Japanese lab-grown brain circuits reveal how the thalamus shapes cortical development

Japanese scientists have engineered miniature human brain circuits in the lab, revealing how a deep brain structure called the thalamus helps organize and mature neural networks in the cerebral cortex. By fusing tiny brain-like tissues grown from stem cells, the team recreated key wiring patterns and activity seen in the developing human brain.

The findings suggest that communication from the thalamus encourages cortical neurons to mature and fire in coordinated patterns, offering a new way to investigate conditions such as autism spectrum disorder and other neurodevelopmental or psychiatric illnesses.

Recreating human brain connections in the lab

The research team in Japan used structures known as assembloids to model how different brain regions connect. Assembloids are formed by combining separate three-dimensional organoids, each grown from human induced pluripotent stem (iPS) cells and designed to resemble a specific brain region.

In this study, scientists at the Graduate School of Pharmaceutical Sciences at Nagoya University, led by Professor Fumitaka Osakada with graduate student Masatoshi Nishimura and colleagues, first generated two kinds of organoids: one mimicking the cerebral cortex and another representing the thalamus.

They then physically fused these cortical and thalamic organoids, allowing the tissues to grow together and establish connections. This fusion created an assembloid that could model how the thalamus and cortex interact during human brain development, a process that is extremely difficult to study directly in living people.

Why cortical neural circuits are so important

The cerebral cortex is responsible for many higher brain functions, including perception, thought and complex cognition. It contains multiple types of neurons that must connect precisely with one another and with other brain regions to build functional circuits.

Disruptions in how these cortical circuits form or operate have been linked to neurodevelopmental conditions such as autism spectrum disorder. Because of this, understanding how cortical networks are wired and how they mature over time is crucial for uncovering the biological basis of these disorders and for guiding future treatment strategies.

The thalamus as a central organizer

Previous studies in animals have shown that the thalamus, located deep within the brain, plays a pivotal role in organizing cortical circuits. Animal research has suggested that thalamic inputs help instruct cortical neurons on how to connect and function.

However, the details of how the human thalamus and cortex cooperate during circuit formation have remained unclear, in part because of ethical and technical barriers to obtaining and studying developing human brain tissue. Organoid and assembloid technology offers a way around these limitations by allowing scientists to model human-specific processes in the lab using stem cell-derived tissues.

Miniature brain circuits that mirror human wiring

When the researchers examined their thalamus–cortex assembloids, they observed that axons, or nerve fibers, from each region extended toward the other. Thalamic fibers grew into the cortical area, while cortical fibers projected into the thalamic region.

These crossing fibers formed synapses—specialized junctions where neurons communicate—resembling the reciprocal connections known to exist between the thalamus and cortex in the human brain.

To find out how this interaction influenced development, the team compared gene activity in the cortical portion of the assembloid with that in cortical organoids grown alone. The cortex that was connected to a thalamus-like region showed molecular signs of more advanced maturation, indicating that input from the thalamus helps drive cortical development.

Thalamic signals synchronize cortical activity

The researchers also studied how electrical signals propagated through the assembloids. They found that neural activity often started in the thalamus and then spread into the cortical region in wave-like patterns. This produced synchronized firing across cortical networks, a hallmark of organized circuit function.

To clarify which cortical neurons participated in this synchronization, the team monitored three major classes of excitatory cortical neurons:

• Intratelencephalic (IT) neurons, which typically connect areas within the cortex
• Pyramidal tract (PT) neurons, which send signals to subcortical targets including the brainstem and spinal cord
• Corticothalamic (CT) neurons, which project from the cortex back to the thalamus

Synchronized activity emerged in PT and CT neurons, both of which send outputs toward subcortical regions including the thalamus. In contrast, IT neurons, which do not directly project to the thalamus, did not show the same level of synchronized firing.

These observations suggest that thalamic input selectively strengthens and coordinates particular cortical neuron types, helping shape functional networks and supporting their maturation.

A new platform for studying brain disorders

By reconstructing human thalamus–cortex circuits using assembloids, the research team has created a powerful experimental system for probing how brain networks form and operate at the cellular level. The model makes it possible to examine how specific neuron types respond to thalamic input and how those interactions may differ in disease.

The authors emphasize that reproducing human brain circuits in the lab allows a “constructivist” approach: building components of the brain step by step to test how each part contributes to overall function. They expect that this approach will speed up the discovery of mechanisms underlying neurological and psychiatric conditions and support the development of new therapeutic strategies.

In the future, similar assembloid models could be created from patient-derived iPS cells, enabling researchers to compare brain circuit development in healthy and affected individuals and to test how potential treatments influence human neural networks before moving into clinical studies.