Birth requires an immediate increase in left ventricular workload and skeletal muscle activity. Although oxygen availability rises dramatically with the onset of breathing, little is known about how the striated muscle microvasculature adapts during this transition.
In this study, we collected hindlimb skeletal muscles and left and right ventricles from late gestation fetal and neonatal lambs to quantify vascular structure and angiogenic signaling.
As expected, heart and skeletal muscle weights and myofiber cross-sectional areas were greater in neonatal compared to fetal lambs. The proportion of slow-twitch oxidative fibers in leg muscles also increased after birth.
In contrast, microvascular measures did not increase in parallel. Vascularity was significantly lower in skeletal muscles after birth compared to immediately before birth. In the left ventricle, vessel density was reduced in neonates, while overall ventricular vascularity was unchanged. Several angiogenesis-related genes were downregulated postnatally, although VEGFA protein expression was higher.
Together, these findings indicate that under normal physiological conditions, striated muscle growth across the perinatal period is equivalent to or greater than its microvascular expansion. Increased VEGFA protein after birth may signal angiogenesis that occurs later in postnatal life to meet ongoing cardiac and skeletal muscle demand.
Find this paper on Pubmed (soon) and The Journal of Physiology.