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We report a rapid hyperspectral fluorescence lifetime imaging (FLIM) instrument that exploits high-speed FLIM technology in a line-scanning microscope. We demonstrate the acquisition of whole-field optically sectioned hyperspectral fluorescence lifetime image stacks (with 32 spectral bins) in less than 40 s and illustrate its application to unstained biological tissue. Microsc. Res. Tech., 2007. © 2007 Wiley-Liss, Inc.

Uptake of circulating macromolecules by the aortic wall is greater downstream than upstream of branch sites in immature rabbits, but the opposite pattern is seen at later ages. The mature pattern is nitric oxide dependent; we tested whether it is also flow dependent. Intercostal arteries of anesthetized rabbits were occluded, sham operated, or left alone. Uptake of rhodamine-labeled albumin was assessed by quantitative fluorescence microscopy of the sections through the aorta. In mature animals, uptake was higher upstream than downstream of the control and sham-operated branches, but the pattern was reversed at occluded branches. In young animals, uptake was not significantly different between regions upstream and downstream of control, sham-operated, or occluded branches. The absence of the normal immature pattern may reflect an influence of anesthesia and will assist in the elucidation of mechanisms underlying this pattern. The data for mature animals provide the first direct evidence that flow determines permeability near arterial branches and may account for the inverse spatial correlation between shear stress and disease prevalence at branches of adult human arteries.

In the microcirculation, the blood vessel wall is the major barrier which limits the transport of materials between the blood and tissues. Mass transport studies in the microcirculation are, therefore, primarily concerned with the mechanisms by which metabolites and catabolites are exchanged across the capillary wall. In contrast, in large vessels, there is more interest in the processes by which materials move into and out of the wall tissue itself. Somewhat paradoxically, the walls of large arteries are relatively avascular, having a slab of intimai and medial tissue which may be several hundred microns thick between the blood in the vessel lumen and that in the capillaries of the vasa vasorum. As thick walled vessels grow, angiogenesis occurs and capillaries penetrate the media from the adventitial side, but with increased thickening of the intima on ageing, the tissue may again become more avascular. For the tissue to undergo efficient metabolism, respiratory gases, nutrients and catabolites must be rapidly exchanged. In the field of large vessel mass transport though, the major interest has been in the movement of larger materials such as plasma proteins because of their putative role in atherosclerosis. Because this condition normally affects only arteries, transport processes in these vessels have been studied far more extensively than in veins or pulmonary vessels.

The blood vessels exhibit wide variations in structure from one site in the body to another. The marked differences in the composition, the ultrastructural organization of tissue components and the thickness of the vessel walls presumably reflect differences in the mechanical forces and chemical environment which prevailed during their growth. As the vessels age, differences also appear in their susceptibility to atherosclerosis. Veins are normally spared from the disease, except when they take on the features of arteries following the develoment of an arterio-venous fistula, or when they are used as arterial grafts. The pulmonary arteries also tend to be affected by atherosclerosis only if there is pulmonary hypertension. Even amongst the systemic arteries there is marked variability in susceptibility; vessels smaller than 1mm are rarely affected and the ascending portion of the aorta is normally affected to a lesser extent than the abdominal region.