Mouse L cells are γ-irradiated at a dose of 1 Mrad, and ultrathin sections of the cells are examined by electron microscopy. The distance between chromatin fibers in diffused chromatin regions in the irradiated nuclei is essentially identical with the non- irradiated control. In contrast, an increase of the distance between the chromatin fibers is observed in the excess of Ca ions in irradiation.
Deoxyribonucleoprotein (DNP) extracted from calf thymus is gamma-irradiated at a dose of 100 krad. Freeze-replicas of the irradiated DNP are made, and examined by means of electron microscopy. The non-irradiated DNP fibers with a mean diameter of 183Å are formed by coiling of finer fibers, 23Å in mean diameter. In contrast, the irradiated DNP fibers appear to be thin fibers, being 49Å in mean diameter.
A human cell line (KONT) producing colony-stimulating activity (CSA) has been established.1) The KONT cell line was isolated from the pleural effusion of a lung cancer patient who had developed marked leukocytosis. The cells produce both human CSA and mouse CSA. Medium conditioned by KONT cells is much more potent in stimulating human bone marrow cell growth than is human leukocyte conditioned medium (HL-CM). Furthermore, the KONT conditioned medium tends to induce granulocytic cells rather than other cell types. The CSA is stable at temperatures ranging from 37°C to 50°C, but is completely inactivated at a temperature of 80°C for 30min. The CSA is also sensitive to digestion by trypsin and pronase, and there is little binding of mouse or human CSA in KONT- CM to concanavalin A-Sepharose. Fractionation of the conditioned medium by Sephadex G-150 chromatography indicated that the maximum activity of the CSA for human marrow cells is eluted at 20, 000-28, 000 dalton fraction.