Stem cell myth: they are homologous to everything

Some stem cell clinics sometimes use questionable ideas to bypass FDA regulations and I’d go far as to say some of these are outright stem cell myths. One such argument that seems like a myth in my opinion relates to the idea of “homologous use”, which is one of the hottest issues in stem cells in the clinical arena right now.

So what is homologous use and what is the clinic myth?

spackle stem cells
Image from web labelled for re-use

The idea with homologous use goes that a stem cell product fitting that definition is similar to the tissue or organ in the body that a provider seeks to treat. For instance, bone marrow used to treat a blood or immune condition is definitely homologous. There’s an undeniable similarity there between the product and the target tissue. It seems so far that the FDA has accepted the notion that marrow cells are homologous to joints too.

In contrast, bone marrow stem cells used to treat the brain would be definitely non-homologous.


Nerves, glia, and other cells in the brain are not at all similar to bone marrow.

The same is true of using fat stem cells to treat the brain because these cells have nothing in common in terms of origin and function with the nervous system.

Another example is that neither blood nor fat stem cells have anything in common with lung tissue. The list goes on and on.

In fact, many stem cell clinics market entire menus of non-homologous offerings. It’s common to find claims, for instance, that one type of fat stem cell product can treat the brain, the lungs, the kidneys, the eyes, the GI tract, sexual dysfunction, and more. All of these are non-homologous uses in my view and this is important because that means that these clinics legally must have FDA approval in advance, but of course they don’t in almost every case.

So how do the clinics handle this?

Some of them just ignore this situation and I guess hope for the best on the regulatory front, but others make a remarkable claim that stem cells are by definition homologous to everything so there never could be a non-homologous use.

This is called “pan-homology” and as a scientist I think it is a myth.

It reflects a fundamental misunderstanding of cell biology. While stem cells are “potent” meaning that they can differentiate into some specialized cell types, this doesn’t make them the same across the board to anything and everything tissue wise.

Why care about non-homologous use?

It makes experimental therapies have higher risk. You can imagine that injecting stem cells of one type into a tissue of a totally different type could lead to trouble. For example, fat stem cells just don’t belong inside the nervous system or in the lungs or in the eye.

But some clinics tout stem cells almost like some kind of universal “fix it” putty that can spackled anywhere with no problem. Spackle it on your face, in your brain, into your lungs, kidneys…it’s all good no matter where you put it and no matter what type of stem cells it is.

I believe that the myth of pan-homology is bad for patients and for the stem cell field.

10 thoughts on “Stem cell myth: they are homologous to everything”

    1. @SammyJo,
      Regarding FDA, fat/fat stem cells, and breast reconstruction, yes it’s not clear to me why fat stem cells in this context would be non-homologous use. Of course there is more to the breast than lactation and a key structural component there is fat tissue. So at this point, if I understand the FDA position correctly then I do disagree with them on this.

  1. “bone marrow stem cells used to treat the brain would be definitely non-homologous.”

    Then why are there human clinical trials happening with BM MSCs to treat MS, ALS, PD, & MSA? Many of them in the U.S. and FDA approved.

    2007-2015 For multiple sclerosis there are 24 pilot and phase 1 studies in pubmed, 227 total subjects, using bone marrow derived and expanded Mesenchymal Stem Cells. All report safety. Many with reduction in EDSS and MRI signs of disease. Except 1 used umbilical cord blood MSCs.

    2003-2016 For ALS, Parkinson’s and MSA, there are 13 studies phase 1/2 in pubmed, 200 total subjects, using bone marrow derived and expanded Mesenchymal Stem Cells. All report safety. No MRI abnormalities (up to 9 year follow up). Many report no or slowed progression, and some studies found improvements.

    1. @SammyJo, I’m guessing some or even most of these trials have INDs and those running them are working with the FDA, meaning those folks acknowledge that their stem cell product is a biological drug. In that context, the homology issue is not at play the same way. Paul

  2. @Paul – fair enough. It would be unfair to insist on every cell being located to its ontogenic source tissue as by comparison to small molecule drugs, not every molecule arrives at the target. One of the first things drug developers do in preclinical studies for small molecules is biodistribution studies with a radiolabelled drug, and such a tracking technology is still in its infancy for cellular therapeutics. This would solve a lot of guesswork on potential off-target effects and the elephant-in-the-room question – where do those cells go?

  3. What you call “pan-homology” may or may not be bad for the stem cell field, but nowhere on your blog have you ever justified the assumption that FDA regulation doesn’t cause more harm to patients by withholding legitimate therapies than it prevents by witholding bad ones.

  4. @Muggles, As to bone marrow it is in fact in a sense continuous with the vascular system and homologous to the cells therein as they arise in the marrow. As to definition in clinical practice, I suppose MDs need to apply a certain common sense approach to what constitutes “homology”, but there will be and in fact already are gray areas. For instance, to me it makes sense that the use of adipose stem cells would be homologous both for use under the skin where there is naturally occurring fat and also in the breast for reconstruction, but apparently it remains debated as to whether the latter is homologous use because the function of the mammary gland is to produce milk. Still, the fat in the breast is an inherent structural component so to me it makes sense that adipose cells could be used in a homologous manner there. The question of whether adipose stem cell products are more than minimally manipulated is a separate question.

  5. Thanks, Roger. There’s real hope for conditions such as yours via stem cells, but this isn’t real today. Unfortunately it’s going to take another 5-10 years I think before we see a lot more stem cell therapies that are really proven get onto the market.

  6. (playing devil’s advocate here) how can non-homologous use be defined in clinical practice? If it means administration of cells to a site other than the tissue from which they were extracted, then all bone marrow transplants will fall under this definition as cells are administered intravenously.

  7. Excellent piece, it answers many of my thoughts about why I backed out of a stem cell clinical trial to treat my Charcot Marie Tooth disease using fat derived stem cells.

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