Morphometric analysis of myofibre development in the adductor femoris muscle of single and twin fetal lambs

Cellular development of the adductor femoris muscle from twin and single fe tuses was studied at 140 days gestation to evaluate the effect of moderate fetal growth retardation on myofibre development. Twin fetuses had lower bo dyweights (13%) and disproportionately small adductor femoris muscle weight s (22%) compared with single fetuses. Reduced muscle mass was associated wi th smaller myofibre cross-sectional areas (CSA) and lower DNA content (22%) , indicative of fewer myonuclei and retarded myofibre hypertrophy. Myofibre number and the phenotypic maturation of the myofibres were similar between twins and singletons. These results indicate that even modest growth restr iction during fetal life can negatively influence myofibre hypertrophy, hig hlighting the importance of fetal nutrition for muscle growth. Large muscle s, such as the adductor femoris, have intrafascicularly terminating myofibr es, which necessitates accurate sampling of the muscle when investigating p ossible perturbations in morphological characteristics (e.g. between single tons and twins). The second objective of the present study was to investiga te the impact of the sampling site on the morphological parameters of the a dductor femoris muscle. The apparent total myofibre number decreased from t he proximal to the distal region of the adductor femoris muscle. The appare nt number of slow-twitch fibres also decreased from the proximal to the med ial region, but was not different between the medial and distal regions of the muscle. Similarly, myofibre CSA differed between the medial and distal regions. These results indicate that, particularly with large muscles, such as the adductor femoris, which has intrafascicularly terminating myofibres , single site sampling for the determination of morphological fibre charact eristics may generate misleading results and that careful selection of the sampling area may be necessary.