Turning on cells that build bone and turning off ones that destroy it
Construction and demolition crews working side by side. It sounds insane, but that's how the human skeleton grows and maintains itself. Each of its 206 bones harbors cells that continually deposit a rigid protein matrix that becomes mineralized. Each bone also contains cells that can break that structure down.
The bone builders keep ahead of the bone destroyers as a child grows, but the balance can shift as a person gets older.
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