Spheres of influence. Presidential scorecard.

A SEQUENTIAL in vivo-ini vitr-o model system has been developed for studying carcinogenesis in the rat brain after transplacental expostire to ethylnitrosotirea (ENU) (Roscoe & Claisse, 1.976, 1978; Claisse et al., 1.978; Hince & Roscoe, 1.978a, b; Roscoe, 1980; Roscoe et al., 1980). Although cultures derived early in the latent period required prolonged passaging before becoming malignant or forming colonies in agar, they exhibited differences in other properties, including plasminogenactivator activity (Hince & Roscoe, 1978b) and survival for long periods suspended in agar (Roscoe & Winslow, 1980) at earlier passages. To characterize these differences more precisely it was desirable to obtain clones from each type of culture. Cholera toxin (CT) has recently been reported to stimulate the proliferation of several cell types (Green, 1978; Pruss & Herschman, 1979; Taylor-Papadimitriou et al., 1980) including Schwann cells from neonatal rats (Raff et al., 1978a, b). It was therefore included in the growth medium (GM) of newly derived brain cultures from ENUand buffer-exposed foetuses, in the hope that it might facilitate the usually difficult procedure of cloning. Female rats of the BD IX inbred strain were injected i.p. with ENU at 40-50 mg/kg body wt, or an equal volume of citrate buffer, on Day 15 or 16 of pregnancv. Cultures were derived from brain tissue and maintained in GM consisting of Dulbecco's modification of Eagle's Medium and 15o% foetal calf serum as Acceptedl 15 October 1f980