Stem Cells


Stem cells are cells in the body with the following characteristics:

  • Ability to develop into different specialised cells types in the body such as the photoreceptors in the retina, liver, nerves and muscles
  • Unlimited self-replication/self-renewal (ability to divide over and over again to produce new stem cells)

These inherent abilities, coupled with an increasing understanding of cell culturing, have made stem cells an attractive option in various facets of eye research which include:

  • Understanding the mechanism of a specific condition by culturing cells harvested from a patient and growing them into little “eye cups”
  • Facilitates the identification and testing of new drugs for a specific condition before proceeding to clinical trials
  • Replacing or regenerating damaged retinal cells
An animation on how stem cells can be generated from our own skin cells

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Type of stem cells

Stem cells can be broadly classified into two types, embryonic and adult stem cells. Human embryonic stem cells (hESCs) are obtained from unused embryos during in vitro fertilisation (IVF) procedures. These cells are donated to science with the consent of the involved parents. ESCs are able to transform into virtually any cell type in the body (pluripotent).

hESCs have always been the main source of pluripotent stem cells until the discovery of human-induced pluripotent stem cells (hiPSCs).[1] iPSCs are generated by reprogramming mature cells (usually skin or blood cells) into pluripotent stem cells. After that, the IPSCs can be directed into becoming any cell type of interest.  In essence, we could generate our own pluripotent stem cells from any mature cells in our bodies.

On the other hand, adult stem cells are more limited (multipotent), which means that they can only develop into some cell types in the body. Their main functions are to supply new cells as an organism grows and to replace damaged cells. For example, bone marrow stem cells (known medically as haematopoietic stem cells) are only able to replace the different blood cells in the body such as red and white blood cells.

A less mentioned but equally important cell type is the progenitor cells. These cells are considered similar to stem cells but have limited ability in self-replication and can only develop into one or a few specialised cell types.[2] For instance, retinal progenitor cells can only develop into photoreceptor cells and other retinal cells but not blood or bone.

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Stem cells as a potential treatment for eye diseases

Nerve cells in our bodies do not regenerate once damaged. The retina is a structure wired with complex nerve connections to the brain which enable us to see when light is detected by the photoreceptor cells. However, the photoreceptors and the supporting retinal pigment epithelium (RPE) cells can be damaged at any age due to either genetic mutations (e.g. inherited retinal degenerations) or wear and tear changes (e.g. age-related macular degeneration/AMD), or even a combination of both. Scientists are exploring the possibility of transplanting retinal cells cultured from stem cells into eyes affected by two conditions, Stargardt disease and AMD.

In both conditions, the RPE cells are damaged which in turn causes secondary damage of the photoreceptors. To replace these cells, RPE generated from different sources including hESCs (hESC-RPE), hiPSCs (hiPSC-RPE) and adult stem cells have been transplanted into the eye in various clinical trials.

1) Pluripotent stem cells

Multiple trials have been undertaken or are currently ongoing investigating the safety of hESC-RPE transplantation. Majority of these trials are in the phase 1 or 2 stages. The scientific summaries of the reported trials are listed as follows:

Comparatively, clinical trials investigating human hiPSC-RPE transplantation are relatively few. The world’s first successfully transplanted iPSC-RPE was reprogrammed from skin cells of a 77-year-old woman with AMD. No major safety issues were reported at 1 year after surgery and the graft (transplanted cells) continues to survive after 4 years. However, the hiPSC-RPE from a second patient in the same trial were found to have genetic changes prior to transplantation that might alter gene function.[3] These might have been instigated during cell reprogramming and as a result the trial was paused. Since then, scientists have moved from using self-donated iPSCs to skin cells harvested from anonymous donors which are screened thoroughly for genetic mutations prior to being deposited in a “cell bank”. Yet, the high cost of maintaining such banks precludes iPSCs from being scaled up into larger clinical trials at present.[2]

2) Adult stem cells

Intravitreal injections (a commonly performed outpatient procedure by eye doctors) of bone marrow stem cells harvested from patients themselves have shown to be safe and potentially beneficial in a multitude of eye conditions, including inherited retinal degenerations, AMD and conditions affecting the blood circulation of the eye such as diabetic eye disease. Instead of replacing the damaged retinal cells, the bone marrow stem cells release proteins (known as neurotrophic factors) that protects the surviving retinal cells from further deterioration (neuroprotection).[4] The reported trials (mostly phase 1 studies) are listed here:

An injection into the clear jelly of the eye called the vitreous. This is a commonly performed outpatient procedure by eye doctors around the world for other conditions.
Intravitreal injection

3) Progenitor cells

Progenitor cells of retinal and neural (eventually becoming nerve tissues) origins are also being investigated as potential treatment for the most common form of inherited retinal degeneration, retinitis pigmentosa. Retinal progenitor cells mainly release neurotrophic factors to protect the surviving photoreceptors but it has been shown in animal models that some are able to replace lost photoreceptors.[5] The preliminary results of current trials for retinal progenitor cells (RPCs) demonstrated that the procedures are safe with some visual benefit though the small number of patients treated precludes drawing conclusions of its effectiveness.[6-8] The ongoing trials are listed as follows:

On the other hand, neural progenitor cells (NPCs) protect the surviving photoreceptors in a slightly different manner. The group heading the ongoing clinical trial (NCT 04284293) investigating this approach has programmed the NPCs (CNS10-NPC) to develop into a type of nerve cell called glial cells, which exerts a neuroprotective effect on surrounding nerves.[9]

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Is it safe?

The main safety concern with stem cell therapy is the risk of tumour formation. Tumour cells are basically cells that have uncontrolled growth and this is of particular concern with stem cells as they possess the unique ability of multiplying itself without any limitations. Hence, stem cells destined for transplantations are checked thoroughly prior to surgery to make sure that they only contain development markers for the cell type of interest. So far, none of the hESC-RPE and hiPSC-RPE trials have reported such occurrence, with the longest follow-up being 4 years after transplantation.[10-13] 

Like all other organ transplant procedures using organs harvested from other individuals (known as allogenic transplant), stem cell transplants sourced from cell banks (either hESCs or hiPSCs) risk inciting an immune reaction in our bodies which could lead to graft rejection. This is because the immune system recognises the graft as foreign and will mount an attack. As a result, patients in initial trials are often put on high dose medications that suppress our immune system (called immunosuppressors) for a period of time (up to 3 months).[11],[14] These medications have a myriad of side effects, which include increasing susceptibility to infections. Such an effect could potentially be harmful for elderly patients. However, a more recent trial conducted by Da Cruz and colleagues have demonstrated that the risk of graft rejection can be adequately controlled with shorter period of immunosuppressor intake (3 weeks) combined with steroid injections and a steroid implant inserted during surgery.[12]

In contrast, such medications can be avoided by using our own stem cells (autologous transplant) as in the case of bone marrow stem cell transplant or iPSCs. Although RPCs are allogenic, they do not seem to cause an immune reaction when injected into the eye.[15]

Other complications or side effects reported in hESC-RPE transplantation trials are mainly associated with the surgical procedure itself, which include worsening of cataract and a case of severe eye infection which was treated successfully with standard treatment protocols.[10]

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Beware of unregulated “stem cell trials”

Many “stem cell trials” are listed on the clinical trials registry provided by the US National Library of Medicine. However, it is important to note that presence of a study on does not mean that the study has been evaluated by an ethics committee or given approval. Furthermore, clinics running some of these trials charge patients exorbitant fees to participate and tend to be euphemistically called “patient-funded research”. Therefore, if you are interested in a particular trial, we strongly advise that you seek the opinion of your doctor on whether they think it is reputable and ethically approved.

One notable example is a “trial” in the US where multiple patients were injected with autologous adipose (fat) tissue-derived stem cells (NCT 02024269—now withdrawn) which resulted in severe visual loss in both eyes of 3 patients.[16] All 3 patients paid $5000 for participating in the “trial” which had supposedly obtained approval from the US Food and Drug Agency (FDA) but this was not stated in the provided patient information leaflets.

Another example is the ongoing Stem Cell Ophthalmology Treatment Study (SCOTS and SCOTS2) for multiple eye conditions. There are no indications that both trials are approved or reviewed by an ethics committee and patients were reportedly charged $20,000 for the treatment.[6] Publications associated with these trials tend to be small case reports (only a small number of patients reported) with follow-up duration ranging from 6-15 months. Most patients reported by the principal investigator of these trials obtain visual improvement with no serious complications.[17-19] However, other groups not associated with the SCOTS or SCOTS2 trials have reported serious complications such as recurrent retinal detachment and formation of epiretinal membranes (a sheet of cells developing on or above the macula) which can lead to visual deterioration.[20],[21]

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Although we have made significant strides in stem cell technology over the last two decades, stem cell therapy for genetic eye conditions still has some ways to go before reaching the bedside as a treatment option. The main challenge facing scientists now is that current stem cell-derived retinal photoreceptor cells are still not fully mature and functional, and therefore having difficulties to replicate or replace the lost cells. The transplanted cells will also need to integrate seamlessly with the other surviving photoreceptors to have a therapeutic effect. Once these issues are solved, we may be one step closer to using cell therapy to treat previously incurable eye conditions.

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  1.  Takahashi K, Yamanaka S. Induction of pluripotent stem cells from mouse embryonic and adult fibroblast cultures by defined factors. Cell. 2006;126(4):663-676
  2.  Rao RC, Dedania VS, Johnson MW. Stem Cells for Retinal Disease: A Perspective on the Promise and Perils. Am J Ophthalmol. 2017;179:32-38
  3.  Mandai M, Kurimoto Y, Takahashi M. Autologous Induced Stem-Cell-Derived Retinal Cells for Macular Degeneration. N Engl J Med. 2017;377(8):792-793
  4.  Park SS, Moisseiev E, Bauer G, et al. Advances in bone marrow stem cell therapy for retinal dysfunction. Prog Retin Eye Res. 2017;56:148-165
  5.  Aftab U, Jiang C, Tucker B, et al. Growth kinetics and transplantation of human retinal progenitor cells. Exp Eye Res. 2009;89(3):301-310
  6.  Terrell D, Comander J. Current Stem-Cell Approaches for the Treatment of Inherited Retinal Degenerations. Semin Ophthalmol. 2019;34(4):287-292
  7.  ReNeuron. hRPCs for Retinal Disease. Accessed 2 June 2020
  8.  jCyte. jCyte Presents Results of Clinical Testing in Retinitis Pigmentosa. Accessed 2 June 2020
  9.  Goldberg NRS, Marsh SE, Ochaba J, et al. Human Neural Progenitor Transplantation Rescues Behavior and Reduces α-Synuclein in a Transgenic Model of Dementia with Lewy Bodies. Stem Cells Transl Med. 2017;6(6):1477-1490
  10.  Schwartz SD, Tan G, Hosseini H, Nagiel A. Subretinal Transplantation of Embryonic Stem Cell–Derived Retinal Pigment Epithelium for the Treatment of Macular Degeneration: An Assessment at 4 Years. Investigative Ophthalmology & Visual Science. 2016;57(5):ORSFc1-ORSFc9
  11.  Mehat MS, Sundaram V, Ripamonti C, et al. Transplantation of Human Embryonic Stem Cell-Derived Retinal Pigment Epithelial Cells in Macular Degeneration. Ophthalmology. 2018;125(11):1765-1775
  12.  da Cruz L, Fynes K, Georgiadis O, et al. Phase 1 clinical study of an embryonic stem cell-derived retinal pigment epithelium patch in age-related macular degeneration. Nat Biotechnol. 2018;36(4):328-337
  13.  Kashani AH, Lebkowski JS, Rahhal FM, et al. A bioengineered retinal pigment epithelial monolayer for advanced, dry age-related macular degeneration. Sci Transl Med. 2018;10(435)
  14.  Schwartz SD, Hubschman JP, Heilwell G, et al. Embryonic stem cell trials for macular degeneration: a preliminary report. Lancet. 2012;379(9817):713-720
  15.  Klassen H. Stem cells in clinical trials for treatment of retinal degeneration. Expert Opin Biol Ther. 2016;16(1):7-14
  16.  Kuriyan AE, Albini TA, Townsend JH, et al. Vision Loss after Intravitreal Injection of Autologous “Stem Cells” for AMD. N Engl J Med. 2017;376(11):1047-1053
  17.  Weiss JN, Levy S. Stem Cell Ophthalmology Treatment Study: bone marrow derived stem cells in the treatment of Retinitis Pigmentosa. Stem Cell Investig. 2018;5:18
  18.  Weiss JN, Levy S, Benes SC. Stem Cell Ophthalmology Treatment Study (SCOTS): bone marrow-derived stem cells in the treatment of Leber’s hereditary optic neuropathy. Neural Regen Res. 2016;11(10):1685-1694
  19.  Weiss JN, Levy S, Benes SC. Stem Cell Ophthalmology Treatment Study (SCOTS) for retinal and optic nerve diseases: a case report of improvement in relapsing auto-immune optic neuropathy. Neural Regen Res. 2015;10(9):1507-1515
  20.  Leung EH, Flynn HW, Jr., Albini TA, Medina CA. Retinal Detachment After Subretinal Stem Cell Transplantation. Ophthalmic Surg Lasers Imaging Retina. 2016;47(6):600-601
  21.  Kim JY, You YS, Kim SH, Kwon OW. Epiretinal membrane formation after intravitreal autologous stem cell implantation in a retinitis pigmentosa patient. Retin Cases Brief Rep. 2017;11(3):227-231

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Updated on December 3, 2020
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