The marmoset brain starts out with the same layout as folded primate brains, but slower-dividing, simpler cells help keep it small and smooth.<\/p>\n
Like most primates, humans have brains covered in deep grooves and ridges. The common marmoset, a small South American monkey, is an exception, with a brain that\u2019s small and almost smooth.<\/p>\n
Early in development, though, a marmoset\u2019s brain has the same basic layout as the big, folded brains of its relatives. Working with lab-grown brain tissue and fetal samples, researchers found that the cells that make neurons divide more slowly in marmosets and take simpler shapes.<\/p>\n
In the team\u2019s view, the marmoset didn\u2019t simply lose the primate program for building a large, folded brain. Instead, several small changes limit its growth.<\/p>\n
The researchers, at the German Primate Center<\/a> (DPZ) in G\u00f6ttingen, Germany, did most of the work in brain organoids<\/a>. These are clusters of brain cells grown from stem cells, and they follow some of the same early steps as a developing brain. The team checked key results against tissue from marmoset fetuses.<\/p>\n \u201cWe wanted to understand which changes at the cellular level cause the marmoset brain to grow less and form fewer folds,\u201d said Lidiia Tynianskaia, a Ph.D. student at the DPZ and one of the study\u2019s two first authors.<\/p>\n Marmoset and human organoids looked alike<\/p>\n Scientists think the common ancestor of all primates probably had a medium-sized brain with at least some folds. In people, those folds<\/a> greatly increase the brain\u2019s surface area and leave room for billions of nerve cells.<\/p>\n The team grew organoids from three human and three marmoset stem cell lines. For some tests they added organoids from rhesus macaques, monkeys with large, folded brains.<\/p>\n Four weeks in, marmoset and human organoids looked much the same under a microscope and held the same kinds of cells.<\/p>\n \u201cAt first glance, human and marmoset brain organoids look remarkably similar. You don\u2019t immediately see major differences,\u201d Michael Heide told Earth.com. Heide leads the DPZ\u2019s Brain Development and Evolution<\/a> group and also works at Hannover Medical School<\/a>.<\/p>\n The differences turned up in the cells that divide to produce neurons, which scientists call neural progenitor cells. \u201cBut once we started analyzing them in detail, it was surprising to see how differently their neural progenitor cells behave,\u201d he said.<\/p>\n Marmoset cells took longer to divide<\/p>\n In marmoset organoids, a full round of cell division took about 68.5 hours. In human organoids, it took about 57.2 hours, so each marmoset cycle ran roughly 11 hours longer.<\/p>\n The team also tracked how many progenitors copied their DNA, a step a cell takes before it divides. That share leveled off at 83.9% in human organoids and at 48.7% in marmoset ones, even though the marmoset cells were watched for as long as 84 hours.<\/p>\n So a large part of the marmoset pool wasn\u2019t dividing much. Some of those cells were cycling slowly, and others seemed to have paused.<\/p>\n The marmoset<\/a> organoids made fewer neurons, too. At 30 days, human and rhesus organoids had produced substantially more neurons for each progenitor than marmoset organoids had.<\/p>\n The growth window is cut short<\/p>\n A developing primate brain first packs its progenitors into a layer around a fluid-filled space in the middle. Later, many of them move outward into a second zone, where in big-brained primates<\/a> they multiply heavily.<\/p>\n In marmoset fetuses, that handover happens abruptly, around day 85 of pregnancy. Neuron production ends after day 105, which leaves the outer zone only a brief window.<\/p>\n The organoids went through the same switch. Between days 30 and 50, the inner layer of marmoset organoids got thinner and fewer of its cells were dividing, while human organoids showed no such drop. Of the stages tested, 50-day-old organoids came closest to tissue from marmoset fetuses at day 90 of pregnancy.<\/p>\n Asked by Earth.com which other results he wanted to highlight, Heide pointed to that timing. \u201cThey reproduce the same sequence of developmental events as the fetal marmoset brain, but on a condensed timescale,\u201d he said.<\/p>\n Simpler cells may multiply less<\/p>\n In big, folded brains, progenitors in the outer zone often grow several branches, and the more branched ones tend to divide more. The team went back to images of marmoset fetal brains at day 100 of pregnancy, taken for an earlier study.<\/p>\n Most of the dividing cells in that zone had a single branch pointing outward. Complex shapes were rare compared with earlier data from humans, macaques, and ferrets.<\/p>\n \u201cOther progenitor cells have a simpler structure than their human counterparts, with fewer processes, and are therefore less proliferative,\u201d explained Cesar Mateo Bastidas-Betancourt, the study\u2019s other first author.<\/p>\n No single change seems to be enough. In an earlier experiment<\/a>, published in 2020, Heide and colleagues added a gene found only in humans to developing marmoset brains.<\/p>\n Those brains grew larger and formed folds, but only in some areas. To the authors, that points to several brakes working at once.<\/p>\n Some questions remain open<\/p>\n Like other lab-grown mini-brains<\/a>, these organoids have limits. Human and marmoset cells may not grow equally well in the same lab conditions, so the authors can\u2019t rule out that some differences reflect how fast each species\u2019 organoids matured.<\/p>\n A small, nearly smooth brain also shows up in some human disorders, such as microcephaly, an unusually small head and brain, and lissencephaly<\/a>, a brain that develops without folds.<\/p>\n \u201cThe important difference is that in marmosets this is the normal developmental program, not the result of malfunctioning neural progenitor cells,\u201d he said.<\/p>\n He added that the study is mainly about how evolution normally sets brain growth. Still, the aggressive brain cancer glioblastoma shares many features with neural progenitor cells. Future research may explore whether the mechanisms that naturally slow those cells in marmosets could help slow the cancer\u2019s growth, he said.<\/p>\n Producing fewer neurons is one reason a brain stays smooth, and the authors say it probably isn\u2019t the only one. Physical forces in the growing tissue and genes switched on where folds will later form, may also play a part. <\/p>\n Future studies will need to test whether organoids can model those features, and whether they differ between marmosets and primates with larger, folded brains.<\/p>\n The full study was published in the journal Science Advances<\/a>.<\/p>\n \u2014\u2013<\/p>\n Like what you read? Subscribe to our newsletter<\/a> for engaging articles, exclusive content, and the latest updates.<\/p>\n Check us out on EarthSnap<\/a>, a free app brought to you by Eric Ralls<\/a> and Earth.com.<\/p>\n \u2014\u2013<\/p>\n","protected":false},"excerpt":{"rendered":"The marmoset brain starts out with the same layout as folded primate brains, but slower-dividing, simpler cells help…\n","protected":false},"author":2,"featured_media":783313,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[26],"tags":[90,56,54,55,4407],"class_list":["post-783312","post","type-post","status-publish","format-standard","has-post-thumbnail","category-wildlife","tag-science","tag-uk","tag-united-kingdom","tag-unitedkingdom","tag-wildlife"],"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/posts\/783312","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/comments?post=783312"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/posts\/783312\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/media\/783313"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/media?parent=783312"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/categories?post=783312"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/tags?post=783312"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}