The first challenge is that growing stem cells in quantity requires space. Like traditional farming, it is a two-dimensional affair. If you want more wheat, corn or stem cells, you need more surface area. Culturing stem cells, therefore, requires a lot of relatively expensive laboratory real estate, not to mention the energy and nutrients necessary to pull it all off.
The second challenge is that once they've divided many times in a lab dish, stem cells do not easily remain in the ideal state of readiness to become other types of cells. Researchers refer to this quality as "stemness." Heilshorn found that for the neural stem cells she was working with, maintaining the cells' stemness requires the cells to be touching.
Heilshorn's team was working with a particular type of stem cell that matures into neurons and other cells of the nervous system. These types of cells, if produced in sufficient quantities, could generate therapies to repair spinal cord injuries, counteract traumatic brain injury or cure some of the most severe degenerative disorders of the nervous system, like Parkinson's and Huntington's diseases.