When a new variant of the COVID-19 virus appeared in the UK as 2020 drew to a close, I didn’t think it…
Remember human embryonic stem (hES) cells? We don’t hear much about them anymore. And so I was surprised to see an application of the controversial cells to grow human embryo-like structures in a recent issue of Nature.
Human embryonic stem cells are not, and have never been, taken from human embryos. Instead, they’re grown in laboratory glassware from cells that are sampled from the inner cell mass. The “icm” is the stage when the prenatal human is a smear of cells hugging the interior of a hollow ball of cells, the blastocyst. The icm expands and contorts, layering itself into embryohood, as the blastocyst gives rise to the nurturing extra-embryonic membranes.
In 2009 the National Institutes of Health issued guidelines forbidding researchers from using government funds to derive new hES cells, but the agency provides nearly 500 already-existing hES cell lines. They represent dozens of inherited diseases, from cancers to neurological conditions to connective tissue disorders – quite an eclectic list.
When the draft guidelines were issued and comments sought, many scientists disagreed with the definition of human embryonic stem cells, and asked that the NIH define them as “originating from the inner cell mass of the blastocyst,” to counter the widely-held public misconception that the cells were parts of embryos. The distinction between embryonic and embryo is important, and it’s lost in much of the public debate over the ethics of research that uses the cells.
To Paraphrase Bob Dylan, Just Like an Embryo
The researchers, from the University of Cambridge and the Hubrecht Institute in The Netherlands, combined observations from experiments on human and mouse ES cells.
Developmentally important genes of human ES cells painted onto grooved surfaces turn on as time goes on, as they should. But the cells don’t organize – they don’t form a top and bottom, nor a front and back – like embryos in a uterus do.
Prenatal development is an exquisitely timed choreography of signals that draws proliferating cells into tissues and folds the tissues into organ buds, which then elaborate during the fetal period that begins eight weeks from fertilization. The human ES cells on the lab dish surfaces just sat there.
But mouse ES cells suspended in fluid, freed from the constraints of their crosstalk that a surface presents, form quasi-organized structures, termed “gastroloids.” Their genes switch on according to the developmental timetable, but unlike their human counterparts struggling on a surface, the freer mouse cells organize and fold and unfold, just like an embryo.
The researchers repeated the mouse experiment with hES cells, fashioning human gastroloids. The structures capture a turning point in prenatal development: formation of the three primary germ layers from which all organs ultimately unfurl. A three-layered embryo is called a gastrula. Hence, gastroloids.
Human Prenatal Development in a Nutshell
A fertilized ovum divides repeatedly during an initial period called cleavage, yielding first a solid ball of cells, called a morula for “mulberry,” which it resembles. During the second week, a space (the amniotic cavity) forms between the inner cell mass and the outer cells anchored to the uterine lining.
Then the icm flattens into a two-layered embryonic disc. The side nearest the amniotic cavity is the ectoderm; the inner layer, closer to the cavity, is the endoderm. Shortly after, a third layer, the mesoderm, forms in the middle. It’s a little like two pieces of bread coming together and then a slab of bologna inserting itself in the middle, creating a sandwich. That’s the gastrula, aka a “primordial embryo.”
The gastrula elaborates itself from days 13 to 19. For those who are going to yell at me for writing this post, the embryo looks nothing like a baby. It isn’t visible. But gastrulation is so critical that it inspired developmental biologist Lewis Wolpert to claim, in 1986, “It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life.”
And, from a strictly biological standpoint, it is.
Gastrulation is when a cell’s address in one of the layers sets its fate, like choosing a major in college.
Cells in the ectoderm, the outermost layer, become skin, nerves, or parts of certain glands. The innermost endoderm cells form parts of the liver and pancreas and the linings of many organs. Finally, the bologna in the middle, the mesoderm, forms everything else: muscle, connective tissues, the reproductive organs, and the kidneys.
Restrictions in Research
As crucial as gastrulation is, in many countries it marks the point after which a prenatal human cannot be studied without prosecution of the researcher. The timing of gastrulation was supposedly chosen because after that a twin can no longer form, and it is also before the first inklings of a brain appear, which might allow pain, although not its perception.
But being unable to view the earliest stages of human prenatal development without risking criminal charges is to ban inquiry that could impact prenatal care and possibly even inform in utero treatments.
Those first two weeks are when congenital defects originate, when teratogens (alcohol, certain drugs, and infections) disrupt development, and when the first signs of impending miscarriage may appear. Genetic diseases may manifest in ways unknown. In fact, identifying the genes that gastruloids express at 72 hours revealed a signature of impending successful sorting into the three layers, which could be used in a new type of very early prenatal test.
A gastroloid isn’t nearly as organized and sophisticated as a gastrula, but it corresponds to a prenatal human at 18 to 21 days from fertilization, and seems a valuable stand-in. “Our model produces part of the blueprint of a human. It’s exciting to witness the developmental processes that until now have been hidden from view – and from study,” said lead researcher Alfonso Martinez-Arias, from the University of Cambridge department of genetics.
Gastruloids shouldn’t bring out the protestors objecting to killing babies. The cell concoctions can’t develop into fully-formed embryos, nor do they have brain cells or the membranes required to nestle into a uterine lining. They can’t develop.
“This is a hugely exciting new model system, which will allow us to reveal and probe the processes of early human embryonic development in the lab for the first time,” said first author Naomi Moris, also from the University of Cambridge. “Our system could prove useful for studying what happens when things go wrong, such as in birth defects.”
(Opening image: A human gastruloid, with green corresponding to the tail end of an embryo and magenta marking the front end, similar to a developing heart. Credit: Naomi Moris.)