The fine structure of tumor blood vessels. I. Participation of non-endothelial cells in tumor angiogenesis.

This report provides fine structural evidence that, dependent upon the malignancy, tumor as well as mesenchymal cells may participate actively in the neovascularization of experimental tumors grown in transparent tissue chambers implanted into skinfolds of syrian hamsters. Such non-endothelial cells may help to promote angiogenesis in two different ways: (1) They are incorporated into capillary sprouts thereby accelerating the growth rate of the latter independent of endothelial cell proliferation. (2) Extravascular cells (tumor and mesenchymal elements) become integrated in varying numbers into the linings of comparatively large blood-perfused vessels. This facilitates the rapid establishment and functional remodelling of the microvascular bed to adapt the microcirculation to the varying local demands of the growing tumor. If these results can be confirmed for other tumors, and if they are independent of the tumor's environment and the experimental protocol, then we will have to reconsider the significance of tumor angiogenesis as a realistic biological model from which general conclusions with regard to neovascularization in non-tumorous tissues may be drawn.