Entry of metastatic malignant cells into the circulation from a subcutaneously growing myelogenous tumor

The initial stage of metastasis formation, i.e., the entry of metastatic cells from a malignant subcutaneously growing myelogenous tumor into the circulation, was observed by means of transmission electron microscopy and scanning electron microscopy in Long-Evans rats. The transmural passage of malignant cells occurred through the walls of intact venules within the tumor. The malignant cell penetrated the endothelial cell body by making a temporary migration pore. The migration pore closed after the malignant cell entered the vascular lumen. The endothelial vascular lining remained continuous. The entry of malignant cells into the circulation was not the result of the general invasive and destructive properties of malignant tumor cells, but the consequence of a specific action of the tumor cells on the abluminal plasma membrane of the endothelium.