Five years in a dish, four years of cortex: the organoid clock runs at human speed

Cortical organoids grown for five years are the longest-lived brain organoids that have been studied in detail.

The team documented the organoids' progress over five years before submitting the research, then continued culturing some for two more years before ending the experiment.

Arlotta's team has grown the same kind of organoids for seven years, but there are too few of these older organoids to reliably analyse them.

Paola Arlotta, a developmental neurobiologist at Harvard University, is senior author of the study.

The expense and manual labour involved mean organoids are usually only grown for a few months, according to Andras Lakatos.

Standard culture conditions did not adequately support neuronal activity over extended periods; adapting the composition of the culture medium supported spontaneous activity, kept neurons active and maintained neuronal populations much longer.

Noelia Anton-Bolanos, PhD, is an assistant professor at UMC Utrecht and worked on the study.

Different brain cell types appeared in the same order as during human brain development, neurons formed increasingly complex connections, and genes became active or inactive at the expected times.

The organoids lack blood vessels, large-scale neural architecture and the sensory input that comes from having a body, all key drivers of development in standard brains; "At some point, you need much more," Arlotta says.

The field aims to improve features that remain incomplete, including vascularization and the layered organization of the cerebral cortex, and the team wants to test whether environmental cues such as light stimulation improve maturation.

Other groups of researchers will not readily be able to do this work, due to the challenges of growing organoids, says Lakatos.

Lancaster says the organoids showed signs of "maturation and even aging like you see in an actual brain" and that the study showcases the power of self-organization.

Finding ways to speed up organoid ageing will be important for research, says Lakatos.

In 2021, researchers reported growing human organoids for up to two years, with those structures mimicking the cerebral cortex from development during pregnancy to about one year old.

At three to six months, the organoids resembled the fetal brain at around three to six months post-conception, compared against measurements from fetal brains in previous research.

The genetic activity of five-year-old organoids mimicked that seen in the cerebral cortex of a typical 4-year-old child.

After approximately one year, the organoids displayed features that normally emerge only after birth.

When an older organoid was dissociated and the cells regrown, they produced cell types associated with a late developmental stage; when older cells were combined with younger cells, the older cells regained the ability to produce neurons but only types associated with later developmental stages.

Madeline Lancaster, a developmental biologist at the University of Cambridge who was not involved in the research, says more mature organoids could be useful for studying neurological disorders that tend to emerge in adulthood, such as schizophrenia, Alzheimer's and Parkinson's disease.

Lakatos says long-lived organoids could offer a way to study how autism and conditions such as epilepsy emerge during the later stages of brain development.

Arlotta says her group is using the organoids to study how autism and epilepsy emerge in later development, and to screen for drugs that may be able to alter the progression of conditions like epilepsy.

Brain organoids pose bioethical concerns including the prospect that they may feel pain; Arlotta says it is considered "highly, highly, highly unlikely" that they are even faintly conscious, and that researchers need to re-examine the question as the systems become more complex.

A little more than two years ago, junior scientist Irene Faravelli brought Arlotta images of the old brain organoids; she had analysed them and then wondered if that had been a grave mistake, and Arlotta said she was pleased somebody had the courage to do it.

The five analysed years extend the previous published two-year maximum by a factor of 2.5, and the unanalysed seven-year cultures extend it by a factor of 3.5.

Sixty months of culture produced tissue resembling a 4-year-old cortex, which is about 57 months post-conception if a term pregnancy is taken as roughly nine months, so developmental time in the dish ran at about 0.95 of wall-clock time.

At that observed rate, ten years of culture would buy roughly nine and a half years of development.