Older animals recover slowly from nerve injury because they take longer to clear debris from damaged nerves than younger creatures do.

Hyuno Kang and Jeff Lichtman at Harvard University in Cambridge, Massachusetts, crushed the nerves that transmit signals to muscles in young and old mice, and used high-resolution time-lapse imaging to follow the nerves' regeneration.

Genetic engineering and staining techniques allowed the duo to distinguish cell types and cell debris by colour. The pair found that nerve growth slowed or temporarily halted when the tips of nerve fibres, or axons, hit lingering debris. Young mice broke down these nerve remnants faster than old mice did.

Axons in aged mice regenerated as quickly as those in young animals if their paths were clear of cell junk, so the authors suggest that nerve-injury therapies for older people could target debris-clearance mechanisms.

J. Neurosci. 33, 19480–19491 (2013)