New Neurons in a “Finished” Brain
For decades, neuroscience textbooks insisted that no new neurons appeared in the adult brain. Yet experiments by Altman and Das in rats overturned this dogma, revealing adult neurogenesis—the generation of functional neurons in maturity.
One key site is the dentate gyrus of the hippocampus, a region central to learning and memory. Here, neural stem or progenitor cells continue to divide and give rise to new neurons that integrate into existing circuits.
Development Never Fully Stops
Adult neurogenesis is modest compared to embryonic growth, but its existence underscores a broader theme: neural development blends into plasticity. Processes first defined in embryos—such as neurogenesis, migration, and synapse integration—persist in restricted adult niches, potentially supporting adaptation and repair.
Watching Brains Change Over Time
Modern imaging and mapping methods now allow scientists to watch circuit structure evolve across development. Using brain mapping, researchers can track how an animal’s brain changes over its lifetime.
By 2021, scientists had mapped and compared whole head ganglia of eight C. elegans worms at different developmental stages, down to the level of individual neurons and synaptic wiring. They also reconstructed the complete wiring of a single mammalian muscle from birth to adulthood, revealing developmental changes in connectivity.
The Human Connectome and Developmental Dynamics
In humans, diffusion-weighted MRI supports reconstruction of large-scale connectomes—graphs where vertices are gray matter regions and edges are axonal tracts between them. Datasets from hundreds of subjects, such as those made available at braingraph.org, allow comparisons across individuals.
A striking phenomenon called Consensus Connectome Dynamics (CCD) emerged when researchers visualized edges that appear in at least k out of 418 subject connectomes and gradually lowered k from n to 1. New edges appeared not randomly but in a pattern resembling a growing tree or shrub.
The hypothesis: this pattern mirrors axonal development. Connections common to most subjects may correspond to early-developing fibers, while those with greater variability represent later-developing connections.
The Takeaway
From newborn neurons in the adult hippocampus to evolving connectomes across populations, the nervous system remains a work in progress. Development leaves signatures that can be read in the architecture of the mature brain.