Microvascular anatomy suggests varying aerobic activity levels in the adipose tissues of diving tetrapods.

Adipose tissue has many important functions including metabolic energy storage, endocrine functions, thermoregulation and structural support. Given these varied functions, the microvascular characteristics within the tissue will have important roles in determining rates/limits of exchange of nutrients, waste, gases and molecular signaling molecules between adipose tissue and blood. Studies on skeletal muscle have suggested that tissues with higher aerobic capacity contain higher microvascular density (MVD) with lower diffusion distances (DD) than less aerobically active tissues. However, little is known about MVD in adipose tissue of most vertebrates; therefore, we measured microvascular characteristics (MVD, DD, diameter and branching) and cell size to explore the comparative aerobic activity in the adipose tissue across diving tetrapods, a group of animals facing additional physiological and metabolic stresses associated with diving. Adipose tissues of 33 animals were examined, including seabirds, sea turtles, pinnipeds, baleen whales and toothed whales. MVD and DD varied significantly (P < 0.001) among the groups, with seabirds generally having high MVD, low DD and small adipocytes. These characteristics suggest that microvessel arrangement in short duration divers (seabirds) reflects rapid lipid turnover, compared to longer duration divers (beaked whales) which have relatively lower MVD and greater DD, perhaps reflecting the requirement for tissue with lower metabolic activity, minimizing energetic costs during diving. Across all groups, predictable scaling patterns in MVD and DD such as those observed in skeletal muscle did not emerge, likely reflecting the fact that unlike skeletal muscle, adipose tissue performs many different functions in marine organisms, often within the same tissue compartment.