Morphological analysis of neovascularization at early stages of rat splenic autografts in comparison with tumor angiogenesis.

This study was undertaken to reveal the neovascularization at early stages of splenic autografts three-dimensionally, to illustrate the differences between it and tumor angiogenesis, and to establish its origin. Early vascular formation after transplantation of the rat spleen or Walker tumor into the major omentum was examined by using a video macroscope, vascular casting methods and the organ culture technique. A complex vascular network layer (vascular cortex) was first formed beneath the capsule of an autograft; later, vascular buds grew from this network toward the necrotic center. They anastomosed and changed into a form resembling withered twigs (vascular medulla). Tumor angiogenesis did not present such morphological features and was characterized by capillary loop formation with a columnar vertex resembling an "inverted V". This fundamental structure did not change throughout angiogenesis except for dilation and irregularity of vascular diameter. The organ culture technique demonstrated that the preliminary vasculature was formed in splenic autografts by regeneration of preexisting vessels in the graft and not by invading capillaries. Transmission electron microscopy showed that the cells present had characteristics of sinus endothelial cells. These results suggest that preexisting sinus endothelial cells rearrange themselves after devascularization and reconstruct a new vasculature that anastomoses with the penetrating capillaries. This mechanism establishes vascular circulation at an early stage, and accelerates regeneration of the splenic autograft before complete necrosis.