Influence of Muscle Phenotype on Local Capillary Supply
Numerous attempts have been made to quantify the capillary supply to muscles of varying metabolic character, e.g. oxidative or glycolytic, using indices based on simple counts. Such studies have mainly been concerned with global estimates, i.e. mean values for individual muscles, using the numerical capillary to fibre ratio (C:F) or capillary density (CD). While one may infer the relative extent of capillarisation to different fibre types from the variation among muscles, this can only be in rather general terms as information about intramuscular heterogeneity is missing. One of the first studies to address the specificity of capillary supply with respect to individual fibres was that of Plyley and Groom (1975), who quantified the number of capillaries around a fibre (CAF) for individual categories or fibre types. However, the limited range of values found among mammalian muscles essentially limits its usefulness to mono-phenotypic muscle such as found in lower vertebrates, or where neovascularisation occurs without any change in the fibre population such as in response to chronic hyperaemia. These authors also attempted to view the microvascular supply from the point of individual capillaries, by quantifying the number of fibres surrounding each capillary or sharing factor (SF). It is unfortunate that variations in such indices are often assumed to directly reflect oxygen demand, attaching undue importance to such data. This ignores the conceptual limitations inherent in these indices; in particular the influence of adjacent fibres, fibre size, and proximity of neighbouring capillaries are not taken into account.
KeywordsFibre Type Capillary Density Fibre Area Oxidative Fibre Sharing Factor
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