Microvascular network topology of the human retinal vessels

A quantitative analysis of blood flow in the human retinal vessels requires a detailed picture of the microvascular network topology. In order to lay the foundation for a quantitative microcirculatory network analysis of the human retina, a novel technique for tissue preparation and network characterization was developed. After injection of hydrogen peroxide into the human bulb, the microvasculature was filled with oxygen produced by endothelial catalase and visualized after embedding in a mixture of cedar oil and gum damar. The vessel topology was documented in the form of photomicrographs, which permitted complete reconstruction of the microvasculature on transparent overlays. By considering the complete capillary system it was possible to divide the retinal network into dichotomous, asymmetric arteriolar and venular trees. The Strahler ordering method, which considers both dichotomous and side branching configurations, was selected and applied to analyze the retinal vascular trees, using the capillaries as the zero order reference vessels. The number of vessel segments was found to be an approximate logarithmic function of the order number, in accordance with Horton's law. Vessel lengths within each order were found to be log-normal distributed, and median lengths for different orders could be approximated by a 2nd degree polynomial curve. Diameters within each order could be approximated by a Gaussian distribution, and the mean values for different orders could be expressed by an exponential curve. These data provide the basis for conductance, pressure and flow computations within the retinal microvessels.