Proliferative effect of platelet on ovarian cancer cells in vivo. The proliferation index was measured in (A) tumors resected from tumor-bearing mice infused with buffer (control; n = 10) or platelets (n = 10) and (B) surgical specimens from ovarian cancer patients with normal platelet counts (n = 10) or thrombocytosis (n = 10), by Ki67 immunostaining. From each tissue block 5 sections were prepared, and from each section 5 fields were counted at original magnification ×200 of a light microscope by 2 different observers as independent blinded assessments. The ratio of positive Ki67 nuclear staining to total number of nuclei was measured for each field. The cumulative results are summarized as bar graphs, and a representative Ki67 staining is shown above each column.