Es Cells

ES cells are derived from the inner cell mass of blastocysts (for review see Smith, 2001). These cells closely resemble EC cells in morphology, growth behavior, and marker expression. Furthermore, ES cells like EC cells are undifferentiated, immortal, and pluripotent cells that have the capacity to differentiate into cell types of all three primary germ layers. However, unlike EC cells, ES cells are diploid, participate in embryogenesis, and are able to differentiate into germ cells in vivo. Great care must be taken in the culturing of mouse ES cells to maintain the undifferentiated stem cell phenotype including the use of feeder cells or growth in the presence of the cytokine leukemia inhibitory factor (LIF). In addition to mice (Evans and Kaufman, 1981; Martin, 1981), ES cells have been prepared from fish (Hong et al., 1996; Sun et al., 1995), chicken (Pain et al., 1996), rhesus monkey (Thomson and Marshall, 1998; Thomson et al., 1995), marmoset (Thomson et al., 1996), and humans (Shamblott et al., 1998; Thomson et al., 1998). A major application of mouse ES cells has been their use in the engineering of transgenic and knockout mice. ES cells also have great potential as a source of cells for transplantation in the treatment of numerous pathologies.

RA can induce differentiation of ES cells into a large number of different cell types including neurons, glial cells, adipocytes, chondrocytes, osteocytes, corneal epithelium, skeletal muscle, smooth muscle, and ventricular cardio-myocytes (Eiges and Benvenisty, 2002; Rohwedel et al., 1999; Schuldiner et al., 2000) (Fig. 3). In all cases, in vitro differentiation begins by removal of LIF from the culture medium and growth of ES cells as small aggregates termed embryoid bodies either in dishes containing a nonadhesive

Embryonic stem cells

Inner cell mass (ICM)

Embryonic stem cells

Inner cell mass (ICM)

Neuron and glia +RA

Corneal endothelial Seeded on +RA

Mouse blastocyst ■xt.

Cells isolated from ICM \

Cultured diploid pluripotent stem cells grown on feeder cells feeder cells

Ventricular +RA following cardiomyocyte " day 5-9 ^

SCSS between day 0 and 5

feeder layer i

Contracting smooth muscle

Chondrocytes

+RA between day 2 and 5

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