A doctor in Colorado found a surprise when removing what he thought was a benign growth from a newborn's brain. Instead of a microscopic tumor, out popped a tiny foot, partially formed hand, a thigh and another partially formed foot.
"It would be a shock to even the most experienced pathologist cutting into a tumor to see this," Dr. Paul Grabb told the ABC affiliate KMGH.
Grabb said he could not tell whether the miniature limbs were from a benign stem cell tumor called a teratoma or the remnants of an identical twin that did not split off and survive, a condition called fetus in fetu.
"It looked like the breach delivery of a baby, coming out of the brain," Grabb told The Associated Press. "To find a perfectly formed structure is extremely unique, unusual, borderline unheard of."
The baby, Sam Esquibel, was 3 days old when doctors at Memorial Hospital for Children in Colorado Springs discovered a microscopic tumor in his brain. A hospital representative said Sam is alive and recovering well after his operation, but his family declined to discuss his case further until its interview with ABC's "20/20" in January.
Whether the growth was the remains of a twin sibling or the work of renegade stem cells in Sam's body is hard to tell.
But stem cell experts who study teratoma believe the body parts are likely the result of fetus in fetu.
"This is pretty bizarre. ... It's certainly possible [it's a teratoma]," said Ed Morrisey, the scientific director of the University of Pennsylvania Institute for Regenerative Medicine in Philadelphia.
"My guess is, it's probably not a teratoma, because the tissues themselves in a teratoma are usually disorganized," said Morrisey.
Morrisey and other stem cell researchers create teratoma in mice to study how one day doctors may coach stem cells into growing into specific types of tissues that doctors need.
By altering embryonic stem cells gene by gene and then watching to see what type of tissue -- nerve, muscle, hair, etc. -- they grow into, researchers can advance toward learning what directs stem cells to form full organs.
But teratoma are known to form recognizable organs or limbs.
"If you take stem cells and inject them into mice they will spontaneously form teratoma," said Morrisey. He said the teratoma that form, and the teratoma that occur naturally, usually grow into disorganized clumps of tissue instead of a fully formed organ.
"You'd form muscle, but it would just be a bunch of muscle cells together," he said. "Not like a foot or a limb."