138 Placental insufficiency induced fetal and skeletal muscle growth restriction in fetal sheep.

Abstract Introduction and Methods: Placental insufficiency (PI) and fetal growth restriction (FGR) increases perinatal mortality and reduces postnatal productivity in livestock. Skeletal muscle constitutes a large proportion of tissue mass in the fetus, which is significantly reduced in the FGR fetus. This study investigates the effects of PI-FGR on musculoskeletal growth in sheep fetuses. Placental insufficiency and FGR (n = 8) was induced in pregnant ewes exposed to heat stress (35 to 40°C; RH 30-40%) between 40 and 95 days of gestation (term: 149 days). Control fetuses (n = 9) were from ewes maintained in thermoneutral conditions (20°C; RH 15-25%). At d120 ± 1, fetal surgeries were performed to place indwelling catheters for blood sampling. At d133 ± 1, umbilical blood flow was assessed before the fetuse was euthanized. Fetal hindlimb muscle satellite cells were isolated, and proliferation rates were assessed both in vivo and in vitro. Results: FGR fetuses had lower plasma glucose, insulin, IGF-1, and blood oxygen content (all P < 0.01). Absolute umbilical blood flow was lower in FGR fetuses (355 ± 41 vs. 619 ± 61 ml/min, P < 0.01), but weight-normalized blood flow was not different between groups. Both fetal weight (2118 ± 243 vs. 3423 ± 229 g) and placental weight (219 ± 41 vs. 426 ± 38 g) were lower (P < 0.01) in FGR fetuses. FGR fetuses exhibited higher brain-to-fetal (1.57 ± 0.06 vs. 1.34 ± 0.06, P = 0.01) and brain-to-liver weight ratios (0.85 ± 0.07 vs. 0.50 ± 0.06, P < 0.01), indicating brain sparing and asymmetrical growth. Hindlimb average muscle mass was reduced in FGR fetuses, including the biceps femoris (11.8 ± 2.2 vs. 18.4 ± 2.0 g, P = 0.04), semitendinosus (3.8 ± 0.7 vs. 6.3 ± 0.6 g, P = 0.01), gastrocnemius (7.7 ± 1.0 vs. 12.6 ± 1.0 g, P < 0.01), and tibialis anterior (3.4 ± 0.5 vs. 5.6 ± 0.5 g, P < 0.01). Muscle weights correlated (P < 0.01) with placental or fetal weights. In vivo, satellite cells proliferation rates were lower in FGR hindlimb muscle (3.8 ± 0.5 vs. 8.2 ± 1.5 %, P < 0.01). However, FGR isolated myoblasts had higher proliferation rates when cultured in nutrient enriched growth media (52 ± 0.5 vs. 48 ± 1.0 %, P < 0.01). Conclusion: FGR fetuses exhibited hypoglycemia, hypoxia, and reduced anabolic hormone concentrations, contributing to reduced hindlimb muscle mass. The contrast between lower satellite cell proliferation in vivo and increased proliferation in vitro under nutrient-enriched conditions suggests that nutrient deficiency and hypoxic stress are primary inhibitors of muscle cell growth in FGR fetal muscle. These results indicate that targeted nutrient or hormonal interventions may help restore muscle growth potential in FGR fetuses.