Federal rules forbid the federal funding of research on human embryonic stem cell lines that aren't listed on the National Institutes of Health's Human Embryonic Stem Cell Registry. There are currently 22 embryonic stem cell lines on the registry, and the demand for these cells is steadily growing.
"We have to find a way to mass-produce them because traditional cell culturing methods can't meet the projected high market demand for stem cells," Yang said.
He and Anli Ouyang, a doctoral student in chemical engineering, grew mouse embryonic stem cells in a bioreactor. Cell growth increased 193-fold in 15 days. At the end of that period, cell density ?the number of cells that had grown in the bioreactor ?was anywhere from 10- to 100-fold higher than the number of stem cells produced by conventional laboratory methods. That's several hundreds of millions more stem cells.
Mass-producing cells like this could reduce stem cell production costs by at least 80 percent, Yang said, as it requires less equipment and monitoring.
Embryonic stem cells are unspecialized, or undifferentiated, cells that can grow into any of the body's 200 different types of cells.
Yang and Ouyang presented their findings in San Diego on March 15 at the national meeting of the American Chemical Society.
They grew some mouse embryonic stem cells in a flask ?a conventional way to grow stem cells ?while other stem cells grew upon strands of polymer threads inside a bioreactor.
The bioreactor used in this study is a tissue-growing device developed by Ohio State scientists. While this bioreactor could be used to produce adult stem cells, the researchers ch ose to look solely at embryonic stem cells for this experiment.
"There's more of a demand for an unlimited supply of embryonic stem cells," Yang said. Also, embryonic stem cells are pluripotent ?they can become any kind of cell in the body, while adult stem cells usually develop into a type of cell based on the kind of tissue that they originated from.
The bioreactor has a chamber that holds the polymer threads on which the stem cells grow and another chamber that holds fluid, or medium. This medium delivers chemical messengers, called cytokines, to the stem cells. The cytokines essentially tell the stem cells to stay in their undifferentiated state.
The difference between growing stem cells in a flask vs. the bioreactor is that the cells grown in the bioreactor could grow in three dimensions, while cells growing on a flat surface ?the bottom of the flask ?could not.
"Cells grown on a flat surface don't act like they would in the body," Yang said. "The growing surface affects how a cell forms, what it looks like and even how it expresses genes."
Cells grown in the bioreactor could grow for a much longer period of time than they could in the flask, as cells had more room to grow in the bioreactor.
"In the same amount of time we could grow up to a billion stem cells per milliliter in the bioreactor, compared to tens of millions of cells per milliliter with conventional systems," Yang said.
Also, it seems that embryonic stem cells grown by conventional methods are more likely to spontaneously differentiate, or change. Researchers aren't sure why or how some stem cells differentiate without prompting.
Yang and Ouyang tested both sets of cells ?the ones grown in a flask and those grown in the bioreactor ?for two key proteins. The presence of these proteins indicates that a stem cell has not differentiated. In the bioreactor experiment, 94 percent of the stem cells tested positive for these proteins, compared to about 85 percent of the cells grown in the flask.
The researchers are now working on ways to program embryonic stem cells so that they differentiate into specific types of cells. In preliminary work, Ouyang has created a network of neurons from undifferentiated embryonic stem cells.
The next step is to use human embryonic stem cells, and the researchers have yet to decide which line of stem cells to use.The mouse embryonic stem cells used in this study were provided by ATCC, an organization that supplies lines of embryonic stem cell lines.