Scientists Built Miniature Brains That Can Experience The Passage Of Time

Our brains contain 86 billion neurons and trillions of connections between them, generating a coordinated tempest of electrical and chemical activity inside a roughly 1.4 kg mass of gel-like fat, protein and water.

The complex biological apparatus that makes us human takes around 20 years to reach maturity – a far longer developmental timetable than that of most species.

Because the developing brain is enclosed inside the skull as well, it is exceptionally difficult to access experimentally.

So scientists are growing brains in the laboratory – no big deal.

Miniature brain organoids that track time

In a breakthrough reported in the journal Nature, scientists have produced miniature brains known as organoids. These can 'sense' time passing and keep a cellular 'memory' of the length of time they have been developing.

Close-up of an organoid immunolabelled with markers for different cell types. (Irene Faravelli and Noelia Antón-Bolaños)

These stem-cell-derived clusters of brain cells are also the longest-lived organoids yet studied, having been monitored for almost six years. That duration is unprecedented: many earlier organoid studies lasted only weeks or months.

"The work demonstrates that a diversity of cells in organoids can develop over considerable time periods and are capable of both recording and recalling their developmental age," the researchers explain.

"These systems provide a wealth of data on largely inscrutable periods of postnatal development of the human brain."

Different sections of an organoid developed to model brain-maturation processes. (Irene Faravelli and Noelia Antón Bolaños)

Scientists have previously created brain-like organoids, including ones that developed basic eyes with brain fibres that share signals.

However, the new research represents at least two important advances: longevity and function.

How the organoids mature over time

The team examined the maturation of dozens of organoids using a range of genetic, imaging and electrophysiological measurements. Crucially, DNA markers used as 'epigenetic clocks' showed that the lab-grown samples matured in a similar way to real brains.

Not every cell type is equally likely to persist in organoids. Nevertheless, most broad cell types continued to mature molecularly throughout the experiment, aligning in vitro brain development with maturation in vivo.

This included the appearance of cells such as star-shaped astrocytes, which help support and maintain the blood-brain barrier, and oligodendrocytes, which form the fatty myelin sheath that insulates nerve fibres.

Although the researchers observed a gradual reduction in the essential but vulnerable neurons responsible for major brain functions, they found that populations of these cells can remain alive in culture for nearly six years.

Organoid evolution over time. (Faravelli et al., *Nature, 2026)*

Importantly, the team also showed how neurons can be better preserved: by allowing them to fire spontaneously, rather like doing regular biceps curls to prevent upper-arm flabbiness.

This neuronal exercise programme is enabled by a specialised culture medium that more closely recreates the chemical conditions of the central nervous system.

Cellular clocks and chimeroids

Perhaps most intriguingly, brain cells appear to mark the passing of time through "a cell-intrinsic clock that sets the pace of brain development, although its molecular mechanisms and functional significance remain to be elucidated," the researchers write.

To investigate this, the scientists separated organoids of varying ages into their individual cells. They then recombined the cells into "chimeroids" – tiny Frankenstein-like entities assembled from components of different ages.

These chimeroids retained a cellular memory of their earlier developmental age despite having first been broken apart and then re-aggregated.

For instance, a chimeroid composed of old cells generated age-appropriate progeny – new cells such as astroglia – even though it had matured for only 15 days after being reassembled.

An organoid composed of younger and older cells. Younger cells, taken from an organoid 15 days after re-aggregation, are red; deep-layer neurons are green; and nuclei are blue. (Faravelli et al., *Nature, 2026)*

When they were mixed with younger cells, chimeroids also moved towards somewhat younger molecular states, while continuing to show traces of their earlier developmental history.

Thus, older progenitors assembled from reconstituted old cells 'remembered' their time in culture, bypassing developmental stages they had already completed. In two weeks, they produced progeny that would otherwise require two months.

By demonstrating that organoids can be sustained for such extended periods, the researchers offer a means of modelling selected molecular, structural and functional aspects of postnatal human brain-cell maturation.

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"Together, organoids cultured over extended timelines and the multimodal wealth of data produced from them represent both a powerful experimental system," the researchers conclude, "and the information to fuel understanding of the largely unexplored mechanisms governing human brain neoteny, maturation and evolution."

This research was published in Nature.

This article was fact-checked by Rachel Garner and edited by Rebecca Dyer. While we take pride in our process, we are only human. If you spot an error, please let us know.