An embryonic stem cell line is a family of constantly dividing cells, deriving from individual parent stem cells extracted from a human embryo.

Scientists have been eager to receive these cells, because many believe they could hold the secret to curing a wide array of diseases and disorders, ranging from cancers to birth defects to paralysis.

The process of developing these cells is complicated. But once established, the original embryonic cells can replicate into millions of stem cells in the controlled environment of a laboratory.

Stem cell lines have been quite limited to date. But now scientists at the Children's Hospital in Boston and the Rockefeller University in New York have produced 11 new lines of stem cells, which were approved Wednesday by the National Institute of Health. They are the first lines to be approved since President Obama loosened the guidelines set down by the Bush administration.

"This is a real change in the landscape," said NIH Director Francis Collins. "This is the first down payment on what is going to be a much longer list ... that will empower the scientific community to explore the potential of embryonic stem cell research."

Embryonic stem cells are controversial because, in order to cultivate the lines, scientists must harvest stem cells from living human embryos. Because the embryos are destroyed in this process, anti-abortion activists have opposed their use. But proponents of the process note that under the new administration's regulations, these stem cell lines have been derived from excess fertility clinic embryos that would otherwise have been discarded.

To harvest a stem cell, researchers use micro-eyedroppers to remove single cells from early-stage embryos, each of which consists of only eight to 10 cells. Those extracted cells are cultivated in petri dishes in a lab environment, where scientists refer to them as a cell culture. In a broth designed to promote growth, the cells divide and spread over the surface of the dish.

The National Institutes of Health note that the process of developing these lines is delicate, and inefficient. If the cells survive, divide and multiply enough to crowd the dish, they are removed gently and plated into several fresh culture dishes, a process that's repeated for many months.

Embryonic stem cells that have proliferated in a cell culture for six or more months without differentiating are referred to as a line. Once established, this line can replicate for long periods of time in the controlled environment of a laboratory.

Obama lifted eight years of restrictions on the master cells last spring. But $21 million-and-counting worth of new projects were on hold until the National Institutes of Health determined which of hundreds of existing stem cell lines were ethically appropriate to use.