A Deep-Brain Signal Told Human Stem Cells to Make More Neurons

Long before the thalamus relays sights and sounds to a mature brain, its growing nerve fibers may already be helping decide what the human cortex becomes. A study published in Science on September 3 found that direct contact from thalamic projections pushed human cortical stem cells to produce more excitatory neurons in laboratory models.

The peer-reviewed study addresses a timing puzzle. Thalamic fibers reach the developing cortex earlier in humans than the mature connections they eventually form with cortical neurons. The team asked whether those early arrivals were already sending instructions to neural progenitors.

The fibers were not merely waiting for future circuitry. In the model, their contact changed which cells the cortex produced.

Building two brain regions together

The researchers grew small three-dimensional models of the thalamus and cortex from human stem cells, then fused them into “assembloids.” This let projections from the thalamic side extend toward cortical radial glia, stem-like cells that generate many of the cortex’s neurons and support cells.

Compared with cortex-only models, thalamic input promoted the proliferation of outer radial glia and increased excitatory neuron production. The change was strongest among upper-layer neurons, which help connect different cortical areas and are especially expanded in the human brain.

A contact point involving NRXN1

The effect required physical proximity rather than only chemicals diffusing through the tissue. The researchers traced part of the interaction to NRXN1, a gene commonly associated with proteins that organize connections between nerve cells.

When the team used patient-derived cells carrying an NRXN1 mutation, the thalamic signals behaved differently and shifted the balance between progenitors and the neurons they made. That result provides a system for studying how one altered contact mechanism can reshape early development.

Why this changes the developmental picture

Brain development is often described as a sequence controlled mainly by genetic programs inside each cell. The new work adds an external instruction: a neighboring brain region can influence cortical cell fate before its familiar adult circuit is complete.

The result also shows why assembloids can reveal relationships that isolated organoids cannot. By connecting separately grown regions, researchers can observe migration, wiring and tissue-to-tissue signals while still manipulating individual genes.