Human cortical organoids grew for five years: their cells reached late developmental stages and retained a “memory of time”

Human cortical organoids grew for five years: their cells reached late developmental stages and retained a “memory of time”

On August 19, Paola Arlotta’s team published in Nature the results of growing human cortical organoids, three-dimensional stem cell-derived models of the cortex, for five years. Across 16 timepoints, cells in these models progressed through a sequence of maturation programs resembling those of the human cortex.

The human cortex develops over several years, but most organoid experiments end after a few months. The previous longest culture lasted 694 days. Arlotta’s team grew organoids for up to five years to test whether their cells would continue to follow the maturation program.

The series included 110 organoids and 424 720 individual cells. The researchers compared gene expression in these cells with data from the human cortex. From nine months to five years, the organoid cells increasingly resembled cells at late prenatal and early postnatal stages of development. DNA methylation, which consists of chemical marks that influence gene activity, provided an independent timescale. Two of the three methylation clocks indicated that the tissue became progressively older as the culture period increased.

The most direct test used mixed organoids. The researchers combined progenitor cells from organoids aged 9 to 12 months with cells from 15-day-old organoids. After 15 days, the younger component produced early cortical cell types, while the older component produced later neuronal and supporting cell types, even though both grew in the same environment. The younger cells shifted some of the older progenitors’ programs toward an earlier state, but the later cell fates remained. Even in a shared environment, the cells retained a record of the developmental stage they had already passed through.

To preserve excitatory neurons, which transmit signals to other cells, during the multi-year culture period, the researchers grew some organoids from day 70 onward in a medium that supported spontaneous electrical activity. After one year, all nine of these organoids produced network bursts, while none of the eight control organoids did. Active networks persisted in this medium for at least two years.

After five years of observation, Arlotta described the next task: “Now we need to understand how to make this happen faster,” she said.