Like many members of his family, Sasai studied medicine. But he soon became frustrated by the lack of basic understanding in the field, especially when it came to neurological conditions. “Without knowing the brain, a doctor cannot do much for the patient and therapeutics will always be superficial,” he recalls thinking. There seemed no better way to know the brain than to study how it emerges and folds in the embryo. “It's complex and usually complex systems are messy,” says Sasai. “But it's one of the most ordered.” He wanted to know how this elaborate system was controlled.
One piece of the puzzle was well known: the Spemann organizer, a node in vertebrate embryos that induces surrounding cells to become neural tissue. How the organizer works had been a mystery since its discovery in 1924; to find out, Sasai accepted a postdoctoral position at the University of California, Los Angeles. The post got off to a difficult start when Sasai was robbed of his money and passports at the airport on his way to California. But his scientific efforts were soon rewarded. “He replaced the passports and within a month produced the clones that gave us the famous gene chordin,” says his supervisor, developmental biologist Eddy De Robertis.
Sasai and his colleagues discovered that the chordin protein is a key developmental signal released by the Spemann organizer5. Rather than pushing nearby cells to become neurons, they found, chordin blocks signals that would turn them into other cell types6,7. The work helped to establish the default model of neural induction: the idea that, without other signals, embryonic cells will follow an internal program to become neural cells.
By the late 1990s, embryonic-stem-cell scientists were also looking at these signals. They wanted to turn stem cells into mature cell types — particularly neurons — that might lead to therapies. The problem, says Sasai, is that scientists generally “push too hard and perturb the system”. Sasai knew that in the embryo, subtracting signals from the system is what counts, not perturbing it. “We tried to minimize external cues,” he says.
Sasai built an experimental system around that philosophy. He threw out the serum usually added to growing embryonic stem cells, which contains a brew of uncharacterized growth factors and other signalling molecules. He also removed physical cues, such as contact with the plastic surfaces of a culture dish, by allowing embryonic stem cells to spontaneously form floating aggregates known as embryoid bodies. “If they're attached, they're like prisoners and can't act out their own desires,” he says. Keeping the cells alive without these support systems was a challenge but, five years of careful experimentation later, Sasai published8 and later patented his serum-free embryoid body culture — a pared-down life-support system with just the right mixture of ingredients for cells to survive. It would go on to form the heart of his brain-tissue factory.
Embryoid bodies in Sasai's system quickly become what he calls “brain balls” — populated with neural precursor cells. Sasai found that balls left entirely alone give rise to cells like those in the developing brain region called the hypothalamus9, but those given just a whiff of growth factors start differentiating into cerebral-cortex cells2. And when Sasai cultured the cells for about two weeks, he got a surprise: the cortical cells spontaneously started to form a layered structure that ended up strikingly similar to the cortex of a 15-day-old mouse. When transplanted into the brain of a newborn mouse, the structure survived. “That's what we do,” says Sasai. “We set up the permissive conditions, selecting the right medium and cell number. But after that we don't do anything. Keep them growing and let them do their job.”