IN-VIVO ANGIOGENESIS IN ADULT-RAT SKELETAL-MUSCLE - EARLY CHANGES IN CAPILLARY NETWORK ARCHITECTURE AND ULTRASTRUCTURE

The individual structural stages in capillary growth have been identif ied during development and under pathological circumstances in adults (wound healing, tumors), but there are no data to indicate whether the se steps are similar when angiogenesis is induced in a fully different iated microvascular bed in normal, uninjured adult skeletal muscle. In this study changes in capillary ultrastructure were correlated with c apillary density and network morphology to elucidate the sequelae of a ngiogenesis in adult rat extensor digitorum longus (EDL) muscle whose activity was increased by stimulation at 10 Hz (8 h/day). This resulte d in an increased capillary/fiber (C/F) ratio (based on staining for a lkaline phosphatase) after 4 days; by 7 days C/F ratio was increased f urther, by approximately 50%. The ultrastructure of capillary endothel ium in both the EDL and extensor hallucis proprius (EHP) was similar t o control muscles after 2 days of stimulation, whereas endothelial cel ls in some capillaries in muscle stimulated for 4 days revealed signs of metabolic activation such as proliferation of organelles (Golgi app aratus, endoplasmic reticulum, ribosomes and mitochondria) and fewer p inocytic vesicles. Luminal surfaces were often irregular with numerous pseudopodial processes. Basement membranes were always present but am orphous regions were observed, particularly near pericyte processes. U nusually small capillary profiles, with either a slit-like lumen or wi th cisternae but no lumen, probably represented capillary sprouts. The interstitium contained increased collagenous and granular extracellul ar matrix surrounding capillaries, and numerous activated fibroblasts which were closely apposed to many capillaries. Capillary growth in EH P was also evaluated by confocal microscopy using whole mounts. The co mplex pattern of vessels underwent remodelling between 2 and 7 days of stimulation, resulting in more tortuous capillaries with numerous spr outs and loops. These combined observations suggest that angiogenesis may occur by a combination of sprouting, intussusceptive growth and el ongation; also, that activation of endothelial cells occurs at the sam e time as disturbance of basement membranes during the earliest phase of growth and remodelling of the capillary bed. These changes are post ulated to occur in connection with increased shear stress and/or capil lary wall tension, which have been demonstrated previously.