Book & CD. The brain has a number of nerve cells estimated at a magnitude of 10 to 100 billion, and 1014 to 1015 synapses, and therefore is the most complex organ of the human body. During fetal development the foundations of the brain are laid as billions of neurons form appropriate connections and patterns. In the adult mammalian brain, most neurons are post-mitotic, and therefore at risk for irreversible damage. As we age, atrophy of the brain occurs. As brain weight declines the volume of the brain in the 8th decade is reduced by 6%-10% versus the third decade, and neuronal loss occurs, up to 10,000 to 100,000 neurons are lost per day, though this estimation is being revised downward with the advance of more sophisticated measurements.