# Your Mom is a Chimera

![image.png](https://res.cloudinary.com/lesswrong-2-0/image/upload/v1775945832/lexical_client_uploads/pq6gr0pjnqbth4abip6y.png)

And so are you!

When you were a fetus, you were sending millions of your cells through the placenta into your mom. And she was sending her cells into you, although to a lesser degree. These cells made themselves right at home, differentiating into heart, blood, and even brain cells. This phenomenon is called [feto-maternal microchimerism](https://pmc.ncbi.nlm.nih.gov/articles/PMC8762399), and is one of the wildest things in placental mammal pregnancy.

What?
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[Microchimerism](https://pmc.ncbi.nlm.nih.gov/articles/PMC6714269) is generally defined as the presence of a small population of genetically distinct cells in an organism. When the fetus sends cells to the mother it's called Fetal Microchimerism (FMc) and when the mother sends cells to the fetus it's called Maternal Microchimerism (MMc). The actual cells are fetal microchimeric cells (FMC) and maternal microchimeric cells (MMC).

![image.png](https://res.cloudinary.com/lesswrong-2-0/image/upload/v1775963366/lexical_client_uploads/urnk8hjca4g8mzv6s7fy.png)

yes somehow this is the official [pubmed diagram](https://pmc.ncbi.nlm.nih.gov/articles/PMC8762399/)

You may have heard that the [placenta](https://pmc.ncbi.nlm.nih.gov/articles/PMC4305167/) is an organ that provides oxygen and nutrients to the fetus during development. Which is true. What's less commonly known is that the placenta also does bidirectional cell and genetic material trafficking, similar to drugs and humans across the US-Mexico border. [](https://pmc.ncbi.nlm.nih.gov/articles/PMC6714269)Most of these cells are [quickly killed](https://pubmed.ncbi.nlm.nih.gov/15251948/) by the mother's immune system immediately after to two weeks after birth, but some have been found in people's brains after [three decades](https://pmc.ncbi.nlm.nih.gov/articles/PMC3654737/). How does a fetus cell cross the blood brain barrier and become a brain cell? No one knows! It's also an open question if these "brain cells" can functionally integrate with the mom's brain circuits and process neuronal activations: They merely "[adopt](https://academic.oup.com/stmcls/article-abstract/23/10/1443/6399855?) locations, morphologies, and expression of immunocytochemical markers" of host neurons but further research is needed to determine if they have physiological significance. Weird!

But Why?
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There are many theories but no one knows for sure why it happens, what these fetal sleeper cells are up to, or if it's net [negative/positive](https://pmc.ncbi.nlm.nih.gov/articles/PMC7543167/) for the mom.

### Pros

There is evidence FMCs can help with [maternal wound healing](http://pmc.ncbi.nlm.nih.gov/articles/PMC4199806/). Unlike adults, human fetuses before the second trimester can [regenerate wounds without scarring](https://storage.imrpress.com/imr/journal/FBL/article/492008/1752765801663.pdf). Fetal cells have been observed homing into and gathering in damaged tissues of [almost all maternal organs](https://www.sciencedirect.com/science/article/pii/S1534580722003756#bib20). For example, a study (n=1230) found that patients with peripartum cardiomyopathy (heart dysfunction occurring in late pregnancy or shortly after birth) had a roughly [70% lower risk of death](https://www.nejm.org/doi/full/10.1056/NEJM200004133421502) than general cardiomyopathy patients (you can literally fix your mom's broken heart!). FMCs have also been found in skin wounds such as [C-section scars](https://www.tandfonline.com/doi/epdf/10.4161/chim.28746?src=getftr&utm_source=sciencedirect_contenthosting&getft_integrator=sciencedirect_contenthosting). They figured this out by using Fluorescent In Situ Hybridization (FISH) to stain X chromosomes red and Y chromosomes green. In the picture below, the blue circles are nuclei and the arrows point to the ones with both X and Y chromosomes (male fetus DNA in a woman!)

![image.png](https://res.cloudinary.com/lesswrong-2-0/image/upload/v1775959677/lexical_client_uploads/pnpiyqnmywepoauakrou.png)

[Source](https://pubmed.ncbi.nlm.nih.gov/24717775/#&gid=article-figures&pid=captionless-figure-uid-0)

### Cons

All sorts of fun things like [preeclampsia](https://www.ahajournals.org/doi/full/10.1161/HYPERTENSIONAHA.113.01486), [spontaneous preterm labor](https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(05)60395-9/abstract), [rheumatoid arthritis](https://www.pnas.org/doi/abs/10.1073/pnas.1904779116) and other [autoimmune disorders.](https://pubmed.ncbi.nlm.nih.gov/29087650/) FMCs also [increase transfer of resources](https://onlinelibrary.wiley.com/doi/full/10.1002/bies.201500059) to the fetus (a con for the mom but pro for the fetus). They route more nutrients through the placenta during the pregnancy, and cause physiological changes in the mother after birth (e.g. in lactation, thermoregulation, and attachment systems). As a fetus, you are kind of at war with your mom. You would totally want her to spend limitless resources on only you, but she might want to not do that and prioritize her own survival and potential future offspring.

What do I do with this information?
-----------------------------------

Call your mom

Conclusion
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When we were kids my mom would often tell my brother and I we were her 心肝 (Xīngān, heart and liver). This might sound weird but it's a common Chinese term for endearment, like "honey" or "darling" in English. It's interesting to learn now that this is also sort of literally true.

Mothers and their children often have a special bond, and it might be cool to know a part of you is always in your mom, and a part of her is always in you (unless you didn't have a good relationship, in which case don't think about it).

There are many open questions in this exciting research field. In the future, perhaps we can study these FMCs and learn how they do their thing, making better organ transplants, stem cell therapies, hybrid animals, or brain replacement for eventual digital uploading.

*Yo momma so fat she has room for two humans worth of allogeneic DNA in her neurons, bone marrow, and organs*