Growing blood vessels are guided by so-called ‘tip cells’. These are differentiated endothelial cells able to sense their environment to induce proliferation of nearby cells necessary for blood vessel formation, known as angiogenesis. They organize directional migration and extend filopodia (protrusions from thecell). We discovered that tip cells also exist in cell cultures, where they can be identified by the presence of a protein named CD34.The image shows a scanningelectron microscope image of one of these cells in culture. You can see a tip cell with filopodia-like extensions which allow the cell to move around. Moving tip cells take on a more 3D appearance than their stationary cousins.
M.J. SIEMERINK et al. Angiogenesis 15 (1), 151–63 (2012)
 DALLINGA MG, BOAS SE, KLAASSEN I, MERKS RH, VAN NOORDEN CJ, SCHLINGEMANN RO. Tip Cells in Angiogenesis. eLS. 2015;1–10.
Image credit: Marchien Dallinga, PhD student at Amsterdam Medical Center (AMC)