Organoids

Organoids are very small pieces of tissue that grow in a dish in the lab. They arise from stem cells – these are immature cells that can still turn into many different cell types. Under suitable conditions, these cells arrange themselves and form a structure that resembles a real organ. An intestinal organoid, for example, has an inner cavity and folded walls, just like a real intestine. However, such structures are not large: usually between half a millimeter and a few millimeters. The name already says it: “-oid” means “similar,” so an organoid is organ-like, but not a functioning organ.

Why the petri dish is not enough

So far, research has mostly examined human cells as a flat layer on the bottom of a dish. But such cells live differently than they do in the body. They lie in a single plane, have no neighborhood of different cell types, and no spatial order. Results from such experiments therefore often translate poorly to humans.

For a long time, the alternative was animal testing. But a mouse is not a small human. Many drugs work well in mice and not in humans – or they damage the human liver, even though the animal tolerates them without any problem. Organoids close this gap because they consist of human cells while still being three-dimensionally structured.

What is especially interesting is that organoids can be grown from the cells of a single person. In the hereditary disease cystic fibrosis, doctors today test on intestinal organoids from a patient which medication works specifically for them. This is personalized medicine in the literal sense: testing is not done on an average person, but on the tissue of the affected individual.

From stem cell clump to mini-intestine

It starts with a source of stem cells. These can be cells from a tissue sample, for example from the intestine. Or one takes normal body cells, such as skin cells, and uses certain signaling substances to reset them to an immature state. Such reset cells are called induced pluripotent stem cells and can, in principle, form any tissue type.

These cells are placed in a soft gel that mimics the connective tissue in the body. The gel provides support so that the cells do not spread out flat but grow in three directions. In addition, there is a nutrient solution containing growth factors – chemical messenger substances that tell the cells what tissue they should become. Which factors are added and when determines whether a liver, kidney, or brain organoid forms.

The cell does the rest itself. No one assembles the tissue cell by cell; the structure arises through self-organization, because cells send signals to one another. But this is also exactly where the limit lies. Organoids lack blood vessels, which is why, from a few millimeters onward, the interior is no longer supplied and dies off. A brain organoid is not a miniature brain: it has no sensory organs, no body, and no discernible perception.

Organoids in pharma headlines and chip factories

In business news, organoids mainly appear in connection with drug development. Bringing a new medication through all trials often costs over a billion euros, and most candidates fail late in the process. If organoids can weed out unsuitable active substances earlier, this saves enormous sums. In 2025, the US drug regulator FDA announced it wants to replace animal testing for certain medications with such lab-based systems.

A growing field is organ-on-a-chip systems. Here, an organoid sits in a stamp-sized plastic chip with fine channels through which fluid flows like blood through vessels. By connecting several such chambers, one can simulate how a substance is broken down by the liver and then acts on the kidney.

The term also comes up in AI topics. Computing systems built from brain organoids are being researched under the keyword “organoid intelligence,” because nerve cells process stimuli using extremely little energy. However, this is still at a very early stage and far from a usable computer. At the same time, an ethical debate is underway: the more complex brain organoids become, the sharper the question becomes of when a tissue becomes more than just a cell culture.