Muscle growth, cell number, type and morphometry in single and twin fetal lambs during mid to late gestation

Muscle growth, myofibre number, type and morphometry were studied in large hindlimb muscles of single and twin fetal lambs during mid to late gestatio n. Placental insufficiency, evident by lower total placentome weight and nu mber per fetus, resulted in reduced fetal weights from 100 to 140 days gest ation in twins compared with singletons (at 140 days: 5016 +/- 108 g v. 575 0 +/- 246 g, respectively; P<0.05). However, competition between littermate s did not consistently reduce muscle mass (15-22%) until 140 days gestation . Apparent myofibre number increased with age, indicating that the full com plement of myofibres in some large hindlimb muscles may be achieved during early postnatal life. Lifter size did not impact on apparent myofibre numbe r in the semitendinosus, plantaris or gastrocnemius muscles. However, a tra nsient effect on myofibre number in the adductor femoris muscle was observe d from 80-120 days gestation. The phenotypic maturation of myofibres was un affected by increasing litter size. Smaller muscle mass in twins was associ ated with smaller myofibre cross-sectional area in the semitendinosus, addu ctor femoris and gastrocnemius muscles at 140 days gestation. A similar tre nd was observed for the plantaris muscle. These results indicate that while competition between littermates for nutrients in late gestation can impact on both fetal and muscle mass, the fetus has the capacity to buffer agains t the effects of restricted nutrient supply on myofibre hyperplasia and phe notypic maturation, but myofibre hypertrophy is compromised.