Podosomes push through

To exit the blood, leukocytes must either squeeze past or go directly through the endothelial cells that line the vessels. According to Christopher Carman, Timothy Springer (Harvard Medical School, Boston, MA), and colleagues, to take the latter transcellular route, leukocytes first palpate the endothelial cell with exploratory protrusions called podosomes. The leukocytes then use one, or sometimes several, of these podosomes to push right on through.

The team found that approximately one-third of leukocytes made their way across an in vitro vascular endothelial monolayer by transcellular migration. Carman suggests that, in vivo, at least in some settings, the proportion might be higher. Transcellular migration might even be favored over the paracellular route, as the latter could potentially weaken cell–cell junctions.

To investigate the mechanism of transcellular migration, the...