified in neurons as guidance molecules, however growing evidence suggests that they also play a role in other cells. The aim of this thesis was to characterise the expression and function of a novel endothelial specific member of this family, Robo4. This study revealed that Robo4 is expressed primarily in vessels but also differentially expressed in tumour vessels. Interestingly Robo4 was primarily located within cytoplasmic vesicles coated with clathrin, suggesting that the presence of Robo4 on the cell surface is being tightly regulated. Overexpression of Robo4 induced filopodia and pseudopodia formation and actin re-organisation into stress fibres. It co-localised with actin and tubulin suggesting an important interaction between Robo4 and the cytoskeleton. Robo4's function in endothelial cells was directly investigated using two approaches, overexpression using adenovirus and knockdown using small interfering RNA. Functional cell-based assays revealed that disrupting Robo4's level of expression negatively affects endothelial cell functions that are required during angiogenesis, such as proliferation, migration and tubulogenesis. Overexpression of a truncated version of Robo4, which lacks the C-terminus, provided clues regarding Robo4's function. The intracellular domain is critical for Robo4's localisation and its association with the cytoskeleton. It is also required for pseudopodia formation. Other findings include possible cleavage of Robo4 and Robo4 homodimerisation and heterodimerisation with Robo1. Taken together, the findings presented in this study strongly suggest a role for Robo4 in endothelial cell guidance. Cell guidance during angiogenesis is poorly understood therefore the identification of a new molecule potentially involved in this mechanism will hopefully help elucidate the process.

The retina is one of the most metabolically active tissue in the body, rendering it sensitive to vascular dysfunction. Consequently, diseases that disrupt normal retinal blood supply, including retinal vein occlusions (RVO) and diabetic retinopathy, are the leading causes of blindness in working-age adults. Despite available therapies, an estimated 50% of patients do not respond to treatment. We employed a mouse model of retinal vein occlusion (RVO), achieved by tail-vein injection of rose bengal, followed by laser photocoagulation of retinal veins. In vivo analyses – optical coherence tomography (OCT), fluorescein angiography, and electroretinograms (ERGs) - were conducted with the Micron IV system (Phoenix Research Labs). RVO induces acute retinal edema, which peaks during the first 24 hours following injury. Over a 7 day time course the edema resolves, revealing a permanent retinal thinning due to death of retinal neurons. We identified caspase-9, a protease traditionally associated with apoptosis, as an essential mediator of edema. Increased levels of activated caspase-9 were detected in vascular endothelial cells 1 hour following RVO. We tested RVO in mice with inducible endothelial-cell-specific deletion of caspase-9 (iC9 ECKO). Compared to littermate controls, iC9 ECKO mice develop less edema, and sustain less retinal degeneration after RVO injury. ERG analysis showed preservation of retinal function in iC9 ECKO mice. To study whether inhibiting caspase-9 would provide protection against RVO we utilized a highly specific caspase-9 inhibitor, which we can deliver to the retina using simple eyedrops. Treatment of wildtype mice with the caspase-9 inhibitor immediately after induction of RVO provided morphologic, biochemical and functional retinal protection. Inhibition of caspase-9 reduces edema, protects retinal morphology, and helps prevent vision loss following RVO injury. Our studies indicate that endothelial caspase-9 plays an essential role in regulating edema pathogenesis. Moreover, our novel cell permeant caspase-9 inhibitor abrogates the edema and may be a potential therapy for individuals suffering from vascular eye disease.

Includes bibliographical references.

Bibliography: leaves 157-177.

Thesis (M. Phil.)--University of Hong Kong, 2006. / Title proper from title frame. Also available in printed format.