Vascular invasion by cancer cells. Images showing injections of human cell lines (red) into zebrafish embryos, an animal model used to study the characteristics and behaviour of human cancer cells. The cell lines are: normal human cells (breast cells 1 and 2, across centre); human tumour cells (adenoid cystic carcinoma of salivary gland, or ACC, upper left); and three forms of non-metastatic tumour cells (MCF7, ACC PDX, and MDA-MB231). The metastatic tumour cells, but not normal or non-metastatic tumour cells, exhibit vascular invasion within 5 to 7 days after injection. The vascular system is labelled green; white arrows indicate sites of human cell injection; red arrows indicate the invasion of tumour cells; and additional red arrows point to the cells that have extravasated.