Can Stem Cells Become Sperm Cells?

Stem Cells and Development, July 7, 2009

Scientists at the Institute of Human Genetics in the Northeast England Stem Cell Institute at Newcastle University in England have announced the creation of human spermatozoa from embryonic stem cells. Their study, published in the medical journal Stem Cell and Development, addresses an issue that influences the lives of approximately 7.3 million Americans: infertility.

In an article entitled, "In Vitro Derivation of Human Sperm from Embryonic Stem Cells", Dr. Karim Nayernia and his colleagues describe a procedure by which human male gametogenesis is modeled in vitro. From human embryonic stem cells (hESCs), the scientists established male germline stem cells (GSCs) which were then stimulated to enter meiosis, thereby generating haploid motile "sperm-like" cells in vitro. Most strikingly, these "sperm-like" cells were able to mimic many of the properties of functional human sperm. While the current study used embryonic stem cells, which thus would not be viable for practical use in the treatment of infertility, the scientists are now trying to generate sperm from iPS (induced pluripotent stem) cells, a type of stem cell derived from somatic (non-stem cell) cells which mimic embryonic stem cells in its pluripotency. (More information about iPS cells may be seen on the video available at this link: iPS cells video). However, a full elucidation has not yet been provided for the precise ways in which these "sperm-like" cells - which the researchers have named in vitro derived (IVD) sperm - may or may not be identical to naturally occurring sperm.

As Dr. Nayernia explains, "We have a system which enables us for the first time to produce human sperm from stem cells. Studying sperm maturation is not accessible in vivo. You cannot follow the system. Now we have a system to monitor the stages of male infertility. This is very amazing and very exciting. They have heads, they have tails and they move. The shape is not quite normal nor the movement, but they contain the proteins for egg activation."

The abnormal motility and morphology of the IVD sperm are of serious concern to a number of other scientists, however, for whom such a procedure raises a number of questions. According to Dr. Edmund Sabanegh Jr., director of the Center for Male Infertility at the Cleveland Clinic, "Some groups have raised questions about the research. There are huge ethical and safety implications of this. In theory, it's exciting for couples that are struggling with this problem." As Dr. Byron Petersen, associate professor in the Department of Pathology at the University of Florida, adds, "I would be very skeptical at this point and really look at what they define as sperm. An actual moving sperm cell or just a haploid cell that can be used to implant into an egg cell? The devil is in the details and it will be how they define their cell phenotype and what-not."

This recent publication is an extension of previous studies that Dr. Nayernia and his colleagues conducted three years ago, at which time they developed a similar in vitro process for mouse sperm. When the IVD mouse sperm were used to inseminate mice in order to produce offspring, however, the newborn mice that were created from the artificial sperm died shortly after birth. A number of scientists therefore question the genetic viability of the IVD sperm as well as the viability of the technique by which each spermatozoon is artificially derived.

Although it is currently illegal in the U.K. as well as in the U.S., among other countries, to try to use the artificially derived human IVD sperm to fertilize human ova in order to create human offspring for reproductive purposes, it is still legal under U.K. law to use the IVD sperm to fertilize human ova in order to create human offspring for research purposes, as long as any embryos created by this method are destroyed by 14 days of age. Even if it were legal to allow such embryos to continue developing for reproductive purposes, the consequences could be disastrous if the IVD sperm were genetically unstable or flawed.

Although embryonic stem cells (ESCs) are so highly coveted because of their alleged pluripotency, in actuality the single celled human zygote - from which all ESCs develop - is totipotent. While totipotency (from the Latin "totus", meaning "total" or "complete") describes cells that are capable of differentiating into embryonic as well as extra-embryonic cell types, by contrast, pluripotency describes cells that are only capable of differentiating into embryonic cell types - namely, all cell and tissue types of the body, from all 3 germ layers - but not extra-embryonic cells and tissue, such as the placenta. Both male and female haploid gametes - spermatozoa and ova - are derivable from a population of "primordial germ cells" (PGCs) and "germline stem cells" (GSCs) which arise in early embryogenesis and later develop into the gonocytes that are responsible for gametogenesis, namely, spermatogenesis in males and oogenesis in females. In males it is not until long after birth that the gonocytes begin differentiating into adult male germline stem cells, known as spermatogonial stem cells (SSCs), which are both self-renewing and capable of producing spermatozoa. As the authors describe, SSCs "are unique stem cells in that they are solely dedicated to transmit genetic information from generation to generation." While somatic (mature, non-stem cell) cells throughout the body are diploid (containing 46 chromosomes), gametes (ova and spermatozoa) are haploid cells (containing 23 chromosomes). The ability to generate such a highly specialized cell as a haploid spermatozoon from a diploid ESC is therefore significant not only because of the possible therapeutic medical applications but also because of the particular pathway by which primordial and germline cells are used for the in vitro acceleration of gametogenesis. As Dr. Nayernia further points out, "Other cell types don't generate the next generation. This makes a very big difference between our study and the study of other cell types from embryonic stem cells."

While the particular focus of this study was male gametogenesis, some scientists have begun to extrapolate similar procedures that may be applicable to female gametogenesis. Although Dr. Nayernia and his team were unsuccessful in trying to differentiate IVD sperm from female embryonic stem cells - as one would logically expect, since the Y chromosome must be present for sperm maturation - nevertheless other researchers are hypothesizing ways in which it might be possible to differentiate IVD ova from female embryonic stem cells, which contain only the X chromosomes.

As more and more people are waiting until later in life to have children, infertility is a growing problem around the world, especially in developed nations. According to a 2002 National Survey of Family Growth conducted by the Centers for Disease Control and Prevention in Atlanta, Georgia, it has been estimated that approximately 7.3 million people in the U.S. alone suffer from infertility, more than one third of whom are male.

If it could be possible to create safe and viable human spermatozoa from an alternate source, such as from embryonic stem cells, the procedure could have widespread therapeutic applications in reproductive medicine. In the wrong hands, however, there is also the risk of commercial exploitation.

Dr. Robert Lanza is adjunct professor at the Institute of Regenerative Medicine at Wake Forest University School of Medicine and chief scientific officer of Advanced Cell Technology Corporation (ACTC) - the company previously directed by the pioneering molecular gerontologist Michael D. West, Ph.D., who served as chairman of the board, chief scientific officer, CEO and President of ACTC from 1998 until 2007. ACTC specializes in the cloning of animals, including transgenic animals (hybrid species containing the genes of different species, usually human genes in combination with nonhuman genes), as well as in the controversial technique known as "therapeutic cloning". Nevertheless, according to Dr. Lanza, and in specific reference to the possibility of creating artificial sperm, "What's most concerning about this potential technology is that anyone, young or old, fertile or infertile, straight or gay, could potentially pass on their genes to a child from just a few cells. For instance, if you had a few skin cells from Albert Einstein - or perhaps even a hair follicle from the Pope or Queen Elizabeth - you could generate pluripotent stem cells. Any couple could go to an IVF clinic and have a child that is half, say, Albert Einstein, or perhaps Brad Pitt or Elizabeth Taylor." It seems as though a number of technical hurdles remain to be overcome, however, before such concerns could materialize. Nevertheless, no doubt it is just a matter of time before such technological glitches are resolved, and many people wonder how the national laws and scientific guidelines of regulatory and oversight agencies will be able to address such changes. As CBS reporter Peter Allen cautions, "What these and other researchers are doing is ripping up the codes of law and morality by which we conduct our lives. The pace of discovery has left our legislators floundering."

Indeed, while Dr. Lanza's concerns may sound like science fiction, the generation of iPS cells has previously been performed from many types of tissues that can easily be stored across decades if not also centuries. Although it is somewhat of a barrier that iPS cells have not yet been made into sperm cells, thus far every cell type that has been generated from embryonic stem cells has also been generated from iPS cells. Accordingly, the ethical questions of using such technology should be addressed as soon as possible, since scholars believe it will be within the next decade that frozen somatic cells such as skin cells will be successfully used for the generation of new sperm.

Dr. Nayernia and his colleagues used a special "cocktail" of growth factors, nutrients and retinoic acid (a derivative of vitamin A), among other chemicals, to create the IVD sperm from embryos that were discarded from IVF clinics. As previously mentioned, Dr. Nayernia and his colleagues are now working on developing a similar method for the creation of IVD sperm from mature somatic skin cells derived from infertile men.

As the authors conclude in their publication, "Understanding the mechanisms of germ cell specification, development and its differentiation to sperm is important for elucidating the causes of male infertility. ... While the full potential of the human ES-derived germ cells and sperm remains to be demonstrated, this in vitro modeling of human gametogenesis provides a new approach for studying the biology of human germ cells and for the establishment of therapeutic approaches in reproductive medicine."

On the other hand, the findings of Dr. Nayernia and his colleagues, although exciting, still have several drawbacks. For example, these cells have not been able to successfully fertilize a human egg and generate offspring. This is actually the vital test, although it has not yet been successfully performed - and given the problems that were encountered when Dr. Nayernia and his colleagues attempted the procedure three years ago in mice, it might be quite awhile before such a procedure is successfully accomplished in humans - with or without the laws that forbid such experimentation.

Perhaps an even more significant and relevant study for male infertility, which did not receive as much media exposure as Dr. Nayernia's, was that reported by Swerdloff's group from Los Angeles. (Lue et al., 'Fate of bone marrow stem cells transplanted into the testis: potential implication for men with testicular failure', Am J Pathol 2007 Mar;170(3):899-908). In this study the scientists found that the administration of bone-marrow-derived adult stem cells into the testicles of mice could generate new cells that look and act like sperm. Implications of such a study might include new procedures for the treatment of infertile males that involve the therapeutic use of the patient's own bone marrow. Additionally, an independent German group (Dursenheirmer et al., 'Putative human male germ cells from bone marrow stem cells', Soc Reprod Fertil Suppl. 2007;63:69-76) has also reported that bone-marrow-derived adult stem cells can be induced to become the cells that give rise to sperm cells based on the expression of specific proteins on the surface of the cells. From studies such as these, one can only conclude that the prospects for developing infertility treatments with stem cells are encouraging, with advances being made both from bone-marrow-derived adult stem cells and from embryonic stem cells, despite the many technological hurdles that still remain.

Certainly no one is more aware of such technological hurdles than Dr. Nayernia, who adds, "We think, for normal structural development, sperm needs the testes environment. In the human, sperm development is a very long process. It takes more than 15 years and is not an accessible system. With this system, we can now watch that development in three months."

But in regard to the ethical and legal dilemmas that are engendered by such procedures, we are only just beginning to see the tip of the iceberg. As Dr. Insoo Hyun, a bioethicist at Case Western Reserve University, explains, "We have the potential therapeutic use of a technology that pushes the boundaries of what people feel comfortable with ethically. This area has potential powerful clinical applications mixed with people's concerns over embryo research. All the ingredients are there for a really, really lively ethical debate."