FIGURE 2. Pre-metastatic clusters are comprised of VEGFR1+ haematopoietic progenitors.
From the following article:
VEGFR1-positive haematopoietic bone marrow progenitors initiate the pre-metastatic niche
Rosandra N. Kaplan, Rebecca D. Riba, Stergios Zacharoulis, Anna H. Bramley, Loïc Vincent, Carla Costa, Daniel D. MacDonald, David K. Jin, Koji Shido, Scott A. Kerns, Zhenping Zhu, Daniel Hicklin, Yan Wu, Jeffrey L. Port, Nasser Altorki, Elisa R. Port, Davide Ruggero, Sergey V. Shmelkov, Kristian K. Jensen, Shahin Rafii and David Lyden
Nature 438, 820-827 (8 December 2005)
doi:10.1038/nature04186
a, VEGFR1 staining in irradiated lung before tumour implantation (left panel and inset; n = 10) and 14 days after LLC cell implantation showing clusters in the lung (right panel, arrows; n = 18, 3.9 
0.2% cells with VEGFR1 staining per 
100 objective field, P < 0.05). b, c, Double immunofluorescence in the lung of an animal with day 14 LLC tumour. b, VEGFR1+ (red) and GFP+ (green) bone marrow cells (left panel), VEGFR1+ (red) and CD133+ (green) (right panel). c, VEGFR1+ (red) and CD117+ (green). d, VEGFR1+ clusters in c-Myc transgenic lymph node at day 40 of life and before tumorigenesis (middle panel and inset showing VEGFR1+ cells (red)) as compared with wild-type littermate lymph node without the transgene (left panel), and day 120 c-Myc transgenic node with lymphoma (right panel). In the inset of the right panel, arrows indicate the VEGFR1+ clusters (red) surrounded by lymphoma (green) (n = 6). Scale bar at bottom right applies to panels a (80 
m; left inset, 40 m), b (20 m), c (20 m) and d (80 m; insets, 8 m).