Cambridge University's Professor Graham Burton explains the rise of the organoids in studying early pregnancy
Miniature functional models of the lining of the womb are helping researchers at the Centre for Trophoblast Research make new discoveries.
The early days and weeks of a pregnancy are critical to its successful outcome and yet, until now, they have been frustratingly difficult to study.
Cambridge researchers at the Centre for Trophoblast Research, which this year marks its tenth anniversary, have succeeded in finding a remarkable way to solve this challenge.
Centre director Prof Graham Burton told the Cambridge Independent: “The events during early pregnancy are really very hidden from view and for obvious reasons it’s very difficult to explore those without disrupting the pregnancy.
“So the first few weeks of pregnancy are very much a black box but there is clearly very complex interaction between the implanting egg and the lining of the womb, the endometrium.
“This has been explored in some animal models. But the form of placentation – the way the placenta develops – in humans is very different. You would be able to recognise a pig heart or a cow heart but placentas are hugely different. We really don’t understand why that is the case but it is, so we don’t have an animal model of early human pregnancy.”
But now the centre’s scientists have successfully grown miniature functional models of the lining of the womb, using cells derived from endometrial tissue, and have maintained them in culture for several months.
Known as organoids, it is hoped that they will provide fresh insight into the early stages of pregnancy, leading to better understanding of fertility problems and conditions such as endometriosis, which affects as many as two million women in the UK.
“For the first time now we have been able to grow these so-called organoids – the groups of cells that really mimic the behaviour or lining of the womb,” explained Prof Burton.
“These are highly secretory cells. They produce a lot of secretions that support the developing embryo. In other species, this is collectively known as uterine milk.
“This has been known since the days of William Harvey in the 16th-17th century. It’s known to be crucially important but in humans we don’t actually know what it contains, what its functions are and what its actions are on the fertilised egg and whether if those secretions are abnormal it leads to problems in later pregnancy or even infertility.
“So these are crucial questions and now that we have these organoids which actually behave in the same physiological way as the lining of the uterus in response to changes in sex hormones and pregnancy hormones, we have the opportunity to start investigating these secretions and interactions.”
While this study has been published with joint senior author Ashley Moffett in the journal Nature Cell Biology, Prof Burton also revealed to the Cambridge Independent a new breakthrough.
“We haven’t actually published this yet but we have been able to do the same with the placental cells. So we have the opportunity to bring together placental and maternal cells in an artificial implantation situation for the first time.
“That’s terribly exciting and opens up a lot of possibilities for looking at these events that are taking place, probably before the woman realises she is pregnant or within a week or two of that.
“This is absolutely critical to the success of the pregnancy and even to long-term health because we now know that if the period of inter-uterine development is abnormal or compromised it can have long-term consequences for your health, with maldevelopment of the heart or the kidneys. That shows up later in life as hypertension or renal failure.
“It’s a critical period and yet it’s inaccessible in real life.”
In those early days of pregnancy, the fertilised egg, or conceptus, undergoes ‘implantation’. There is growing evidence that complications that show themselves later, such as restricted growth of the foetus, stillbirth and pre-eclampsia, could have their origins around this time.
“By 10 days after fertilisation, the embryo has moved from the cavity of the uterus into the wall of the uterus. This is totally different from the sheep or the pig and many other species where the embryo remains within the cavity of the uterus,” said Prof Burton.
“People thought that this more invasive form of implantation in humans brought the two circulations into contact earlier and that’s why we have a bigger brain, but we’ve shown that’s quite untrue.”
The purpose of these different forms of implantation is not fully understood but probably enables species to maximise their reproductive success in different environments. What is known is that the fertilised egg effectively stimulates its own development by sending messages to the mother.
“That’s the fascinating thing and one of the big questions we will be able to address with our culture systems now,” said Prof Burton. “It’s clear that there’s an enormous dialogue going on between the mother’s cells and the foetal cells at the time of implantation.
“It seems from animal studies that the placenta and the embryo is able to signal to the wall of the uterus to secrete more of these uterine proteins, which stimulate the development.”
Intriguingly, until 10-12 weeks, there is no significant blood flow from the placenta – it is the nutrient supply from glands in the uterus that nourishes the foetus.
“This is some work I was involved in and again if you look at animal species they would have a similar period where nutrition comes from the glands. We believe that these secretions can provide everything that the embryo needs but it does mean you need a relatively low-oxygen environment. We think that benefits the embryo because very important decisions on organ differentiation are being taken at that time and oxygen is actually quite disruptive to these processes.
“Our normal metabolism produces a level of free radicals that is in proportion to the amount of oxygen in the environment so I think if you keep the embryo in a relatively low oxygen state, you reduce the levels of free radicals, which are capable of damaging the DNA, altering signalling pathways and affecting the differentiation of these organs.”
It is known that congenital malformations are more likely among women with diabetes, where the burning of high levels of glucose leads to increased oxygen and therefore more free radicals.
“You are looking at a very protective environment for that first 10-12 weeks when all the major systems are differentiating,” said Prof Burton.
“At the end of that period you are a miniature baby – you are still only about 2cm in size but all the organs are differentiated and you have arms, legs and everything. At that point, you just need to grow and for that you need a good blood supply.”
From this point, the placenta acts like a kind of umpire between mum and developing baby.
“The placenta is sitting between the maternal nutrient supply and the foetal demand,” explains Prof Burton. “It not only transfers nutrients but it has a major endocrine function – it produces a lot of hormones, which affect almost every aspect of maternal physiology, including the mother’s metabolism.
“Those hormones will increase her appetite, they will stimulate her to lay down fat deposits early in pregnancy and then later in pregnancy they will mobilise those deposits and push up her circulating level of nutrients so they can go across the placenta to the baby.
“It’s a fantastic system. The baby really is controlling the mother’s metabolism, but she fights back to a certain extent by not allowing herself to be drained of all nutrients because, for one thing, the baby would get too big and couldn’t be delivered and, secondly, she needs to be in a condition appropriate for nursing the baby and for having subsequent children.”
In nutrient-deprived mothers, it is known that the placenta will increase its transport capacity in an attempt to protect the foetus.
The centre is going through an ethical approval process to conduct trials with Bourn Hall, the fertility clinic, to help explore why some couples have trouble conceiving.
“It might tell us that a woman is not producing key proteins or key factors that assist in implantation and the early growth of the placenta or they are not responding to the normal cues from the placenta and the mother,” said Prof Burton.
Meanwhile, Dr Margherita Turco has succeeded in growing organoids from cancer cells. She hopes it will help lead to a better understanding and ultimately treatments for diseases of the endometrium, including cancer of the uterus and endometriosis.
As the centre marks its anniversary next month, it is clear it has an exciting future.
“There are centres that focus on individual complications of pregnancy but I don’t think there is anywhere else that has the same breadth of expertise,” said Prof Burton. “I think it is unique actually both in the UK and across the world.”
A decade of progress at The Centre for Trophoblast Research
The Centre for Trophoblast Research is named after cells that form the outer layer of the blastocyst, providing nutrients to the embryo and developing into a large part of the placenta.
“We’ve come an enormous way in 10 years and the next 10 are going to be equally exiting,” said Prof Burton.
“We now have a much better appreciation of that dialogue between the embryo and the uterus at all sorts of levels.
“We know much more about the way cells signal to each other – between the placenta and the mother, both in terms of the hormones but also in the way cells release very small particles which contain RNA and DNA, so there is much more incorporation of these messengers from one to the other. These are circulating in the maternal blood so you now have the opportunity of being able to sample from the placenta and from the embryo just by taking maternal blood samples. People have done the full genotyping of the embryo from those samples.
“That is tremendous for diagnosis but also it’s very exciting for us as researchers. It’s changing our way of thinking, getting away from the idea of an antagonistic mother into a much more co-operative dialogue type of setting. The tools are improving all the time. We are now able to look at the genes that are expressed in individual cells.”