Cell Culture Advances Yield Brain Organoid
By Anette Breindl
In recent years, culturing methods have enabled researchers to generate model tissues that are far more life-like than the single sheets of single cell types that were once the only game in town for cell culture research.
Such tissues include duct tissue from intestinal cells, retinae and, most recently, artificial livers. (See BioWorld Today, July 8, 2013.)
But “so far, the most complex of all human organs, which is the human brain, has not been susceptible to such methods,” Juergen Knoblich told reporters at a press conference.
Today, however, Knoblich, first author Madeline Lancaster and their co-authors reported success in generating a so-called organoid of the brain – a three-dimensional tissue culture containing “discrete regions that resemble brain areas of the developing brain,” Knoblich said.
Though the organoid may initially bring to mind works of fiction from Roald Dahl’s “William and Mary” to the Cybermen of Dr. Who, the organoids are tissue cultures, not human brains in any sense.
Knoblich and Lancaster, who are at the Austrian Institute of Molecular Biotechnology, consulted with the Austrian Ethics commission as well as individual ethicists while developing their culturing methods.
“Among ethicists, there are very active considerations about how complex . . . a structure [can] be before we have to apply ethical rules to it,” Knoblich said, although currently, “much of this work is going into complex computer networks.”
Wherever that complexity barrier is, the organoids, which are described in today’s online issue of Nature, don’t rate.
They resemble 9-week-old human brains in terms of the brain structures that are present, but those structures cannot be said to think by any stretch. Nor does Knoblich believe that the culturing methods could be developed to enable cognitive processes, since “the way cognitive processes organize is in response to sensory input,” which the organoids lack.
Nor could the method be used for regenerative medicine, which is one of the big goals of growing other, less complex artificial organs. In the central nervous system, each individual part’s ability to function depends absolutely on its being wired correctly to its neighbors.
“You cannot take out a piece of spinal cord” – much less a piece of brain – “and just put another one in, because they have to be connected in the proper way,” Knoblich said.
Instead, “what our organoids are good for is to model development of the brain, and to study anything that causes a defect in development. . . . [They] model very nicely what’s going on in real development,” he said.
In the paper, the team used the organoids to gain new insights into the causes of microcephaly, a developmental defect that results in a very small brain, which in turn leads to severe mental deficiency. Growing organoids from induced pluripotent stem cells (iPSCs) of patients with microcephaly, the team was able to show that individuals with microcephaly have a stem cell population that differentiates earlier in development than that of control subjects – too early.
“The stem cells were not expanding as they should and instead already making neurons, which leads to the depletion of the stem cell population” and an overall inability to make enough neurons, first author Lancaster told reporters.
In addition to studying developmental questions, Knoblich said, “these cultures may also be very useful for pharmacology. . . . [They] offer the possibility to test drugs directly in a human setting.”
They might also be adapted to study neurodegeneration, though Knoblich pointed out that neurodegeneration is at the other end of the developmental spectrum: “It affects mainly old people.”
“I would not preclude that one could ultimately use [organoids] for neurodegeneration,” he added. “One would have to play some tricks, though.”
Methodologically, perhaps the most striking thing about the paper is that less turned out to be more as far as the culturing was concerned.
Despite the fact that optimizing brain development in children has become something of a competitive sport for parents, the simple truth is that it is a process that will basically happen on its own.
The organoid cultures turned out to be the same way – the best way to get good model brains was fresh air, good food and a hands-off approach.
Lancaster described her method to make organoids to reporters. In various experiments, the team started with either embryonic stem cells (ESCs) or iPSCs and “used these stem cells to generate what are known as embryoid bodies.”
After allowing those cells to develop for a time, the team cultured the resulting embryoid bodies under conditions that allowed only neuroectoderm cells to survive.
The key to coaxing those cells into forming brain organoids was to transfer them onto scaffolding and then to a spinning bioreactor that was able to supply the cells with oxygen and nutrients better than classical cell culture methods.
While specific individual cell types from embryonic stem cells are usually generated by applying different growth factors in precisely controlled cocktails, the team found that creating their complex organoids worked best when they took advantage of the cells’ “amazing ability to self-organize” in ways that were very similar to the developing brain.
“We don’t need to apply a lot of extrinsic cues, and if we do so . . . we actually end up with tissues that don’t model the human brain as well,” Lancaster explained.
And Knoblich added that “scientists realize more and more that if you leave the cells alone, they just set up their own signaling. . . . Cells have intrinsic knowledge of how to set up their own extrinsic signaling centers.”
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