y are removed and plated into several fresh culture dishes. The cells are then sub-cultured for many months to produce a large yield. Embryonic stem cells that have proliferated in cell culture for six or more months without differentiating, are pluripotent, and appear genetically normal, are referred to as an embryonic stem cell line. Once cell lines are established, or even before that stage, batches of them can be frozen and shipped to other laboratories for further culture and experimentation. As long as the embryonic stem cells in culture are grown under certain conditions, they can remain undifferentiated. But if cells are allowed to clump together to form embryoid bodies, they begin to differentiate spontaneously. Although spontaneous differentiation is a good indication that a culture of embryonic stem cells is healthy, it is not an efficient way to produce cultures of specific cell types. So, to generate cultures of specific types of differentiated cells scientists change the chemical composition of the culture mediu
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