A study published in Science looks at the generation of functional primordial germ cell–like cells from pluripotent stem cells in rats.
Prof Robin Lovell-Badge, Group Leader, The Francis Crick Institute, said:
“The paper from Mami Oikawa and colleagues is impressive. It represents a huge amount of work, where they have essentially replicated in the rat several studies that were originally carried out in mice claiming to show that they can derive functional male gametes from pluripotent stem cells grown in culture. Each of the steps, from working with the pluripotent stem cells to deriving primordial germ cell like cells, to combining these with gonadal cells to encourage their further differentiation and their introduction back into testes of rats, involved the generation of specific genetically altered cells or animals, and this is a challenge in the rat compared to the mouse.
“Along the way they reveal similarities and differences with mouse germ cell development, and I agree with the authors that this may help define better ways of obtaining in vitro-derived sperm (and eggs) in other mammals, including humans.
“The generation of liveborn animals from the cells originally grown in culture is not very efficient, but the rates are high enough to now begin to refine each of the steps, and it was encouraging that the animals that were born mostly survived well and were apparently healthy.
“This bodes well for attempts to obtain human sperm – and perhaps eggs – from pluripotent stem cells, which will be important to understand how they develop and as a potential solution to overcome problems of fertility, such as in adults who had radiotherapy or chemotherapy for childhood cancers.”
Prof Dorothy Bennett, Director, Molecular and Clinical Sciences Research Institute, St. George’s, University of London, said:
“The press release seems a reasonable summary of the science, and the science seems rigorous and of high quality.
“Pluripotent stem cells (PSCs) are cells grown in culture from very early embryos, which are capable of dividing and developing into all three layers of the early embryo as well as the placenta in mammals.
“It has previously been achieved in mice to use bioactive factors in the culture medium to induce PSCs to develop into cells equivalent to functional primordial germ cells (PGCs), the precursors of gametes, which could then develop into normal functional gametes within a mouse.
“What is novel about the present study is that ten years later it is the first to achieve the production of PGC-like cells from PSCs from a mammal other than mouse, namely rat. Various differences in the culture methods were needed. The PGC-like cells were shown to be functional as PGCs by implantation into the testes of sterile male rats (lacking their own sperm), who could then produce normal, fertile offspring.
“Implications: The authors propose this to be helpful for the study of genetic diseases. It can make it easier to generate genetically engineered rats, to mimic and study the effects of specific mutations found in human families with a genetic disease – as already done in mice. They say that rats are physiologically more similar to humans than mice are, so more relevant. We might question whether they are much more relevant, since even zebrafish have been useful in such studies. But it can be said that studying a given gene mutation in more than one mammal could make conclusions more firm.”
Prof Azim Surani, Director of Germline and Epigenetics Research, Gurdon Institute, University of Cambridge, said:
“This study represents another example of rapid advances in mammalian germline biology, demonstrating the possibility of making viable rat gametes from pluripotent stem cells and induced pluripotent stem cells, for instance, from skin cells.
“In this field of science, derivation of early human precursors of gametes and the onset of early gametogenesis have in other studies also been demonstrated in cultures at very low frequencies.
“However, we are far from making viable human gametes from human stem cells. But studies such as this one on the rat advance knowledge of germline biology and help us find ways to address infertility and alleviate some of the barriers to human fertility.
“Germline is ‘immortal’ through successive generations and how this is achieved has broader implications for other human diseases, including age-related diseases.”
Dr Harry Leitch, Stem cell biologist and group leader at the MRC London Institute of Medical Sciences, and Academic Clinical Lecturer in Clinical Genetics, Imperial College London, said:
“This is a technical tour de force and extremely important study for the field. Previous groundbreaking work in mice has shown that early germ cells generated in vitro from pluripotent stem cells can give rise to functional gametes – the sperm and egg. The authors make some adjustments to previous protocols and successfully produce functional gametes from rat pluripotent stem cell lines grown in culture.
“Rats are close relatives of mice, but this first step into a different species is important and raises the possibility that similar discoveries might be possible in bigger animals such as live stock species and even humans. Indeed, some progress in the first steps of this process have already been made using human pluripotent stem cells.
“This work highlights that careful experimentation across a range of different species will be key to future advances – especially if we are to realise the translational potential of this work, for instance as a system to study or even treat infertility in the future.”
Prof Petra Hajkova, Interim Director, and Professor of Developmental Epigenetics, MRC London Institute of Medical Sciences, said:
“This is an interesting story and an important step in the field of germline biology. As the authors suggest, generation of mature sperm from pluripotent embryonic stem cells has been achieved in the mouse a decade ago by Katsuhiko Hayashi and colleagues – however, the same has not been achieved in other mammalian species so far.
“The presented research is an extension of the pioneering work in the mouse and although important, the results are perhaps not that surprising given the close similarities between the germline development in those two species.
“It should however be noted that there are important differences between the early germline specification in rodents and in primates (including humans).”
Dr Darius Widera, Associate Professor in Stem Cell Biology and Regenerative Medicine, University of Reading, said:
“The study by Oikawa and colleagues is an important milestone for developmental and reproductive biology, and potentially for animal breeding, and reproductive medicine. Although generation of mouse sperm cells in a dish has been achieved a decade ago, this initial success could not be replicated in any other mammalian species so far. In the present study, functional rat sperm cells have been generated from rat embryonic stem cells. These engineered sperm cells were very similar to naturally developed sperm cells and could generate viable and fertile rat offspring after in vitro injection into unfertilised egg cells. However, the animals produced through this method shown some morphological abnormalities. Moreover, the study did not yet prove whether these engineered sperm cells can give rise to viable offspring through natural mating.
“Since rats are physiologically closer to humans than mice, the findings could pave the way for development of protocols for generating human sperm cell from pluripotent stem cells. This would have substantial implications for human developmental biology and in long term, for reproductive medicine – but we are not there yet and much more research would be needed first.”
‘Functional primordial germ cell–like cells from pluripotent stem cells in rats’ by Mami Oikawa et al. was published in Science at 19:00 UK time on Thursday 7 April 2022.
Prof Robin Lovell-Badge: “I have no financial conflicts of interest although some of our research is close to the topic of the paper.”
Prof Dorothy Bennett: “I have no conflict of interest.”
Prof Azim Surani: “I have no conflicts of interest.”
Dr Harry Leitch: “None.”
Prof Petra Hajkova: “I declare no competing interest.”
Dr Darius Widera: “I have no conflicts of interest to declare.”