Development of striated muscle microvasculature across the perinatal period in lambs

Transition of the fetus to extrauterine life requires increased cardiac workload and skeletal muscle activity, yet little is known about microvasculature growth during the perinatal period. We collected hindlimb skeletal muscles and cardiac left (LV) and right ventricles (RV) from fetal (135 days of gestational age; 135D) and neonatal (postnatal days 1 and 5; PD1 and PD5) lambs to measure vascular structures by immunofluorescence and expression of angiogenesis regulators. Heart and skeletal muscle weights and myofibre cross-sectional areas were greater in neonatal compared to fetal lambs. The proportion of slow-twitch oxidative myofibres in tibialis anterior (TA) and flexor digitorum superficialis (FDS) was greater in neonatal compared to fetal lambs. Vascularity in TA was 34% lower on PD1 (P = 0.0005) and 26% lower on PD5 (P = 0.00522) compared to 135D, and capillary density was 36% lower on PD5 compared to 135D (P = 0.0007). Similarly, vascularity in FDS was 40% lower on PD1 (P = 0.0003) and 45% lower on PD5 (P = 0.0001) compared to 135D. In RV and LV, vascularity was similar among age groups, but vessel density was 29% lower in LV on PD1 (P = 0.0001) and 40% lower on PD5 (P < 0.0001) compared to 135D. Several genes involved in angiogenesis were downregulated in neonatal compared to fetal muscle and LV, though VEGFA and VEGFR1 protein expression was higher. Striated muscle growth across the perinatal period is equivalent or greater than its microvascular expansion. Postnatal VEGFA protein expression may herald an increase in angiogenesis known to occur beyond the first week of life to meet ongoing striated muscle demand. KEY POINTS: Physiological changes at birth support increased cardiac workload and skeletal muscle activity in the neonate. Previous work in vivo showed that striated muscle perfusion was reduced in neonatal lambs compared to late gestation fetuses in the context of a marked increase in the partial pressure of oxygen upon breathing. Despite an increase in striated muscle size and a greater proportion of slow-twitch oxidative myofibres across the perinatal period, vascularity and microvessel density were either unchanged or reduced in several skeletal muscles and left and right cardiac ventricles of neonatal compared to late gestation fetal lambs. Our results indicate that under normal physiological conditions, striated muscle growth across the perinatal period is equivalent or greater than its microvascular expansion. Future investigations are warranted to determine how an adverse intrauterine environment or an abnormal birth transition may impact skeletal and cardiac microvascular growth.