Abstract:
Muscle growth, myofibre number, type and morphometry were studied in largehindlimb muscles of single and twin fetal lambs during mid to late gestation.Placental insufficiency, evident by lower total placentome weight and numberper fetus, resulted in reduced fetal weights from 100 to 140 days gestation intwins compared with singletons (at 140 days: 5016 108 gv. 5750 246 g, respectively;P<0.05). However, competition between littermates didnot consistently reduce muscle mass (15-22%) until 140 daysgestation. Apparent myofibre number increased with age, indicating that thefull complement of myofibres in some large hindlimb muscles may be achievedduring early postnatal life. Litter size did not impact on apparent myofibrenumber in the semitendinosus,plantaris or gastrocnemiusmuscles. However, a transient effect on myofibre number in theadductor femoris muscle was observed from 80-120days gestation. The phenotypic maturation of myofibres was unaffected byincreasing litter size. Smaller muscle mass in twins was associated withsmaller myofibre cross-sectional area in thesemitendinosus, adductor femorisand gastrocnemius muscles at 140 days gestation. Asimilar trend was observed for the plantaris muscle. These results indicatethat while competition between littermates for nutrients in late gestation canimpact on both fetal and muscle mass, the fetus has the capacity to bufferagainst the effects of restricted nutrient supply on myofibre hyperplasia andphenotypic maturation, but myofibre hypertrophy is compromised.