Serum-free mouse embryo (SFME) cells, derived in medium in which serum is replaced with growth factors and other supplements, display distinctive properties: (i) SFME cells do not lose proliferative potential or show gross chromosomal aberration upon extended culture, (ii) these cells depend on epidermal growth factor for survival; and (ii) SFME cell proliferation is reversibly inhibited by serum. Treatment of SFME cells with serum or transforming growth factor j3 led to the appearance of glial fibrillary acidic protein, a specific marker for astrocytes. The appearance of glial fibrillary acidic protein in cultures was reversed upon removal of transforming growth factor (3 or serum. Cells with properties similar to SFME cells were also isolated from adult mouse brain. These results suggest a role for transforming growth factor (3 in astrocyte differentiation in developing organisms and in response to injury and identify the cell type that has the unusual properties of SFME cells.Serum-free mouse embryo (SFME) cells initiated and maintained multipassage in a rich basal nutrient medium supplemented with insulin, transferrin, epidermal growth factor (EGF), high-density lipoprotein, and fibronectin display several distinctive properties (1)(2)(3)(4)(5)(6). Mouse embryo cells derived in conventional serum-supplemented medium undergo a period in which proliferative potential is lost or reduced, and then genetically altered cells with indefinite proliferative potential emerge (2,7,8). SFME cells do not exhibit any loss of proliferative potential or gross chromosomal aberration and maintain a relatively stable karyotype when cultured for >10 times the population doublings achieved with karyotypically normal mouse embryo cells in serum-containing medium. Although the period in which reduced growth rate can be detected may be minimal or absent in cultures in serumcontaining medium maintained in a passaging protocol involving high plating densities, the protocols used to derive SFME cells lead to a well-defined period of limited proliferative potential in cultures maintained in serum-containing medium (2, 7) that is not seen when the cells are cultured in serum-free medium. SFME cells also depend on EGF for survival, and cycloheximide or actinomycin D prevents death from EGF deprivation (5), suggesting that cell death requires the synthesis of RNA and protein, a phenomenon of programmed cell death similar to that reported for neuronal cell death in the absence of nerve growth factor (NGF) (9). In addition, proliferation of SFME cells is reversibly inhibited by serum or platelet-free plasma (1, 4, 6), and thus these cells would not be propagated under conventional culture conditions using serum-supplemented medium. As for most mouse embryo cell lines, the precise nature of the cells giving rise to the continuously growing SFME cultures was unknown. The unusual characteristics of SFME cells led us to attempt to determine the cell type represented by these cells.We found that treatment of SFME cells with serum or We a...

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Mouse embryo cells derived in a serum-free medium formulation (SFME cells) do not exhibit growth crisis or chromosomal abnormalities and are nontumorigenic in vivo; these cells are also reversibly growth inhibited by serum or platelet-free plasma (Loo et al.; Science, 236:200-202, 1987). A portion of the inhibitory activity of serum could be extracted by charcoal, a procedure that removes steroid and thyroid hormones. Both L-3,5,3'-triiodothyronine (T3) and hydrocortisone inhibited growth of SFME cells in a reversible manner. The inhibitory activity of serum also was partially removed by treatment with anion exchange resin in a procedure designed to deplete serum of thyroid hormone. However, the effect of serum on untransformed SFME cells could not be prevented by addition of the antiglucocorticoid RU38486, and ras-transformed clones of SFME cells, which are capable of growing in serum-containing medium, retained inhibitory responses to glucocorticoid and, with some clonal variability, to T3. These results suggest that glucocorticoid or thyroid hormones may contribute to the inhibitory activity of serum on SFME cells, but additional factors are also involved.