Understanding Teratomas and Their Historical Context

Teratomas are tumors that consist of a chaotic mix of tissues normally found in mature organisms. These tumors can sometimes contain organ-like structures such as teeth, skin with hair and sebaceous glands, and even limbs. The term "teratoma" was coined by Rudolf Virchow, deriving from the Greek word τέρατος (monster) combined with a suffix indicating a tumor. The earliest documented reference to such tumors in human pathology can be traced back to a clay tablet from the Chaldean Royal Library of Nineveh, dating between 600 to 900 BC.

In 2006, Kazutoshi Takahashi and Shinya Yamanaka made headlines with their groundbreaking research titled "Induction of Pluripotent Stem Cells from Mouse Embryonic and Adult Fibroblast Cultures by Defined Factors." This pivotal study, which has since garnered over 6,000 citations, revealed that adding just four specific molecular factors to differentiated fibroblast cells could revert them to a pluripotent state, capable of transforming into any cell type. The four transcription factors—Oct3/4, Sox2, c-Myc, and Klf4—were all that was necessary to reprogram these previously specialized cells.

The significance of this discovery lies in its implications for understanding cellular identity. Induced pluripotent stem cells (iPSCs) derived from patients offer valuable models for investigating human diseases, and they hold promise for personalized cell therapies aimed at treating conditions ranging from macular degeneration to heart disease.

A particularly striking experiment detailed in their paper involved injecting cells from their pluripotent stem cell populations into immunocompromised mice. After four weeks, these injected cells had formed discernible tumors, as anticipated. The tumors were then carefully removed, preserved, sliced, and stained with hematoxylin and eosin, allowing for a detailed examination of the cellular components. Under microscopic observation, the researchers identified a chaotic amalgamation of various tissue types, characteristic of teratomas. The stem cells had proliferated rapidly and differentiated into recognizable tissue structures, including muscle, epithelium, cartilage, and neural tissues, thus demonstrating their pluripotency by generating tissues from all three germ layers.

The teratoma assay, as this transplantation experiment is known, was not a novel approach; it was originally explored by Leroy Stevens and CC Little, who studied naturally occurring testicular teratomas in mice. They discovered a tumor that continued to grow and differentiate for 16 generations of serial transplantation, attributing this capability to embryonic-type cells that could both differentiate and self-renew.

Since the publication of Takahashi and Yamanaka's 2006 study, there has been a surge in interest regarding induced pluripotent stem cells. Various research groups have sought to create human stem cells from various starting cell types, each needing to validate that their generated cells were indeed pluripotent, typically via the teratoma assay.

However, as time has passed, some scientists have begun to question the teratoma assay's status as the definitive test for pluripotency. Critics argue that the assay is not only time-consuming but also ethically questionable, given the number of animals involved. Owen Witte, director of the Broad Stem Cell Research Center at UCLA, described the teratoma assay as "the most ridiculous assay on the planet."

Currently, the Human Pluripotent Stem Cell Registry lists 1,880 cell lines, with each line requiring at least three mice for the teratoma assay to confirm pluripotency. This results in the use of 5,640 mice, all of which endure a significant burden before being euthanized for tissue analysis. The actual number of mice used is likely much higher, as only the most thoroughly vetted and successful cell lines are included in the registry.

For patient-specific cell therapies, generating pluripotent cells from the patients' own cells is ideal. However, if a teratoma assay were required for every patient-specific induced pluripotent stem cell line, the implications would be staggering. While the teratoma assay might be justified if it were the sole method available for confirming pluripotency, it is not the only option and does not perform flawlessly.

At the 2010 International Stem Cell Initiative workshop, scientists voiced their concerns regarding the teratoma assay but did not reach a consensus on its efficacy. Instead of discarding the assay, the prevailing thought was to enhance it while simultaneously exploring alternative methods. This led to the publication of "A call to standardize teratoma assays used to define human pluripotent stem cell lines" by Müller and Loring et al. in Cell.

In 2013, Buta et al. questioned the necessity of the teratoma assays, publishing "Reconsidering pluripotency tests: do we still need teratoma assays?" in Stem Cell Research. By 2018, during the International Stem Cell Banking Initiative meeting, the consensus shifted further, with many agreeing that the teratoma assay is no longer the most effective method for assessing pluripotency.

Despite these discussions, a search on PubMed indicates that the teratoma assay remains in use, lacking standardization. This raises questions: Are researchers ignoring the call for change, concerned that their findings will not be published without the teratoma assay, or do they still find value in it that isn’t reflected in the broader consensus?

The situation evokes a familiar challenge: we convene leaders in various fields, conduct extensive research, and issue recommendations, only to have these suggestions largely disregarded.

Reflecting on broader issues, such as climate change or health guidelines regarding antibiotic use in agriculture, we see similar patterns. Despite having the necessary information and expert recommendations, meaningful change occurs at a glacial pace, with many individuals failing to heed the evidence presented.

A lingering question remains: Why does this happen? I am eager to hear your thoughts.