In vivo Analysis of Tumor Vascularization in the Rat

By using transparent chambers in rats, we have directly observed tumor‐induced neovascularization in the early stage and the formation of intricate networks in Yoshida rat ascites hepatoma AH109A and Sato lung carcinoma at high magnification. We counted branching point numbers per unit area in the microvascular network with and without tumors in order to clarify the sites from which new vascular sprouts originate. Branching point number per unit area in normal tissue was 13.6 ± 7.4/0.1 mm(2) in the field near a terminal arteriole, and 12.9 ± 7.3/0.1 mm(2) in the field distant from a terminal arteriole. There was no significant difference between these two fields in the normal vascular network. On the other hand, in the tumor vascular network, the branching point number in the field near a terminal arteriole was 50.4 ± 12.6/0.1 mm(2), and 30.1 ± 11.5/0.1 mm(2) in the field distant from a terminal arteriole. The difference is highly significant (P<0.001). The frequency with which new capillaries originated from veins and venules was very low. We concluded from these results that the position from which tumor vessels originated was usually the terminal portion of a terminal arteriole.