Infertility Treatment with Stem Cells is Unlikely


Because several laboratories have managed to differentiate embryonic stem cells into cells that look very much like human eggs and sperm, many have predicted that infertility will be treated with stem cell treatments (see Volarevic V, et al., Biomed Res Int. 2014;2014:507234). However, new work from the University of Gothenburg and Karolinska Institute has cast doubt on this hope.

At about 24 days of life, large, spherical sex cells are recognizable among the endodermal cells of the umbilical vesicle close to the allantois. These cells are the primordial germ cells and they are the progenitor cells of the sperm in men and eggs in women. As the embryo folds during about the late 4th week, the dorsal portion of the umbilical vesicle is incorporated into the embryo. This incorporation of the umbilical vesicle occur concurrently with the migration of these primordial germ cells along the dorsal mesentery of the hindgut to the gonadal ridges. During the 6th week of life, the primordial germ cells enter the underlying mesenchyme and are incorporated into the gonadal cords. Primordial germ cell migration is mainly regulated by three genes: Stella, Fragilis, and BMP-4.

PGC migration

These primordial germ cells divide as they migrate, and by five months of gestation, embryonic ovaries contain about six to seven million oogonia. Most of these oogonia experience cell death before birth, but the remaining oogonia begin meiosis toward the end of gestation. At this time, the oogonia are called primary oocytes. Meiosis is arrested in prophase of the first meiotic division, and this is the same stage at which spermatogenesis in the male is blocked. Primary oocytes decrease in number throughout a woman’s life. The ovaries of a newborn girl contain about two million primary oocytes and these are all the gametes she will ever have. Each primary oocyte is contained within a hollow ball of cells called the ovarian follicle. By the time a woman reaches puberty, that number of primary oocytes has been reduced to 400,000. Only about 400 of these cells will ovulate during a woman’s reproductive years. The rest will die by means of programmed cell death. Once all the primary oocytes are gone, ovulation stops and the woman undergoes menopause.

Kui Liu from the University of Gothenburg said: “Ever since 2004, the studies on stem cell research and infertility have been surrounded by hype. There has been a great amount of media interest in this, and the message has been that the treatment of infertility with stem cells is about to happen. However, many researchers, including my research group, have tried to replicate these studies and not succeeded. This creates uncertainty about whether it is all possible to create new eggs with the help of stem cells.”

In collaboration with Outi-Hovatta’s laboratory at the Karolinska Institute and Jan-Åke Gustafsson’ research team at the university of Houston in the US, Lui’s research team carried out experiments on mice that failed to demonstrate that functional gametes could be formed from pluripotent stem cells. Essentially, the only gametes that could that the female mice had were the ones they were born with.

In Liu’s opinion, fertility clinics should place their attention on using the eggs that women still have in order to treat infertility.

Mesenchymal Stem Cells Derived from Induced Pluripotent Stem Cells are Epigenetically Rejuvenated


Earlier this year, Miltalipov and his research group published a paper in Nature that compared the genetic integrity of embryonic stem cells made from embryos, to induced pluripotent stem cells and embryonic stem cells made from cloned embryos.  All three sets of stem cells seemed to have comparable numbers of mutations, but the induced pluripotent stem cells had “epigenetic changes” that were not found in either stem cell line from cloned or non-cloned embryos.

Genetic characteristics have to do with the sequence of the DNA molecules that make up the genome of an organism.  Epigenetic characteristics have nothing to do with the sequence of DNA, but instead are the result of small chemicals that are attached to the DNA molecule.  These small chemical tags affect gene expression patterns.  Every cell has a specific epigenetic signature.

During development, the cells that will form our eggs and sperm in our bodies, the “primordial germ cells,” begin their lives in the outer layer of the embryo.  During the third week of life, these primordial germ cells or PGCs move like amoebas and wander into the yolk sac wall and collect near the exit of a sac called the “allantois.”  The PGCs are outside the embryo at this time or extraembryonal.  Incidentallyyolk sac is a terrible name for this structure, since it does not produce yolk proteins.  Therefore other textbooks have renamed it the “primary umbilical vesicle,” which is a bit of a mouthful, but it probably better than “yolk sac.”

 

1 - Primordial germ cells 2 - Allantois 3 - Rectum 4 - Ectoderm 5 - Foregut 6 - Primordial Heart 7 - Secondary yolk sac 8 - Endoderm 9 - Mesoderm 10 - Amniotic cavity
1 – Primordial germ cells
2 – Allantois
3 – Rectum
4 – Ectoderm
5 – Foregut
6 – Primordial Heart
7 – Secondary yolk sac
8 – Endoderm
9 – Mesoderm
10 – Amniotic cavity

The embryo around this time undergoes a bending process as a result of its growth and the head bends toward the tail (known as the cranio-caudal curvature) and then the sides of the embryo fold downwards and eventually fuse (lateral folding).  This bending of the embryo allows the PGCs to wander back into the embryo again between the fourth and sixth week.  The PGCs move along the yolk sac wall to the vitelline and into the wall of the rectum.  After crossing the dorsal mesentery (which holds the developing intestines in place) they colonize the gonadal or genital ridge (which is the developing gonad). During their journey, and while in the gonadal ridge, the PGCs divide many times.

1 - Rectum 2 - Vitelline 3 - Allantois 4 - Nephrogenic cord (pink) 5 - Gonadal ridge (green) 6 - Primordial germ cells (red dots) 7 - Heart prominence
1 – Rectum
2 – Vitelline
3 – Allantois
4 – Nephrogenic cord (pink)
5 – Gonadal ridge (green)
6 – Primordial germ cells (red dots)
7 – Heart prominence

When the PGCs move into the developing gonad, the chemical tags on their DNA are completely removed (rather famous paper – Lee, et al., Development 129, 1807–1817 (2002).  This epigenetic erasure proceeds in order for the PGCs to develop into gametes and then received a gamete-specific set of epigenetic modifications.  These epigenetic modifications also extend to the proteins that package the DNA into chromosomes – proteins called histones.  Specific modifications of histone proteins and DNA lead to gamete-specific expression of genes.  Once fertilization occurs, and the embryological program is initiated, tissue-specific epigenetic modifications are conveyed onto the DNA and histones of particular cell populations.

This is a long-winded explanation, but because many cancer cells have abnormal epigenetic modifications, these epigenetic abnormalities in induced pluripotent stem cells (iPSCs) have been taken with some degree of seriousness.  Although, there is little evidence to date that links the cancer-causing capabilities of iPSCs with specific epigenetic modifications, although it certainly affects the ability of these cells to differentiate into various cell types.

A paper has just come from the laboratory of Wolfgang Wagner from the Aachen University Medical School, in Aachen, Germany that derived iPSCs from mesenchymal stem cells from human bone marrow, and then in a cool one-step procedure, differentiated these cells into mesenchymal stem cells (MSCs).  These  iPS-MSCs looked the same, and acted the same in cell culture as the parent MSCs, and had the same gene expression profiles as primary MSCs.  However, all age-related and tissue-specific epigenetic patterns had been erased by the reprogramming process.  This means that all the tissue-specific, senescence-associated, and age-related epigenetic patterns were erased during reprogramming.  Another feature of these iPS-MSCs is that they lacked but the ability to down-regulate the immune response, which is a major feature of MSCs.

Thus, this paper by the Wagner lab shows that MSCs derived from iPSCs are rejuvenated by the reprogramming process.  Also, the donor-specific epigenetic features are maintained, which was also discovered by Shao and others last year.  This suggests that epigenetic abnormalities are not an inherent property of the derivation of iPSCs, and that this feature is not an intractable characteristic of iPSCs derivation and may not prevent these cells from being successfully and safely used in the clinic.  However, this might be a cell type-specific phenomenon.  Also, the loss of the immune system regulatory capabilities of these iPS-MSCs is troubling and this requires further work.

iPS-MSCs