SUBSTANCES AFFECTING ADULT TISSUE IN VITRO : III. A STIMULANT (THE "A FACTOR") IN SERUM ULTRAFILTRATE INVOLVED IN OVERCOMING ADULT TISSUE DORMANCY

1. The growth of fresh adult tissue in plasma medium suggested that plasma contained a substance active in overcoming the dormancy of adult tissues. 2. Incubation of adult chicken aorta in serum (from chickens or other species) before it was planted in a plasma medium, resulted in a much shorter lag period and a faster initial growth than that of control tissue incubated in Tyrode solution (or of fresh untreated tissue). In other words, serum helped overcome the dormancy of adult tissue while Tyrode solution preserved the tissue in a dormant state. This activity of serum was shown to be due neither to its albumin or globulin nor to its lipase or other enzymes. 3. The ultrafiltrate from serum was highly active in overcoming adult tissue dormancy, while the dialyzed residue was inactive. The ultrafiltrate was not species specific. It withstood 100°C. in neutral solution for 10 minutes, but not for 3 hours, and was destroyed at pH 2 and pH 12. It was active after 10 months in the ice box. It does not affect tissues in the cold. 4. Copper acetate precipitated a fraction from serum ultrafiltrate which, when freed from copper, was stimulating to the initial growth of adult aorta tissue. 5. Calcium chloride precipitated fractions from serum ultrafiltrate (and also from urine) which were stimulating. 6. Attempts to concentrate the A factor, the active agent, by vacuum evaporation and subsequent fractionation with different solvents were not satisfactory. 7. Lymph was highly stimulating and ventricular fluid slightly stimulating. 8. No growth was obtained in the absence of the A factor, but we doubt that it is alone responsible for overcoming the dormancy of adult tissue.