Slipping through the middle

In the 1960s scientists saw evidence in electron micrographs that leukocytes migrate through the middle of endothelial cells in vivo. But when others couldn't replicate the findings in vitro and instead saw the immune cells slip between the endothelial cells, the field largely abandoned the idea of transcellular movement. Finally, on page 377, Carman and Springer show in vitro evidence that leukocytes can pass through the middle of cells as they leave the blood vessels and move into the tissue.

The team found that once a leukocyte attaches to the surface of an endothelial cell, microvilli protrude from the vascular cell surface, partially surrounding the immune cell. Adhesion proteins in the microvilli appear to realign the integrin molecules in the leukocytes, providing them with directional information. Meanwhile, a pore in the membrane of the endothelial cell forms and the...