Cell cultures established from cerebrum of 1-day-old rats were used to investigate hormonal regulation of the development of oligodendrocytes, which synthesize myelin in the central nervous system. The number of oligodendrocytes that developed was preferentially increased by insulin, or by insulin-like growth factor I (IGF-I), also known as somatomedin C. High concentrations (5 Pg/ml) of insulin were required for substantial induction of oligodendrocyte development, whereas only 3.3 ng of IGF-I per ml was needed for a 2-fold increase in oligodendrocyte numbers. At an IGF-I concentration of 100 ng/ml, oligodendrocyte numbers were increased 6-fold in cultures grown in the presence of 10% fetal bovine serum, or up to 60-fold in cultures maintained in serum-free medium. IGF-I produced less than a 2-fold increase in the number of nonoligodendroglial cells in the same cultures. Type I IGF receptors were identified on oligodendrocytes and on a putative oligodendrocyte precursor cell population identified by using mouse monoclonal antibody A2B5. These results indicate that IGF-I is a potent inducer of oligodendrocyte development and suggest a possible mechanism based on IGF deficiency for the hypomyelination that results from early postnatal malnutrition.Oligodendrocytes play a critical role in nervous system function by synthesizing and maintaining myelin in the central nervous system (CNS) (1). Very little is known about the stimuli or regulatory signals that promote the development of oligodendrocytes, but such information might provide the basis for approaches to promote repair in demyelinating disorders.We studied tissue cultures established from cerebrum of MATERIALS AND METHODS Cell Explantation and Culture. Cells were explanted by mechanical dissociation of cerebrum of 1-day-old strain LEC rats as described (3, 4) and were inoculated into polystyrene tissue culture dishes, glass microscope slide chambers, or dishes containing glass coverslips. Glass surfaces were previously treated with D-polylysine (10-50 pug/ml) or polylysine followed by fibronectin (1 Atg/cm2; purified by method B of ref. 9). Cells were maintained as mixed cultures without separation of individual cell types. Unless otherwise noted, culture medium was Eagle's minimum essential medium supplemented with glucose to a total of 6 g/liter, 0.1 mM of each nonessential amino acid, antibiotics, and 10% fetal bovine serum. For serum-free culture, cells were maintained in the presence of 10%6 fetal bovine serum for the first 16-24 hr of culture to allow the cells to attach (10, 11), and the cells were then rinsed and refed with serum-free culture medium. Serum-free medium (modified from ref. 10) consisted ofequal parts of Dulbecco's modified Eagle's medium (high-glucose formula) and Ham's F-12 medium, supplemented with glucose to a total of 6 g/liter/15 nM triiodothyronine/30 nM sodium selenite/transferrin (50 pug/ml). Insulin was absent except where noted. In all cases, medium was changed on the fourth day after explantation and every second da...

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