For the first time, researchers have observed the formation and evolution of new adult-born neurons.

Until 15 years ago, scientists thought adult brains ceased growing new neurons. Around the turn of the millennium, neuroscientists discovered that small portions of the adult animal brain -- the hippocampus in humans and olfactory bulb in rodents -- continue regenerating neurons.

Because these regions are rather inaccessible, watching the regeneration in action has proven extremely difficult. Scientists in France used new bioluminescent technology to illuminate these adult-born neurons in mice. They then watched their formation using electron scanning microscopes.

Their observations proved the newly born adult neurons don't just assume old pathways, but rapidly and ceaselessly forge ever-changing connections, or synapses, with neighboring neural cells.

"We expected to see the synapses gradually stabilizing, as happens during brain development," Kurt Sailor, a researcher at the Institut Pasteur in Paris, explained in a news release. "But astonishingly, these synapses proved to be highly dynamic throughout the life of the new neurons."

Sailor and his colleagues found that 20 percent of synaptic connections between new and pre-existing neurons changed on a daily basis. Scientists believe the constant rearrangement allows the new neurons to adapt to sensory changes in the environment.

The novel findings were detailed in a new paper, published this week in the journal Neuron.

"Our findings suggest that the plasticity of this constantly regenerating region of the brain occurs with continuous physical formation and elimination of synaptic connections," researchers wrote in the paper. "This structural plasticity reveals a unique dynamic mechanism that is vital for the regeneration and integration of new neurons within the adult brain circuit."