As an active biological researcher, I'm often amazed at just how impressive nature is, but we discover cool things all the time so I don't often get that "wow" factor any more. However, I recently learned about a phenomenon that actually completely blew my mind: microchimerism. For reasons that are obvious to most of you by now, I've been taking more of an interest in developmental biology in recent weeks. I work in a developmental neuroscience lab but my focus was widened somewhat from interneurons in the hippocampus to how a little human is actually made. Before I explain microchimerism, we need a quick embryology primer for those few friends I have who didn't do a biology degree...
Most people know the basics: sperm meets egg, buys it a few drinks, they sneak off somewhere quiet, fuse their pronuclei (the half-genome that each of them brings to the party), and create a stem cell. This stem cell is pluripotent, meaning that it can form any type of tissue. The cell begins to divide and multiply and, before you can have a panic attack after peeing on a little stick that changes your life forever (*), you have a little ball of cells that have burrowed their way into mummy's uterus and are marching towards the path of becoming a living creature. As the ball of cells gets bigger, some of the cells start to differentiate to form specific lineages of cells that will become brain cells, immune cells, liver cells, etc. The early stem cells can form any type of tissue but you get more specialised stem cells that can form a subset of tissues and these can persist into adulthood: new brain cells are created from adult stem cells that migrate into your hippocampus where they form dentate granule cells (kind of important for memory and learning), and other adult stem cells in your bone marrow give rise to, um, immune cells and other stuff I know nothing about (hey, I'm a brain guy...).
With me so far? Okay, let's explain what microchimerism is and why it's so mind-bogglingly awesome. Well, it turns out that some of those fetal stem cells will go on a jolly and cross the placenta into the mother's body. This in itself isn't so unexpected - there are lots of diagnostic tests where medically-inclinded folks measure fetal DNA from the mother's blood. But what is insane is that these fetal stem cells not only enter the mother's body but migrate to random tissues where they establish permanent cell lines and integrate into the mother's organs. Crazy, eh? The Wikipedia page on microchimerism has this detailed and highly technical diagram:
Another study (in mice) found that, using green fluorescent cells, they could detect fetal cells in the brains of the mousey mother. The really cool thing is that they found more fetal cells 4 weeks after birth than they did at the time of delivery, implying that the fetal cells had established permanent populations in the brain.
Fetal-maternal microchimerism has been known to occur in most mammals for quite some time. Is there any evidence for it in humans? Well, this PLoS study from 2012 found evidence of male DNA in multiple brain regions of 63% of female autopsied brains, all of whom had had male offspring. What is really interesting is that those women who suffered from Alzheimer's disease were significantly LESS likely to harbour male DNA. Obviously, this doesn't imply that the fetal cells were neuroprotective, but it does raise interesting questions. As an aside, this phenomenon also occurs with female fetuses, but it's much easier to detect male DNA in a female brain, which is why most studies focus on this. The pretty picture at the start of this post comes from this review article, showing male cells detected in a female liver.
If you're more interested in this (and who wouldn't be!?), it's worth looking at these non-specialist articles in The Scientist or Scientific American to find out more than the brief foray that I've made into the subject here.
Most of you know that my lovely wife Dom is a big fan of cake and sleeping. But since she's been pregnant with the twingers, her sweet tooth has been replaced with a craving for bread, cheese and beef jerky. Instead of sleeping in until 10am, she wakes up at 5, ready to eat all the food in the world. We like to joke that this is due to my genes being expressed in her brain, via our future progeny. If she starts making dirty jokes and chasing deer in our local park, we might start to believe this...
Last word? Microchimerism is pretty bloody amazing. And... look what I did (**):
(*) or at least seeing the stick after someone else pee'd on it...
(**) my wife may have done some of the hard work too...
Fascinating--I had read once that when women are pregnant they get a "boost" of youth --- maybe this is what it boils down to. Makes evolutionary sense if you need to be strong and available to raise the children.
ReplyDeleteWell, my wife is certainly glowing at the moment, although that's just the light from the fridge as she raids it for food. Microchimerism does make sense from an evolutionary perspective: fetuses with genes that allow stem cells to cross the placenta and repair any damage in the mother are more likely to survive to have their own progeny. I've never thought about prenatal selection before, but I guess it's plausible.
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