ES cells can now be used to induce structures with regionalised maxillary and mandibular primordia through the neural crest cell state, allowing for the recapitulation of jaw development in vitro. (POU3F3+ for maxillary and HAND2+ for mandibular)
CREDIT: KyotoU/Mototsugu Eiraku and Yusuke Seto
The highly complex shapes of animal faces originate from their respective transient neural crest cells. These embryonic pluripotent cells within the facial primordium – the early development form – may be necessary for forming proper facial structures. They migrate from their dorsal origin to the ventral craniofacial primordium and contribute to the cartilage, bones, and connective tissues. Analysing the molecular mechanisms in such early stages of development however poses many technical challenges.
Now, a group of Kyoto University researchers have produced neural crest cell-rich aggregates from human pluripotent stem cells and also developed a method to differentiate them in cell populations with a branchial arch-like gene expression pattern. Their research is published in Nature Communications.
“After the cell populations differentiate into precursors of maxillary and mandibular cells in response to external signalling factors, these populations spontaneously form patterns of the facial primordium,” explains Yusuke Seto of KyotoU’s Institute for Life and Medical Sciences.
This cartilage-like structure, reminiscent of Meckel’s cartilage, is formed locally within the aggregates.
“We aim to establish a model for studying early facial development by using the properties of human pluripotent stem cells to generate in vitro tissue resembling the bronchial arch of the primordial face,” adds Ryoma Ogihara, also of the Institute.
Researchers are examining the various developmental processes that cause interspecific and individual differences in facial structure to explain conditions such as craniofacial disorders.
“Using our in vitro model could help us better understand and control signal integration during the fate determination of the branchial arch and cartilage formation in the face and elsewhere. We hope our technology can contribute to the development of cellular materials for new regenerative medicine,” adds Mototsugu Eiraku, also of the Institute.
Scientists studying gene activity data of the early human embryo have discovered an overlooked type of cell which self-destructs within days of forming, as part of a quality control process to protect the developing foetus. The findings, published in PLoS Biology, give insights on what happens at the very first stages of life after fertilisation which could in the future help improve IVF or regenerative medicine treatments.
Self-destructing embryonic cell
As a zygote develops, cells start to specialise, and like trains sent to different end stations, some will be shunted off to become the placenta while others will become the embryo.
The team of scientists analysed previously published data on gene activity of each individual cell from 5-day old embryos and discovered around a quarter of the cells didn’t fit the profile of any of the known cell types (pre-embryo, pre-placenta etc).
Investigating further, they discovered that these cells contained so-called “Young transposable elements” or “jumping genes.” These are rogue elements of DNA that can copy themselves and insert themselves back into our DNA, often causing damage in the process.
Staining of embryos by project collaborators in Spain confirmed the existence of the cells with proteins derived from the jumping genes.
Looking a little further forward in time, the team found their descendants both have DNA damage and undergo a process of programmed cell death.
Quality control mechanism
This process, the researchers suggest, looks like a form of quality control: selection between cells in favour of the good ones.
Dr Zsuzsanna Izsva?k, co-senior author from the Max Delbrück Center and an expert on mobile DNA, said: “Humans, like all organisms, fight a never-ending game of cat and mouse with these harmful jumping genes.
“While we try and suppress these jumping genes by any means possible, very early in development they are active in some cells, probably because we cannot get our genetic defences in place fast enough.”
Co-lead author Professor Laurence Hurst, from the Milner Centre for Evolution at the University of Bath, said: “If a cell is damaged by the jumping genes – or any other sort of error such as having too few or too many chromosomes – then the embryo is better off removing these cells and not allowing them to become part of the developing baby.
“We are used to the idea of natural selection favouring one organism over another. What we are seeing within embryos also looks like survival of the fittest but this time between almost identical cells. It looks like we’ve uncovered a novel part of our arsenal against these harmful genetic components.”
Using old genetic enemies to fight new ones
Conversely, the single-cell data showed that the key cells that will become the embryo (the inner cell mass or ICM) don’t contain jumping genes but instead express a virus-like gene called human endogenous virus H. This helps suppress the young jumping genes in the inner cell mass, fitting with an emerging pattern that we use our old genetic enemies to fight our new ones.
The authors suggest that if the quality control process is too sensitive, the embryo as a whole may die. This might explain why some mutations in our system to detect damage in early embryos are also associated with infertility.
Neuroscientists published in the Journal of Neurochemistry, shows that maternal levels of vitamin D are key in the development of dopaminergic neurons, which are thought to be involved in schizophrenia.
Professor Darryl Eyles has built on past research out of his laboratory at the Queensland Brain Institute linking maternal vitamin D deficiency and brain development disorders, such as schizophrenia, to understand the functional changes taking place in the brain.
Schizophrenia is associated with many developmental risk factors, both genetic and environmental. While the precise neurological causes of the disorder are unknown, what is known is that schizophrenia is associated with a pronounced change in the way the brain uses dopamine, the neurotransmitter often referred to as the brain’s ‘reward molecule’.
Professor Eyles has followed the mechanisms that might relate to abnormal dopamine release and discovered that maternal vitamin D deficiency affects the early development and later differentiation of dopaminergic neurons.
The team at the Queensland Brain Institute developed dopamine-like cells to replicate the process of differentiation into early dopaminergic neurons that usually takes place during embryonic development.
They cultured the neurons both in the presence and absence of the active vitamin D hormone. In three different model systems they showed dopamine neurite outgrowth was markedly increased. They then showed alterations in the distribution of presynaptic proteins responsible for dopamine release within these neurites.
“What we found was the altered differentiation process in the presence of vitamin D not only makes the cells grow differently, but recruits machinery to release dopamine differently,” Professor Eyles said.
Using a new visualisation tool known as false fluorescent neurotransmitters, the team could then analyse the functional changes in presynaptic dopamine uptake and release in the presence and absence of vitamin D.
They showed that dopamine release was enhanced in cells grown in the presence of the hormone compared to a control.
“This is conclusive evidence that vitamin D affects the structural differentiation of dopaminergic neurons.”
Leveraging advances in targeting and visualising single molecules within presynaptic nerve terminals has enabled Professor Eyles and his team to further explore their long-standing belief that maternal vitamin D deficiency changes how early dopaminergic circuits are formed.
The team is now exploring whether other environmental risk factors for schizophrenia such as maternal hypoxia or infection similarly alter the trajectory of dopamine neuron differentiation.
Eyles and his team believe such early alterations to dopamine neuron differentiation and function may be the neurodevelopmental origin of dopamine dysfunction later in adults who develop schizophrenia.
Craniofacial region of a 13-day old mouse embryo by transmitted light microscopy. Credit: Craig Rhodes and Kenneth Yamada, National Institute of Dental and Craniofacial Research, National Institutes of Health
A team of researchers have found evidence of mouse and human germline cells that suggest they can reset their biological age.
As animals age, cell divisions run into replication errors and other external factors (such as exposure to pollutants) lead to gradual decay in cell quality; all of this is part of the natural ageing process. Eventually, cells become senescent and no longer able to divide in response to injury or wear and tear. In a new effort to understand this, researchers have found evidence that shows germline cells have a mechanism to effectively reset this process, enabling offspring to reset their ageing clocks.
Germline cells pass on genetic material from parent to offspring during the reproductive process. For many years, scientists have wondered why these cells do not inherit the age of their parents. And for many years, they assumed that the cells were ageless, but recent work has shown that they do, in fact, age. So that raised the question of how offspring are able to begin their lives with fresh cells.
To find out, the researchers at Brigham and Women’s Hospital and Harvard Medical School used molecular clocks to track the ageing process of mouse embryos. These clocks measure epigenetic changes in cells, and using them, the researchers continuously compare the biological age of embryos (apparent age based on reactions to epigenetic changes) with their chronological age. They found that the biological age of the mouse embryos remained constant through initial cell division after an egg was fertilised. However, about a week later, after embryo implantation in the uterus, the biological age of the embryos dropped. Some mechanism, it seems, had reset the biological age of the embryo back to zero.
Turning to human embryos, the team was unable to track ageing in human embryos because ethics rules forbid such research, but they still managed evidence suggesting that human embryos also reset their clocks. They plan to continue seeking the mechanism behind the reset process. The team’s findings were published in the journal Science Advances.
Journal information: Csaba Kerepesi et al, Epigenetic clocks reveal a rejuvenation event during embryogenesis followed by aging, Science Advances (2021). DOI: 10.1126/sciadv.abg6082
A study published in the journal Cell has announced the creation of human-monkey chimeric embryos, igniting renewed debate over ethics.
The embryos are known as chimeras, organisms whose cells come from two or more “individuals”, and in this case, different species: a long-tailed macaque and a human. The research confirmed that the cells can survive and multiply.
Natural human chimeras do exist, and can involve humans cells from two embryos in the same womb fusing to produce a single individual, or a combination of maternal and foetal cells, or monozygotic twins sharing blood cells from a shared placenta.
Previously, researchers had produced pig or sheep embryos that contained human cells, in an effort to one day develop a way to grow human organs for transplant inside the animals.
The researchers, led by Prof Juan Carlos Izpisua Belmonte from the Salk Institute in the US, said that the results shed new light on the communications pathways between cells of different species. This could help with future efforts to engineer chimeras using more distantly related species.
“These results may help to better understand early human development and primate evolution and develop effective strategies to improve human chimerism in evolutionarily distant species,” the authors wrote.
In 2019, the Spanish newspaper El País reported on rumours that a team of researchers led by Belmonte had created monkey-human chimeras.
Specific human foetal cells called fibroblasts were reprogrammed to become stem cells, and were then introduced into 132 embryos of long-tailed macaques, six days after fertilisation.
“Twenty-five human cells were injected and on average we observed around 4% of human cells in the monkey epiblast,” said Dr Jun Wu, a co-author of the research, and now at the University of Texas Southwestern Medical Center.
The embryos were grown in petri dishes and were terminated 19 days after the stem cells were injected. The human cells were engineered with a fluorescent protein to enable identification.
The researchers reported that 132 embryos contained human cells on day seven after fertilisation. However, the proportion containing human cells fell over time.
“We demonstrated that the human stem cells survived and generated additional cells, as would happen normally as primate embryos develop and form the layers of cells that eventually lead to all of an animal’s organs,” Belmonte said.
The researchers also found differences in intercellular interactions between human and monkey cells within chimeric embryos, compared to the normal monkey embryos.
The researchers hoped the research would help develop “transplantable human tissues and organs in pigs to help overcome the shortages of donor organs worldwide”, said Wu.
Prof Robin Lovell-Badge, a developmental biologist from the Francis Crick Institute in London, said at the time of the El País report he was not concerned about the ethics of the experiment, noting the team had only produced a ball of cells. But he noted conundrums could arise in the future should the embryos be allowed to develop further.
While not the first attempt at making human-monkey chimeras – another group reported such experiments last year – the new study has reignited such concerns. Prof Julian Savulescu, the director of the Oxford Uehiro Centre for Practical Ethics and co-director of the Wellcome Centre for Ethics and Humanities at the University of Oxford, said the research had raised all sorts of ethical concerns.
“These embryos were destroyed at 20 days of development but it is only a matter of time before human-nonhuman chimeras are successfully developed, perhaps as a source of organs for humans,” he said, and added that a key ethical concern was the moral status of such organisms.
“Before any experiments are performed on live-born chimeras, or their organs extracted, it is essential that their mental capacities and lives are properly assessed. What looks like a nonhuman animal may mentally be close to a human,” he said. “We will need new ways to understand animals, their mental lives and relationships before they are used for human benefit.”
Others were less concerned, and rather pointed out that all the study found was that the creation of such chimera was simply ineffective.
Dr Alfonso Martinez Arias, an affiliated lecturer in the department of genetics at the University of Cambridge, said: “I do not think that the conclusions are backed up by solid data. The results, in so far as they can be interpreted, show that these chimeras do not work and that all experimental animals are very sick.
“Importantly, there are many systems based on human embryonic stem cells to study human development that are ethically acceptable and in the end, we shall use this rather than chimeras of the kind suggested here.”
