"To label the cells" means to assign or categorize different cells with specific identifying marks or names for easier recognition or differentiation. Full definition
This will involve labeling cells in the growing embryo tail and using microinjections of dye to follow the pathways that these cells take as the tail develops. (discovermagazine.com)
Mouse heart with two sets of labeled cell clones (red and green). (gladstone.org)
Perin also used antibodies to label cells expressing vimentin and desmin, proteins found on developing, not mature, heart muscle cells. (discovermagazine.com)