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 fetuses was studied at 140 days gestation to evaluate the effect of moderate fetal growth retardation on myofibre development. Twin fetuses had lower bodyweights (13%) and disproportionately small adductor femoris muscle weights (22%) compared with single fetuses. Reduced muscle mass was associated with 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 restriction during fetal life can negatively influence myofibre hypertrophy, highlighting the importance of fetal nutrition for muscle growth. Large muscles, such as the adductor femoris, have intrafascicularly terminating myofibres, which necessitates accurate sampling of the muscle when investigating possible perturbations in morphological characteristics (e.g. between singletons and twins). The second objective of the present study was to investigate the impact of the sampling site on the morphological parameters of the adductor femoris muscle. The apparent total myofibre number decreased from the proximal to the distal region of the adductor femoris muscle. The apparent number of slow-twitch fibres also decreased from the proximal to the medial 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 characteristics may generate misleading results and that careful selection of the sampling area may be necessary. Extra keywords: fetus, morphometry, sheep.