Histologic analysis and evolution of the MVD. (A) Histologic analysis and CD31 staining of BM samples during 5T2MM disease progression. From the onset of the tumor inoculation 3 mice were analyzed each week until the end stage of the disease was reached. From each mouse one hind leg was processed for CD31 staining. (i) Preangiogenic stage: Small tumor aggregates were formed (upper left corner) without increase of the MVD. The right part of the figure illustrates tumor-free normal BM tissue. This section was made from a mouse killed at day 35. Bar = 100 μm. (ii) Early angiogenic stage: The angiogenic stage started when large confluent aggregates of MM cells were formed (left). On the right side, in the noninvaded part of the BM, the MVD is lower than on the left side. This section was made from a mouse killed at day 49. Bar = 200 μm. (iii) Late angiogenic stage: Massive BM invasion was accompanied by a high MVD. Most of the microvessels are small and intensely stained for CD31. This section was made from a mouse at day 77. Bar = 200 μm. Each section is representative for 3 mice. (B) Evolution of the MVD during 5T2MM disease progression. MVD (number of blood vessels per 0.20 mm²) was quantified as described in “Materials and methods.” Each point represents mean values of 3 mice ± SD.