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.

They differ in number and type differentiated cell types they can become.

Differentiation Capacity

While embryonic stem cells can become all cell types of the body because they are pluripotent, adult stem cells are  limited to differentiating into their tissue of origin.

Growth

Embryonic stem cells can be grown relatively easily in culture. Adult stem cells are rare  and isolating these cells from an adult tissue is difficult. Moreover ways to increase their population in cell culture is yet to be found.

This is an important difference, as large numbers of cells are needed for stem cell replacement therapies.

Rejection Chances

Embryonic and adult stem cells may differ in the likelihood of being rejected after transplantation. Though not supported by any scientific evidence, adult stem cells, and tissues derived from them, are currently believed less likely to initiate rejection after transplantation. This is because a patient’s own cells could be expanded in culture, coaxed into assuming a specific cell type  and then reintroduced into the patient.

The use of adult stem cells and tissues derived from the patient’s own adult stem cells would mean that the cells are less likely to be rejected by the immune system.

This represents a significant advantage, as immune rejection can be circumvented only by continuous administration of immunosuppressive drugs, and the drugs themselves may cause seroius side effects.

Specifically, embryonic stem cells are derived from embryos that develop from eggs that have been fertilized in vitro—in an in vitro fertilization clinic—and then donated for research purposes with informed consent of the donors.

They are not derived from eggs fertilized in a woman’s body. The embryos from which human embryonic stem cells are derived are typically four or five days old and are a hollow microscopic ball of cells called the blastocyst.

The blastocyst includes three structures

• Trophoblast, which is the layer of cells that surrounds the blastocyst
• Blastocoel, which is the hollow cavity inside the blastocyst
• Inner cell mass, which is a group of approximately 30 cells at one end of the blastocoel.

Embryonic Stem Cells They can develop into each of the more than 200 cell types of the adult body when given sufficient and necessary stimulation for a specific cell type.

Most of the research on embryonic  stem cell has been done on  mouse embryonic stem cells or human embryonic stem cells .

Both  require very different environments in order to maintain an undifferentiated state. Without optimal culture conditions or genetic manipulation, embryonic stem cells will rapidly differentiate.

A human embryonic stem cell is also defined by the presence of several transcription factors and cell surface proteins. The transcription factors Oct-4, Nanog, and SOX2 form the core regulatory network that ensures the suppression of genes that lead to differentiation and the maintenance of pluripotency.

The cell surface antigens most commonly used to identify hES cells are

• Glycolipids SSEA3 and SSEA4
• Keratan sulfate antigens Tra-1-60 and Tra-1-81.

Embryonic stem cells, being pluripotent cells, require specific signals for correct differentiation. If injected directly into another body, ES cells will differentiate into many different types of cells, causing a teratoma. A teratoma is a tumor with tissue or organ components resembling normal derivatives of all three germ layers.

Differentiating ES cells into usable cells while avoiding transplant rejection are just a few of the hurdles that embryonic stem cell researchers still face. Because of their combined abilities of unlimited expansion and pluripotency, embryonic stem cells remain a theoretically potential source for regenerative medicine and tissue replacement after injury or disease.

Stem cells are present in most multi-cellular organisms.

The two broad types of mammalian stem cells are

• Embryonic stem cells
  These cells are isolated from the inner cell mass of blastocysts in embryo.In a developing embryo, stem cells can differentiate into all of the specialized embryonic tissues.
• Adult stem cells
  These are found in adult tissues and participate in  repair system for the body, replenish specialized cells and maintain the normal turnover of regenerative organs, such as blood, skin or intestinal tissues.

Stem cells can now be grown and transformed into specialized cells with characteristics consistent with cells of various tissues such as muscles or nerves through cell culture.

Highly plastic adult stem cells from a variety of sources, including umbilical cord blood and bone marrow, are routinely used in medical therapies. Embryonic cell lines and autologous embryonic stem cells generated through therapeutic cloning are promising candidates for future therapies.