Human embryonic stem cells that have been propagated for more than two  years also demonstrate a stable and normal complement of chromosomes. This is in contrast to the unstable and abnormal complement of embryonic cancer cell lines used in the past.

Research on human embryonic stem cells is very recent. The data about properties of stem cells comes from mice.

The embryonic stem cells in mice  have been studied for  more than two decades  and provide a critical basic knowledge.

Types of cells derived from cultured mouse embryonic stem cells are

• Fat cells
• Brain and nervous system cell
• Insulin-producing cells of the pancreas
• Bone cells
• Hematopoietic cells
• Yolk sac
• Endothelial cells
• Primitive endodermal cells
• Smooth and striated muscle cells including  heart muscle cells.

Mouse embryonic stem cells, to proliferate in an undifferentiated state require

• The presence of  leukemia inhibitory factor
• A layer of mouse embryonic fibroblasts (feeder cells) in a medium containing serum from cows.

The feeder cells produce growth factors that sustain the embryonic stem cells.

if feeder cells are not present,  human embryonic stem cells form aggregated balls of cells called embryonic bodies  that has cells from all the three layers.

Human embryonic stem cells injected into mice form a type of benign tumor called a teratoma that is made up of tissues from all three embryonic layers forms tissues like teeth, gut, hair follicles, skin, epithelium, muscle, bone, cartilage, lung tissue, and neural cells.

The experiments showed the capability of ESCs to produce a variety of tissues but also  also highlight danger of causing malignancy. It has been possible to create a lineage of mouse embryonic stem cells that generate neural cell precursors.

Embryonic stem cells have a definite role in regenerating the tissues. We would know better as the time unfolds.