Genetically programmed:
Most neurons are created during embryonic development and have no “backup” after birth. Researchers have generally believed that their survival is determined nearly extrinsically, or by outside forces, such as the tissues and cells that neurons supply with nerve cells.
A research team led by Sika Zheng, a biomedical scientist at the University of California, Riverside, has challenged this notion and reports the continuous survival of neurons is also intrinsically programmed during development.
The study, published in the journal Neuron, identifies a mechanism the researchers say is triggered at neuron birth to intrinsically decrease a general form of cell death — or “apoptosis” — specifically in neurons. When this genetic regulation is stopped, continuous neuronal survival is disrupted and leads to the death of the animal.
Iqbal Pittalwala, “Neurons are genetically programmed to have long lives” at University of California – Riverside
Paper. (paywall)
Funny how all those interlocking layers of specified complexity just sort of happen. Hmm.