In grade school you may remember your science teacher explaining what chromosomes were. Human cells generally have two sets of 23 chromosomes, one from the mother and the other from the father. (One exception is haploid cells, also known as reproductive cells, which only have a single set.) Until now, haploid cells weren't able to divide or be divided.

"These stem cells have the ability, like diploid cells, to transform into other cell types."

In an extraordinary first-time study, scientists, from the Hebrew University of Jerusalem and the Colombia University Medical Center, were able to successfully develop embryonic stem cells that only carry one set of chromosomes. We should point out that while the haploid cells do share analogous characteristics to diploid cells, they have different sizes and properties. However, they do have the distinct ability, like diploid cells, to transform into other cell types such as pancreatic, nerve and heart cells.

To conduct the study, scientists split an unfertilized human egg cell. As they expected, when the cell divided, it formed two carbon copies of itself, each with 46 chromosomes. However, they also believed haploid cells could be present. They screened the cells with a fluorescent marker and found that two out of the 2,000 cells were haploid. From there, the scientists sought to figure out if these haploid cells could then divide further, which they did.

In one regard, the study is significant because it could allow scientists to more easily analyze diseases.

"We have two genes of everything and if one is mutated the effect is not so obvious," said Egli in a telephone interview. "Because these cells reduce the number of possible combinations and reduce the number of variance, it should be easier to get the answers."

Dr. Benvenisty also noted the study will further help in better understanding how humans develop.

"This study has given us a new type of human stem cell that will have an important impact on human genetic and medical research," said Dr. Benvenisty. "These cells will provide researchers with a novel tool for improving our understanding of human development, and the reasons why we reproduce sexually, instead of from a single parent."

Stem cells have always been unique in that their flexibility to transform into other cells gives scientists the opportunity to treat a wide variety of conditions. Now, with their ability to do so with only one set of chromosomes, they can be even more versatile medical tools.