MICROVASCULAR PATTERN IN THE METAPHYSIS DURING BONE-GROWTH

Background: Little is known about the three-dimensional micromorpholog y of vessels in the growth zone of long bones, where significant vascu logenesis occurs, Therefore, we examined the microvascular pattern of the femoral metaphysis. Methods: Six-week-old normal rats of either se x were used, We cast the femurs of 14 rats with Mercer for scanning el ectron microscopy (SEM), and in 10 rats we prepared tissue sections of femurs for light (LM) and transmission electron microscopy (TEM). Res ults: In the LM, calcified cartilage was found to define cylindrical c ompartments beneath the last row of hypertrophied chondrocytes of the metaphyseal growth plate, These compartments ran in the bone's longitu dinal axis and contained a single capillary profile. Endothelial cells of these capillaries often showed increased cytoplasmic volume and lo ose texture of nuclear chromatin. Cast metaphyses by SEM showed numero us parallel vascular loops with nodular protrusions 10-12 mu m in diam eter at their tips, The loops had ascending and descending limbs with a luminal diameter of 10-14 mu m. Small projections 4-5 mu m in diamet er and delicate crests were sometimes found on the tip of the larger n odes. In a 100 x 100 mu m area, there were 14-17 large nodes. By TEM, capillary sprouts were identified at the level beneath the last row of hypertrophied chondrocytes, These capillaries had voluminous endothel ial cells rich in free ribosomes and rough endoplasmic reticulum, Endo thelial cell nuclei were rounded and showed loose chromatin texture, E ndothelial cells were connected by intermediate junctions and there wa s no basal lamina, Deeper into the metaphysis, arterioles and sinusoid s were present. Conclusions: We conclude that the metaphyseal plate of the growing rat offers an optimal model to study vasculogenesis, Capi llary sprouts can be readily identified, measured, and counted because they are located within a plane bordering against avascular cartilage . In addition, by using microvascular corrosion casting in SEM not onl y capillary sprouting per se but also different stages of neovasculari zation, indicated by differently sized nodular projections at the tip of vascular loops, can be studied in the growing long bone. (C) 1995 W iley-Liss, Inc.