Assembloid: combining organoids to model connected brain regions

An assembloid is a three-dimensional structure made by combining two or more separately grown organoids, or an organoid with another cell type, so that they fuse and interact. In brain research, assembloids join organoids patterned to resemble different regions, letting scientists study how human neurons migrate and form connections between regions in a way a single organoid cannot.

Key takeaways

• Assembloids are built from parts: each organoid is patterned toward a specific identity first, then combined.
• Birey et al. (Nature, 2017) fused dorsal and ventral forebrain spheroids and watched interneurons migrate and integrate into cortical circuits.
• Andersen et al. (Cell, 2020) linked cortex, spinal cord and muscle in a cortico-motor assembloid in which cortical stimulation could cause muscle contraction.
• Assembloids model circuits and interactions, while standard organoids mostly model the development of one region.
• For biocomputing, assembloids hint at modular, structured living networks, though this use is early.

What is an assembloid? #

A brain organoid is a self-organizing cluster of neural tissue grown from stem cells, usually induced pluripotent stem cells. Early “whole-brain” protocols, such as the cerebral organoids described by Lancaster et al. in Nature (2013), let cells choose their own fates, producing several regions in an unpredictable arrangement. Region-specific protocols instead use signaling molecules to guide cells toward one identity, such as dorsal cortex.

An assembloid combines these approaches. Researchers grow region-specific organoids separately, then place them in contact. Over days, the tissues fuse, cells migrate across the boundary and axons grow from one part into another. In his 2018 Nature review, “The rise of three-dimensional human brain cultures,” Sergiu Pasca described assembloids as a way to capture interactions between brain regions and cell lineages.

Types of assembloids #

Assembloids can be classified by what is combined:

• Region-region assembloids: two or more brain regions, for example cortex with ventral forebrain, or cortex with thalamus or striatum.
• Multi-tissue assembloids: neural tissue with non-neural tissue, such as muscle.
• Cell-type assembloids: an organoid combined with added cells it normally lacks, such as microglia or vascular cells.

How are assembloids made? #

The general workflow looks like this:

1. Derive stem cells, typically from a donor’s skin or blood cells reprogrammed into iPSCs.
2. Pattern each part separately using growth factors and small molecules that mimic developmental signals.
3. Grow each organoid to a stage where its identity is established, often several weeks.
4. Combine the parts, commonly by placing them together in a small well or tube so they touch and adhere.
5. Let them integrate for weeks, then measure migration, connectivity and activity.

Fluorescent labels in one part are often used to track which cells cross into the other, and calcium imaging or microelectrode arrays are used to test functional connections. Our guide on how brain organoids are grown covers the earlier steps in more detail.

What have assembloids shown so far? #

Interneuron migration in the forebrain #

Birey et al. reported in Nature (2017) that when human dorsal (cortex-like) and ventral (subpallium-like) forebrain spheroids were fused, GABAergic interneurons migrated from the ventral side into the dorsal side and integrated into circuits there, mirroring a key step in human brain development. Using cells from people with Timothy syndrome, a genetic condition linked to autism and epilepsy, they also observed abnormal interneuron migration, showing how assembloids can model disorders of circuit formation.

A circuit from cortex to muscle #

Andersen et al. (Cell, 2020) combined cortical, spinal cord and skeletal muscle spheroids into cortico-motor assembloids. They reported that cortical neurons projected into the spinal region, motor neurons connected to muscle, and stimulating the cortical part could drive muscle contraction. This demonstrated a functional, multi-step pathway assembled entirely in vitro.

How do researchers know the parts are connected? #

Fusing two organoids so that they stick together is not the same as connecting them functionally. Researchers typically look for several kinds of evidence, moving from structure to function:

• Cell tracking: labeling one part with a fluorescent protein shows whether cells migrate into the other part and how far.
• Axon projections: imaging reveals whether axons from one region grow into the other and where they terminate.
• Synapse markers: staining for synaptic proteins indicates whether contacts have formed at the boundary.
• Correlated activity: calcium imaging or electrode recordings test whether activity in one part is followed by activity in the other.
• Causal tests: stimulating one part, electrically or optogenetically, and recording a response in the other is the strongest evidence of a working pathway.

For biocomputing, the last two tests matter most. A modular living network is only useful if a signal delivered to one module reliably changes activity in another, and that relationship can be measured and logged over time.

Organoid vs assembloid vs organ-on-chip #

These approaches are complementary. An assembloid could, in principle, be housed in a microfluidic device and supported by organoid vascularization strategies.

Why do assembloids matter for biocomputing? #

Organoid intelligence research currently relies mostly on single organoids or flat cultures. These networks tend toward spontaneous, synchronized network bursts and lack the layered input-to-output architecture seen in real brains. Assembloids suggest a few possibilities:

• Modularity: separate “input” and “output” modules joined by defined pathways, closer to how sensory and motor regions interact.
• Richer dynamics: combining excitatory cortex with inhibitory interneuron sources may produce more balanced activity.
• Quality control: each module can be checked before assembly, which could reduce variability.
• Interfacing: a module dedicated to receiving stimulation could be placed near electrodes while another is recorded separately.

None of these are established computing results yet. They are research directions, discussed further in the future of biocomputing.

What are the limitations of assembloids? #

• Variability: fusion outcomes vary between batches and cell lines.
• Size and nutrient limits: larger combined structures face the same diffusion constraints as organoids, often worsening internal cell death.
• Incomplete anatomy: assembloids capture selected connections, not the full wiring of a brain.
• Maturity: like organoids, they resemble fetal rather than adult tissue.
• Ethical questions: as assembloids become more connected and complex, questions about sensation and moral status, discussed in the ethics of organoid intelligence, become more pressing.

Assembloids are one of the clearest examples of moving from “growing tissue” to “engineering circuits.” At Organoid Programming, we think the software side of that shift, recording which modules were combined, from which donors and with what consent, will matter as much as the biology.

Frequently asked questions

What is an assembloid?

An assembloid is a 3D cell culture made by placing two or more separately grown organoids (or an organoid plus another cell type) together so they fuse and interact. In neuroscience, assembloids usually combine organoids patterned to resemble different brain regions, allowing researchers to study cell migration and long-range connections between regions in human tissue.

What is the difference between an organoid and an assembloid?

An organoid is a self-organizing 3D culture grown from stem cells that resembles one tissue or region. An assembloid is built from two or more such units, each patterned separately, then combined. The key advantage is control: each part can be guided toward a specific identity before fusion, so interactions between defined regions can be studied.

Who first made brain assembloids?

A widely cited early example is Birey et al., published in Nature in 2017 by Sergiu Pasca’s group at Stanford. They fused human dorsal and ventral forebrain spheroids and observed interneurons migrating from the ventral to the dorsal part, where they integrated into circuits. The Pasca lab has since popularized the term assembloid.

Can assembloids control muscles?

Andersen et al. (Cell, 2020) reported human cortico-motor assembloids that combined cortical, spinal cord and skeletal muscle spheroids. Cortical neurons projected into the spinal part, spinal motor neurons connected to muscle, and stimulating the cortical part could trigger muscle contraction. This showed that assembloids can model multi-step circuits across tissues.

Why are assembloids interesting for biocomputing?

Biocomputing needs networks that are structured, not just large. Assembloids offer a way to build modular living networks, such as connecting a region that receives input to one that produces output, with defined pathways between them. They may also improve reproducibility because each module can be quality-checked before assembly. These uses are still exploratory.

Sources

1. Birey et al. (2017). Assembly of functionally integrated human forebrain spheroids. Nature.
2. Andersen et al. (2020). Generation of functional human 3D cortico-motor assembloids. Cell.
3. Pasca (2018). The rise of three-dimensional human brain cultures. Nature.
4. Lancaster et al. (2013). Cerebral organoids model human brain development and microcephaly. Nature.