Development of the vascular supply in rat skeletal muscles

The vascular development in the extensor muscles of rat forelegs from 12-day-old embryos up to 9-day-old animals was investigated with a light microscope using semithin plastic sections. On late day 12, blood vessels are distributed over the whole cross sectional area of the foreleg. On early day 13, a rearrangement of the blood vessels is visible. Now an avascular centre is enclosed by a vascularized tissue mantle, in which the muscle anlage starts developing. Few capillaries are visible within the muscle anlage. After differentiation of the muscle anlage into individual muscles the epimysium is passed by a netlike capillary plexus enwrapping the first avascular muscles. With the establishment of the perimysium on day 16 of embryonic development some capillaries are also visible within the individual muscles. The diameter of the capillaries is by far larger than the diameter of the myotubes. In older embryos the perimysium shows an increasing number of such large capillaries, whereas in the epimysium the capillaries are less frequent. In the perimysium a few additional capillaries with a small diameter of about 5 micron occur. On day 3 postpartum, small capillaries are also visible within the muscle fibre bundles. In the perimysium many large capillaries are still obvious. The vascular supply on day 9 postpartum, resembles that in adult muscles. The vascular supply of the muscles progresses from their periphery to more and more smaller compartments within the muscles, in close correlation to the establishment of the muscular connective tissue.