Researchers from the University of Michigan and the Weizmann Institute of Science have for the first time developed a central nervous system on a chip that faithfully simulates that of the human fetus - from the end of the spinal cord to the forebrain
At some point we were all a bunch of compressed stem cells. With embryonic development, the collection of cells lengthened and grew limbs at its sides, buttocks at its back, abdomen at its front and a raised head. The division of cells into the various organs, which saved us from a shapeless fate, is attributed to morphogens - molecules that are secreted at different times and places in the embryo, disperse in the environment and determine the location and shape of the organs. The changing concentration of the morphogens in the environment serves as a map that tells the cells where they are and what their purpose is.
Maps of morphogen concentrations are the bread and butter of organoid technologies - those miniaturized laboratory versions of living organs that have conquered the world of developmental biology in the last decade. Until now, the common use of a uniform concentration of morphogens in the petri dishes where the organoids are grown has limited researchers, who have had to grow small sections of the organ in each dish instead of a miniaturized version of the full organ. Now, researchers from the University of Michigan led by Prof. Jianping Fu and dr Casso rye, from the Weizmann Institute of Science and the University of Pennsylvania They created a miniature model of the central nervous system of the fetus - from the brain to the tip of the spinal cord - using a microflow chip that simulates the distribution of morphogens during development.
"Studies have already been published that exposed organoids to varying concentrations of morphogens, but they were limited to short sections of the central nervous system - only the spinal cord, or only the forebrain, but not both," explains Prof. Orly Rayner From the Department of Molecular Genetics at the Weizmann Institute of Science. Prof. Rainer, who has been researching diseases that affect the developing brain for 30 years, and who began growing organoids in her laboratory about a decade ago, says that the new chip will allow her and others to ask a completely new kind of questions - both regarding the development of the healthy fetus and regarding diseases and damage to the tissue.
The chip allows researchers to inject morphogens from almost any direction and at any time they want, along the length and width of the organoid. In the center of the chip there are narrow and sticky surfaces, and similar to the length of the central nervous system of a month-old fetus, the length of each surface is 4 millimeters. Stem cells are seeded along the surfaces, and they are coated with a gel that simulates the extracellular environment that allows the cells to develop into a three-dimensional tissue. In a short time the cells spontaneously organize into a hollow tube. Three days later, the researchers begin to inject morphogens from one end of the chip, and these diffuse through the length or width of the tissue.
In a short time, the researchers saw that the cells in the chip differentiated into a variety of different cell types of the embryonic central nervous system - at the end where the concentration of morphogens was high, cells appeared that would develop into the end of the spinal cord, followed by cells destined for the center of the spinal cord, the hindbrain, the midbrain, and at the furthest end - the forebrain. "When we characterized the new organoids, we saw a masterful arrangement along the entire length of the central nervous system, as it appears in the early embryonic period," says Prof. Reiner.
After creating the longitudinal axis of the nervous system, the researchers decided to take on another challenge - and imitate the development of the forebrain along the ventral-dorsal axis of the fetus, along which two populations of cells develop that are essential to the functioning of the adult brain: the stimulating or excitatory cells which encourage neural activity and the regulating cells or the inhibitors that inhibit this electrical activity. "Until now, we did not have the tools to grow in one tissue both the production center of the stimulating cells and the connecting nerve cells that are responsible for the regulation action. Instead, we had to grow two types of organoids and try to connect them," explains Prof. Rainer. This is because the regulating cells are formed in a unique tissue in the inner part of the forebrain, and from there they migrate to the rest of the brain.
The new technology allowed the researchers to spread the concentrations of the morphogens that cause the cells to differentiate into the two different production centers, thus growing them in the same tissue. In the first stage, they created the longitudinal axis of the nervous system, as we described, and on the seventh day they began to inject morphogens near the forebrain - away from the spinal cord. In a short time, the cells destined to become the regulating nerve cells appeared in the inner part of the tube, and the cells from which the excitatory nerve cells develop appeared in the outer part of the tube - as they appear in embryonic development.
The researchers emphasize that the chip does not mimic the earliest stages of central nervous system development: "We actually skip the early stages and push the stem cells into developmental stages that are typical of a four-week-old fetus." However, after a few days, a three-dimensional tissue is formed that is remarkably similar to the central nervous system of the fetus, both in the type of cells it contains and in the order in which they appear. Thus, for example, the researchers could study genes involved in the differentiation of a population of cells in the spinal cord whose differentiation and ultimate identity had not been understood before.
The chips are already being used by researchers to examine questions related to the development of the human nervous system. For example, Prof. Rainer's laboratory has already implemented the technology in its laboratory and is using the chip to study how a genetic disease affects the longitudinal development of parts of the brain. The researchers hope that more researchers will use the technology to expand our understanding of a wide variety of diseases that affect the nervous system.
Also participating in the study were Dr. Jung Soo Kim, Dr. Norio Kobayashi, Dr. Yue Liu, Dr. Jason Spence, Dr. Robin Zhexuan Yan, Dr. Yu-Hwai Tsai, Shiu Sun Wei Zeng of the University of Michigan; Dr. Rami Yair Tsheva and Alfredo-Isaac Pons-Arias from the department of molecular genetics at the institute; Prof. Hongjun Song, Prof. Guo-Li Ming and Dr. Richard O'Loughlin from the University of Pennsylvania; and Prof. Azim Sorani and Dr. Frederick C.K. Wong of the University of Cambridge.
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