Skin capillary metrics and hemodynamics in the hairless mouse

The transparency of the homozygous hairless mouse ear permits detailed study of the intact skin microcirculation without surgical interventions to the skin tissue. It is useful to study many microvascular phenomena and has the potential to provide data to clarify issues related to human skin microcirculation. The aim of this investigation was to quantify the normal capillary geometric and hemodynamic parameters. In each of 36 capillary pairs (10 mice), capillary diameter (D), length (L), and velocity (V) were measured and blood flow (Q) and shear rate (S = 8V D) were calculated. Loops were chosen such that each branch of the capillary pair had a common arteriolar origin, venule confluence, and thus a common pressure difference across each branch, thereby eliminating the confounding effects of these variables on perfusion differences in each of the branches. Temporal and overall distributions for each parameter were determined and comparisons between paired capillaries made. Overall mean ± SEM were for D, L, V, Q, and S, respectively, 4.8 ± 0.2 μm, 161 ± 5 μm, 192 ± 5 μm/sec, 3.6 ± 0.3 pl/sec, and 43 ± 3 sec^-1. Symmetry between paired capillaries was assessed by parameter ratios (smaller/larger); for D, V, Q, and S, respectively, these were 0.85 ± 0.02, 0.66 ± 0.03, 0.60 ± 0.04, and 0.64 ± 0.04 with corresponding distribution medians of 0.86, 0.72, 0.63, and 0.64. Similar comparisons were made for parameters in smaller/larger diameter capillary pairs yielding for V, Q and S; 1.14 ± 0.12, 0.82 ± 0.09, and 1.37 ± 0.14 with corresponding medians of 0.9, 1.07, and 0.69. These composite results provide baseline data on the naturally occurring animal-to-animal variability, temporal variation, and overall parameter value distributions in the capillary network of this experimental model of skin microcirculation. They thus provide the necessary initial framework for subsequent assessments of pharmacological interventions and the study of various pathological processes on capillary perfusion parameters.