Muscle fibre differentiation and vascularisation in the juvenile European eel (Anguilla anguilla L.).

The differentiation of the lateral musculature in the elver stage of the European eel (Anguilla anguilla, L.) has been studied using histochemical staining techniques. Extracellular lipid deposits constitute 12% of the body cross-sectional area. Two fibre type may be distinguished on the basis of myofibrillar ATPase activity. Fibres with an alkaline-labile (pH 10.2) ATPase occur as a two-fibre layer around the trunk circumference, with invaginations along the horizontal septum and fin insertions. These pale-yellow fibres correspond to the slow ("red") fibres of other fish and comprise around 7% of the body cross-section (mean size, 167 micrometer2). Slow fibres show a moderate staining for PAS (glycogen), but a relatively weak reaction of Sudan black (lipid) and the aerobic enzyme markers succinic dehydrogenase (SDH) and cytochrome oxidase (COX). The bulk of the trunk muscle is composed of fast fibres (68%; 328 micrometer2). These are characterised by an alkaline (pH 10.2)-stable ATPase activity, and their innervation. Each fibre is innervated by a single "en-plaque" type endplate at one myoseptal end. Fast fibres adjacent to the slow fibre layer (2-4 fibres deep) show a moderate staining for PAS and a weak reaction for SDH and COX. Deeper regions show a wide range of fibre size, with about 5% being greater than 1,400 micrometer2. Fibres greater than 200 micrometer2 show no significant staining for glycogen, lipid, SDH, or COX. Small fast fibres less than 120 micrometer2 (up to 5% of the white muscle mass) show a strong staining reaction for PAS and a slight reaction for SDH and COX activities. Parameters of vascularisation were calculated from low-magnification electron micrographs (x 1,900). The number of capillaries/fibre and % fibre perimeter in contact with capillaries were, respectively, 0.98, 6.33% (slow fibres); 0.33, 1.96% (superficial fast fibres); and 0.12, 0.71% (deep fast fibres). These values are low in comparison with other fish species. It is suggested that the low aerobic capacity of elver slow muscle reflects a relatively restricted aerobic scope for activity associated with the anguilliform mode of locomotion.