Heterogeneity of pericyte populations in equine skeletal muscle and dermal microvessels: a quantitative study

The objective of this ultrastructural investigation was to determine if populations of pericytes in equine dermal and skeletal capillaries increase in a head-to-foot direction, as has been reported in human skeletal muscles. Samples of equine microvessels were obtained from the longissimus dorsi skeletal muscle 150 cm from the ground, from the dermis above this muscle, from the extensor carpi radiali muscle at 55 cm from the ground, from the dermis adjacent to that muscle, and from dermis 15 cm from the ground, just above the hoof wall. Tissues were processed for transmission electron microscopy. Electron micrographs were analysed with a digitizing tablet and computer, to determine the ratios of endothelial cell outer circumference and pericyte inner lengths. Pericytes were separated into two classes; those closest to the endothelial cells were defined as covering capillaries. Those separated from endothelial cells by another layer of pericytes were termed enveloping pericytes. There was much greater coverage and envelopment of dermal capillaries (85% and 135%) than skeletal muscle capillaries (27% and 31%). Regression analysis of the pericyte coverage and envelopment of dermal capillaries revealed a significant increase in pericytes toward the ground. Similarly, the two skeletal muscle tissues differed significantly in their pericyte coverage and envelopment (25/27% at 150 cm, 31/55% at 55 cm). It is concluded that, as in humans, capillary pericytes are not homogenously distributed within the same tissues, but are more numerous closer to the ground. Differences in pericyte populations could affect studies of microvessel function.