Arteriolar network growth in rat striated muscle during juvenile maturation

To better understand normal microvascular network growth, we conducted a longitudinal study on the spinotrapezius muscle of Sprague-Dawley rats of 3 ages: weanling (3-4 weeks), juvenile (7-8 weeks) and mature (11-12 weeks). Morphometric analysis revealed that despite dramatic growth of the muscle (from 117 +/- 60 mg in weanling to 417 +/- 112 mg in mature rats), there is no significant change in the total number of arcade arteriole segments per network over this period (from 140 +/- 35 to 181 +/- 51). The mean arcade segment length increased over this period (from 0.96 +/- 0.17 to 1.45 +/- 0.32 mm), but not in proportion to tissue growth. Consequently, the total arcade segment length per unit muscle volume significantly decreased (from 1.36 +/- 0.48 to 0.66 +/- 0.12 mm/mm3). The estimated number of transverse arteriolar trees per muscle (approximately 600) did not appreciably change with growth, leading to a decrease in the number of trees per millimeter arcade arteriole (from 4.4 +/- 1.2 to 2.3 +/- 0.6). Transverse arteriolar trees underwent age-dependent increases in the number of segments within each branch order and in mean segment length. These observations suggest that arteriolar network growth during juvenile maturation occurs by elongation of pre-existing arcade and transverse vessels with angiogenesis occurring in the distal segments of transverse arteriolar trees.