Not born in the brain: Bone marrow-derived microglia in human aging.

Bone marrow-derived cells in the aging human brain are linked to microglia-like populations, supported by somatic mutation evidence and a clonal composition signal.

• Why it matters: Aging microglia may be shaped by blood-derived inputs, but the extent and clonal makeup of that contribution are unclear.
• What they did: Belk et al. used somatic mutations to track bone marrow-derived cells entering the aging human brain and assessed whether they resemble microglia.
• The result: The aged microglial compartment may have a substantial hematopoietic contribution, and its clonal composition could matter for stem-cell-based microglial replacement.

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