X Li

Every morning the city wakes up and starts repairing itself: workers replace asphalt, painters coat facades, street sweepers haul away trash. You can picture skin as just such a city, where worker cells constantly renew the streets and buildings.

In the bone marrow, as in a barracks, there is a constant roll call. Rookie T cells wait for their first assignment, and for a while they all look alike. But with age, soldiers with a special mark on their uniform start to appear among them: the protein annexin A1. This mark is not a decoration but a brake shoe that keeps the cell from charging into battle.

In a fibrotic liver, as in a city after a long siege, neighborhoods appear where life has stalled: working cells are replaced by scar tissue, and hiding inside it are senescent macrophages. These cells have stopped dividing but have not retired. They release substances that push their neighbors to build even more scar and drive off immune patrols.

In the aged skin of a male mouse, every third hair follicle is left without a nerve, like a house whose electrical line has been pulled away. The socket itself is intact: the neurons, tucked away in nerve ganglia along the spine, have not gone anywhere. They are alive and have no intention of dying.