MLS ovarian epithelial carcinoma multicellular spheroids xenografted subcutaneously in CD-1 nude mice displayed growth delay, or dormancy, of up to 52 days. In the study reported here, implanted MLS spheroids were used for testing the role of angiogenesis and vascular maturation in triggering the initiation of tumor progression. The kinetics and impact of neovascular maturation and functionality, in dormancy, and growth of MLS spheroid xenografts were studied noninvasively by BOLD contrast MRI. MR data were supported by histologic staining for biotinylated albumin as a blood pool marker and alpha-smooth muscle actin (alpha-SMA) as marker for perivascular mural cells. Although the tumor periphery showed higher levels of total and mature vasculature than normal skin, the fraction of mature out of the total vessels as detected by MRI vascular maturation index (VMI(MRI)) was significantly lower in the tumor both before and after tumor exit from dormancy. The neovasculature induced by the implanted spheroid was unstable and showed cycles of vessel growth and regression. Surprisingly, this instability was not restricted to the immature vessels, but rather included also regression of mature vessels. During dormancy, neovasculature was predominantly peripheral with no infiltration into the implanted spheroid. Infiltration of alpha-SMA positive stroma cells into the spheroid was associated with functional vascularization and tumor growth. Thus, stroma infiltration and vascular maturation are an important checkpoint linking the angiogenic switch with initiation of tumor progression.
Copyright 2003 Wiley-Liss, Inc.