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Types and characteristics of stem cells for culture

Embryonic stem (ES) cells are obtained from the inner cell mass and cultured as illustrated:

ES cells from mouse embryos have been cultured since the 1980s by various groups of researchers working independently.10 These pioneers established murine embryonic stem cells lines that could differentiate into several different cell types.11 ES cell lines have been established from other mammals (hamsters, rats, pigs, and cows). Thompson and colleagues at the University of Wisconsin reported isolation of primate ES cells in 1995 and human ES cells in 1998.12

ES cells are the best characterized of all the cultured stem cells. Properties of ES cells:13
(i) ES cells are pleuripotent, i.e. they have the ability to differentiate into cells derived from all three germ layers, but not the embryonic membranes.
(ii) ES cells are immortal i.e. cells proliferate in culture and have been maintained in culture for several hundred doublings. The advantage of maintaining stem cells in culture is that they are a source of a large number of cells in the undifferentiated state. So far other adult stem cells have not been maintained indefinitely.
(iii) ES cells maintain a normal karyotype (there are no major structural changes in the chromosomes)
(iv) ES cells display Oct-4 protein and other unique markers on the cell surface.

Generally, ES cells are maintained in culture on feeder cells (mouse fibroblast cells) There have been recent reports of ES cultured on feeder cell-free medium.14

ES cells can be induced to differentiate in vitro by culturing in suspension to form three-dimensional cell aggregates called embryoid bodies (EBs).15 The cells spontaneously differentiate into various cell types, e.g. neurons, cardiomyocytes, and pancreatic beta cells. The addition of growth factors to the culture directs differentiation to specific cell types. However, it is still challenging to isolate pure differentiated cell types.

Following injection of ES cells into immunodeficient mice, teratomas develop with derivatives of all three germ layers. This is a major disadvantage of using ES cells for cell therapy since any contaminating undifferentiated cells could give rise to cancer.

Embryonic germ cells Gearhart and colleagues originally derived stem cells from primordial germ cells.16 Cells cultured from the genital ridge of the human embryo have been isolated and cultured. These cells have a lesser capacity of proliferation than ES cells but have an advantage in that they are not tumorigenic, unlike ES cells.17

Embryonal carcinoma cells Embryonal carcinoma cell lines were first developed in 1967 by Ephrussi and colleagues from mouse teratomas, followed in 1975 by Fogh and Tempe from a human testicular teratocarcinoma. These cells are malignant relatives of ES and EG cells, which were used in many of the techniques to cultivate them. EC cells can differentiate under the right conditions and have a potential to be used for research and perhaps clinical applications.18 Once they differentiate they would not be expected to cause cancer, but these cells have not been studied as well as ES cells and are of limited use at present.

Adult or somatic stem cells The existence of hematopoietic stem cells was discovered in the 1960s, followed by the discovery of stromal cells (also called mesenchymal cells). Only in the 1990s did scientists confirm the reports of neural stem cells in mammalian brains. Since then stem cells have been found in the epidermis, liver and several other tissues.19
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