A part of the placenta called the amnion, or the outer membrane of the amniotic sac, is comprised of cells that have strikingly similar characteristics to embryonic stem cells, including the ability to express two key genes that give embryonic stem cells their unique capability for developing into any kind of specialized cell, the researchers report. And according to the results of their studies, these so-called amniotic epithelial cells could in fact be directed to form liver, pancreas, heart and nerve cells under the right laboratory conditions.
"If we could develop efficient methods that would allow amnion-derived cells to differentiate into specific cell types, then placentas would no longer be relegated to the trashcan. Instead, we'd have a useful source of cells for transplantation and regenerative medicine," said senior author Stephen C. Strom, Ph.D., associate professor of pathology at the University of Pittsburgh School of Medicine and a researcher at the university's McGowan Institute for Regenerative Medicine.
According to U.S. census figures, there are more than 4 million live births each year. For each discarded placenta, the researchers calculate there are about 300 million amniotic epithelial cells that potentially could be expanded to between 10 and 60 billion cells relatively easily.
"Provided that research advances to the point that we can demonstrate these cells' true therapeutic benefit, parents could conceivably choose to bank their child's amniotic epithelial cells in the event they may someday be needed, as is sometimes done now with umbilical cord blood," commented Dr. Strom.
The amnion is derived from the embryo and forms as early as eight days after fertilization, when the fate of cells has yet to be determined, and serves to protect the developing fetus. According to the researchers' studies using placentas from full-term pregnancies, amniotic epithelial cells have many of the telltale surface markers that define embryonic stem cells, and also express the Oct-4 and nanog genes that are known to be required for self-renewal and pluripotency - the ability to develop into any type of cell.
Yet the authors are careful to point out that despite their remarkable similarities to embryonic stem cells, amniotic epithelial cells are not stem cells per se, because they can't grow indefinitely. This may be due to the fact that these amnion-derived cells do not express a certain enzyme, called telomerase, that is important for normal DNA and chromosome replication, and by extension, ultimately, cell division.
"Perhaps it's to their advantage that the amnion epithelial cells lack telomerase expression, because telomerase is associated with many cancers and one of the main concerns about stem cell therapies is that transplanted stem cells would replicate in the recipient to form tumors," noted Toshio Miki, M.D., Ph.D., first author of the paper and an instructor in the department of pathology at the School of Medicine.
To help determine if amnion-derived cells that are delivered directly to tissues would cause tumors, the researchers conducted studies in immune system-deficient mice and found no evidence that tumors had developed seven months after the cells were injected into multiple sites.
While amniotic epithelial cells do not share the same capacity for unlimited replication as do embryonic stem cells, they still can double in population size about 20 times over without needing another cell type serving as a feeder cell layer. This is significant, because to replicate, the currently available embryonic stem cell lines require a bed of mouse cells, traces of which can end up in each new generation of stem cells. Amniotic epithelial cells, on the other hand, create their own feeder layer, with some cells choosing to spread out at the bottom of the culture dish thereby giving those cells just above them the best environment for replicating and for retaining their stem cell characteristics.
With the addition of various growth factors, the authors report the amnion-derived cells could differentiate to become liver cells, heart cells, the glial and neuronal cells that make up the nervous system, and pancreatic cells with genetic markers for insulin and glycogen production.
"In this first paper we sought to determine if amniotic epithelial cells have the potential to differentiate into many different cell types rather than focusing on ways for optimizing this potential for a specific cell type. Further studies will be required to better understand if and how they may be useful in a clinical setting," Dr. Strom added.
The researchers say their original motivation was, and still is, to identify cells with the same therapeutic promise as embryonic stem cells. To this end, they began looking at the viability of amnion as a cell source in late 2001, obtaining discarded placentas from full-term births under an Institutional Review Board-approved protocol. In 2002, the University of Pittsburgh licensed the technology to a company now called Stemnion, LLC, and as part of the agreement, and in keeping with university patent policy, Drs. Strom and Miki will receive license proceeds. Both have served as paid consultants and hold equity in Stemnion.
The research was supported by the Alpha-1 Foundation and the National Institute of Diabetes and Digestive and Kidney Diseases, a part of the National Institutes of Health. In addition to Drs. Miki and Strom, other authors are Thomas Lehmann, Ph.D., and Hongbo Cai, M.D., Ph.D., both from the department of pathology, and Donna Stolz, Ph.D., of the department of cell biology and physiology.