Skip to content

When you choose to publish with PLOS, your research makes an impact. Make your work accessible to all, without restrictions, and accelerate scientific discovery with options like preprints and published peer review that make your work more Open.


Leroy Stevens: Fairwell to The Unsung Hero of Stem Cell Research

Yesterday a Google Alert popped up with a blast from my past, an obituary for Jackson Lab researcher Leroy C. Stevens. It quoted me calling him “The unsung hero of stem cell research” in an article I wrote 15 years ago for The Scientist. Dr. Stevens passed away on March 28.

Leroy Stevens was born in 1920 in Kenmore, New York, earned a B.S. from Cornell University in 1942 and a doctorate in embryology from the University of Rochester in 1952. I’d discovered his contribution while writing an article for The Scientist published on September 27, 1997, “Embryonic Stem Cells Debut to Little Media Attention.” Unfortunately the headline left off the word “human,” so hardly anyone noticed the article; hES cells blasted into public consciousness some 14 months later.

Back in those pre-electronic days, I trudged to the upper floors of the library at Albany Medical College, where I ironically teach online today, and found a dusty volume that held the treasures of Dr. Stevens’ serial images of developing mouse embryos.

IMG_1086The Scientist has graciously agreed to let me post my tribute from 2000. A more detailed version is in my essay book, “Discovery: Windows on the Life Sciences” (Blackwell Science, 2001). Many thanks to Anne Wheeler for permission to use the photos of her father. He spent his later research years developing mouse models to test chemotherapeutic agents. An update from her follows the post.

A Stem Cell Legacy: Leroy Stevens
The Scientist, March 6, 2000

When Science voted stem cell research its 1999 Breakthrough of the Year, the congratulatory article traced the field’s origin to the 1981 successful culture of mouse embryonic stem (ES) cells. But the roots of exploring these multipotential cells go back considerably farther, to a little-mentioned researcher who worked with mice at the Jackson Laboratory in Bar Harbor, Maine.

Leroy Stevens arrived at the lab in 1953, a newly-minted developmental biologist in a premolecular era when the tools of the trade were mostly one’s eyes. The young scientist found himself with an initial assignment that he calls “crazy.”

“The founder of the lab had gotten a grant from a tobacco company, and they wanted him to show that it wasn’t tobacco that was the problem–it was the paper in cigarettes!” recalls Don Varnum, a long-time technician in the lab. So Stevens dutifully dissected cigarettes and exposed mice to the components. One day, he noticed a mouse with a huge scrotum. “We killed it, and looked at the testes, and they had strange things inside.”

This teratoma from the chest has fat blobs, a tooth, a hair, and bone bits.
This teratoma from the chest has fat blobs, teeth, a hair, and bone bits.

The growth included a mishmash of tissue, including hair and teeth. It was a teratoma, and other mice of strain 129 had them too. Intrigued, Stevens pursued federal funding to explore teratomas, and the support lasted a professional lifetime. “This stuff was extremely interesting, and it sure beat studying cigarette papers!” he recalls from his home in rural Vermont, where he has lived near family since suffering a stroke 10 years ago.

Stevens bred strain 129 to select for the teratoma tendency. Then he developed a serial transfer technique to be able to continuously study the rare cells in some of the growths that rendered them cancerous. When and if a teratoma ran out of these “embryonal” cells, it would just sit there, sporting its peculiar mix of hair and teeth, cartilage, and tiny tubules. “We just wanted to keep the tumor alive longer so we could study it. But it took a long time to succeed,” says Stevens.

By passaging teratomas through many hosts and removing them from time to time and cataloging the tissues, the researchers witnessed all that these cells could become. And the odd tissue struggling for identity wasn’t as haphazard as it seemed. When Stevens and Varnum transplanted teratoma bits into the peritoneal cavities of mice, curious growths resembling inside-out embryos formed, called embryoid bodies. The tissues from the teratomas, given signals from the ascites fluid of the peritoneal cavity, seemed to be trying to organize.

The researchers continued to describe the tissues of teratomas and embryoid bodies, supplementing their publications with spectacular atlases of photographs that captured the temporal unfolding of this deranged development. Although the anatomy was clearly off, both out of place and out of time, Stevens and Varnum noted that the events followed a sequence of sorts. “Roy looked at thousands and thousands of mice. He noticed that for tissues to develop, several tissues must be in contact, so that the cells know whether to become liver or kidney,” Varnum recalls.

256px-JacksonLabsSignAlthough Stevens’ and Varnum’s work was purely basic research, the events that they so carefully chronicled, and the biochemicals from without and within that fueled the choreography of early development, are what stem cell researchers and tissue engineers are deciphering today.


With a steady supply of teratomas and their tissues identified, Stevens moved on to other questions–where, and when, did development take a wrong turn? And so he began looking backward, seeking the earliest stage when cells in the testes looked different, and then going back a few more days to account for changes not obvious to a human observer. This approach took him to the genital ridge in a 12-day prenatal mouse, which houses primordial germ cells, sperm precursors.

800px-Lab_mouse_mg_3140Then in 1970, Stevens made a leap that would profoundly affect stem cell technology a decade later–he noticed that the primordial germ cells that gave rise to teratomas looked a lot like the cells of considerably earlier embryos. So he transplanted cells from various stages of early strain 129 embryos, including inner cell mass cells (a very early embryo, minus the cells that become extra-embryonic membranes), into testes of adult mice. Some of the early embryo cells gave rise to teratomas! These induced growths looked and acted like spontaneous teratomas, yielding embryoid bodies when transplanted into mouse bellies and displaying an impressive range of tissue types.

Stevens called these cells from early strain 129 embryos that could support differentiation “pluripotent embryonic stem cells”–the origin of the term “ES cell.” But because these cells could give rise to cancerous as well as normal cells, they became known as embryonal carcinoma, or EC cells.

The rest, as they say, was history. But quite a lot transpired between Stevens’ identification of the developmental potentials of primordial germ cells and inner cell mass cells, and the unveiling of the role of the human versions of these cells as ES cells.

Human ES cells (NHGRI)
Human ES cells (NHGRI)

First, Beatrice Mintz and Karl Illmensee, from the Institute for Cancer Research in Philadelphia, visited Stevens to learn his techniques and borrow mice; then they demonstrated that ES cells could give rise to organisms, not just teratomas. (Their surprise announcement of this feat at a meeting floored Stevens, a story unto itself.) Then Martin Evans at Cambridge University and Gail Martin at the University of California, San Francisco, and their coworkers showed that inner cell masses from normal mice could support development too.

The field detoured into genetically altering mouse ES cells, which evolved into knockout technology with the harnessing of homologous recombination to target the genetic changes. Then in the late 1980s, Brigid Hogan, a professor of cell biology at the Vanderbilt University School of Medicine in Nashville, with Peter Donovan’s group at the National Cancer Institute, reignited interest in primordial germ cells by devising culture methods. These cells became known as embryonic germ (EG) or  embryonic stem-like cells.

It wasn’t until November 1998, with publication of the two human ES cell papers, that the media and public were finally diverted from Dolly the cloned sheep sufficiently to notice this much more powerful technology. Attention centered on the two cell sources, and still does.

James Thomson’s group at the Wisconsin Regional Primate Research Center used inner cell masses from in vitro fertilization clinic “leftovers,” while John Gearhart’s group at the Johns Hopkins University School of Medicine used primordial germ cells from aborted fetuses. And the work that led to their success began in a mouse lab nearly half a century ago, with a man with gifted hands and alert eyes who personifies Louis Pasteur’s oft-quoted observation that “chance favors only the prepared mind.”

P.S. A few words from Dr. Stevens’ daughter, Anne Wheeler:

Leroy old“My dad had a major stroke a few days before he retired. It took years for him to ‘come back’, but he did…much to everyone’s surprise. In the past 25 years, he must have been ‘near death’ at least 50 times…strokes, heart attack, bleeding ulcers, etc. He was tenacious, strong, had good genes (apparently!!), and I think his joie de vie, positive attitude and sense of humor brought him back each time. He finally died of congestive respiratory failure. He died peacefully in his sleep, luckily.

He lived on his own, cut his own firewood, had a garden until about 15 years ago. Then, he moved to a ‘retirement’ place where he lived on his own in his own cottage, until just 9 months ago, when he moved to a memory loss center. Every winter, we traveled to Florida (“not that interesting”- LCS), then Costa Rica and Belize. Watching monkeys, sloths, and the ocean waves were some of his favorite things in life.”

He loved being with his 7 grandchildren…lots of birthday parties, weddings, celebrations.”

I’m honored to have had the chance to interview and learn from this great man, who founded a field.

  1. It is always nice celebrating work of a very amazing person, especially if the person contributed to making change in the world we live in. Now, stem cells are used for so many things, but most importantly they are used to treat cancer. So this man is truly a hero. Thank you


Leave a Reply

Your email address will not be published. Required fields are marked *

Add your ORCID here. (e.g. 0000-0002-7299-680X)

Back to top