Human cortical neural cell balls

Cells cultured in embryoid bodies self-organize into specific neural subtypes

Embryonic stem cells can be readily coaxed into becoming neural progenitors, but neurons from the cerebral cortex have proved extremely difficult to make in vitro, presumably because forming these neurons requires input from other cells in a developing brain. Earlier this year, a group led by Pierre Vanderhaeghen of the Institute for Interdisciplinary Research at the University of Brussels, Belgium showed how to make these cells in a flat monolayer culture¹. Now, a team lead by Yoshiko Sasai at the RIKEN Center for Developmental Biology in Kobe, Japan, shows that both mouse and human embryonic stem cells can organize themselves in culture into three-dimensional structures that recapitulate processes in early development to form cortical neural cells.²

“Both are fantastic papers,” says Stewart Anderson, a developmental neurobiologist turned stem cell scientist at Weill Cornell Medical College in New York, but he added that the culture system from the Sasai team promises more applications. “They can get these growing balls of cortical cells such that they can try to make different regions of cells depending on extracellular signals,” he says. “If the Vanderhaeghen group has made a really nice BMW, the Sasai group has made a Ferrari.”

Anderson cautions that these culture-derived cells still need to be more thoroughly compared to their endogenous counterparts. Sasai identified cell types with markers and used transplantation studies to show that cells generally migrate to the appropriate parts within the cortex, but Anderson says eventually the analysis should be more specific. “If you drive the cells to being visual cortex cells, do they selectively innervate the superior colliculus?”

The Sasai technique allows isolated embryonic stem cells to aggregate quickly in a culture media that does not contain serum. The balls of cells that subsequently form produce a high percentage of cortical precursors, and they even assemble into layers showing interactions of the type observed in the cerebral cortex: wave-like oscillations in calcium ions and regional pattern induction caused by signaling proteins Wnt and fibroblast growth factor.

Cell-surface markers indicate that these embryoid bodies even seem to form neurons according to a pattern dubbed 'inside early, outside late', in which outer layers of cells contain the neruons that form in later stages of development. Both human and mouse cells formed qualitatively similar structures, displaying four layers characteristic of the early cortex but not the two final, more mature layers. However, the human cells formed mushroom-shaped aggregates about ten times wider than those formed by the mouse cells. They also, as expected, took much longer to grow.

The organization and differentiation of the embryonic stem cells seems sensitive to cell density and culture media, and a different set of parameters may allow the formation of all six layers found in the neocortex. Sasai now plans to use real-time imaging to watch cell events and figure out what those parameters might be.

“This is the first time that such self-organization in cortical structures has been demonstrated for mammals,” says Luc Leyns, who studies embryonic patterning and cell differentiation at the Vrije Universteit Brussel in Belgium. He notes that while the spatial organization of the early embryonic cortex is nicely recapitulated in culture, the temporal sequence of the structures is different, and that this should be further investigated. Nonetheless, he sees several potential biomedical applications of Sasai's work. If induced pluripotent stem cells created from particular patients were used, for instance, structures formed by cells from people with and without neurodegenerative diseases could be compared. He also believes the assay could be adapted to screen out drugs with potential neurotoxic effects.

Sasai himself says he was both surprised and impressed by what he calls “the self-formation of a highly ordered pattern from patternless cells.” He thinks that this could lead to a shift in thinking for both research and medical applications; instead of focusing on producing certain cells, scientists could instead strive to make functional tissues.

Related articles

An intrinsic program of brain development