Microvessel density, lipid chemistry and N2 solubility in human and pig adipose tissue

Decompression sickness (DCS) occurs when nitrogen gas (N2) comes out of solution too quickly, forming bubbles in the blood and tissues. These bubbles can be a serious condition; thus it is of extreme interest in the dive community to model DCS risk. Diving models use tissue compartments to calculate tissue partial pressures, often using data obtained from other mammalian species (i.e., pigs). Adipose tissue is an important compartment in these models because N2 is five times more soluble in fat than in blood; at any blood/tissue interface N2 will diffuse into the fat and can lead to bubble formation on ascent. Little is known about many characteristics of adipose tissue relevant to diving physiology. Therefore, we measured microvessel density and morphology, lipid composition, and N2 solubility in adipose tissue from humans and pigs. Human adipose tissue has significantly higher microvascular density (1.79 ± 0.04 vs. 1.21 ± 0.30%), vessel diameter (10.25 ± 0.28 vs. 6.72 ± 0.60 µm), total monounsaturated fatty acids (50.1 vs. 41.2 mol%) and N2 solubility (0.061 ± 0.003 vs. 0.054 ± 0.004 mL N2 mL⁻ ¹ oil) compared to pig tissue. Pig adipose tissue has significantly higher lipid content (76.1 ± 4.9 vs. 64.6 ± 5.1%) and total saturated fatty acids (38.8 vs. 29.5 mol%). Though two important components in gas kinetics within adipose tissue during diving (blood flow rates and degree of perfusion) are not well understood, our results indicate differences between the adipose tissue of humans and pigs. This suggests data from swine may not exactly predict gas dynamics for estimating DCS in humans.