Morphogenesis of blood vessels in the head muscles of avian embryo: spatial, temporal, and VEGF expression analyses

Abstract

Adult skeletal muscle is a highly vascularized tissue, but the development of intramuscular endothelial networks has not been well studied. In quail embryos, QH1-positive angioblasts are present and moving throughout myogenic head mesoderm before the onset of primary myotube formation. On day 5 of incubation, concurrent with early myotube formation and aggregation, angioblasts establish a transient vascular plexus surrounding the myogenic condensations. Between days 5 and 9, the intramuscular vessels form an irregular network of endothelial cords and patent channels and only later are the parallel arrays of capillaries characteristic of adult muscles established. Microinjections using India ink, QH1, and Mercox resin reveal that these intramuscular capillaries are typically not connected to systemic vessels of the head until day 10, which is near the end of primary myogenesis and corresponds to the onset of muscular function. Morphometric analyses performed during primary myogenesis stages show a decrease in muscle cell density but no significant changes in intramuscular vascular density between days 5 and 9. This finding was surprising, as it is generally assumed that muscle growth requires elevated oxygen and nutrient levels. Moreover, there are no significant morphometric differences in vascular supply to embryonic fast and slow muscles. Endothelial tissue density is similar in slow muscles (oculorotatory, e.g., lateral rectus), fast muscles (mandibular depressor), and mixed muscles, in which the fiber types can be interspersed (jaw adductors) or segregated (branchiomandibular). Vascular endothelial growth factor (VEGF) protein is abundant in myotubes but not endothelial cells within both fast and slow head muscles at days 7 and 9. However, in some mixed muscles, only a minority of myotubes, which do not correspond to one specific fiber type, express VEGF. These results document a dynamic set of intramuscular and perimuscular angiogenic reorganizations during avian head myogenesis. Thus far, no vasculogenic distinctions between fast and slow muscles have been observed, although muscle heterogeneity in VEGF expression is evident.