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Figure 3. Role of cell-autonomous and host-dependent CXCR4 signalling in experimental metastasis formation in the zebrafish xenograft model. A: inoculation of human tumour cells into the blood circulation of zebrafish embryos results in experimental metastasis formation, characterized by tumour cell aggregates in the blood vessels, and extravasation and invasion in the surrounding tissue, in the region of the caudal hematopoietic tissue (CHT). During early metastatic events, endothelium alteration takes place and neutrophils localize in the surrounding of the tumour. The CHT is a vascular plexus in the tail fin between the DA and the CV and is a hematopoietic site; B: upon disruption of the tumour cell-autonomous CXCR4 signalling, cancer cells are unable to initiate early metastatic events, while surrounded by immune cells; C: the same inhibition of experimental metastasis formation occurs upon disruption of the host-dependent CXCR4 (Cxcr4b) signalling. Neutrophils are preferentially retained in the CHT and their recruitment at the metastatic site is impaired upon Cxcr4 signalling inhibition