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 si
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5-Aug-2005