Abstract
Explants of mouse mammary glands were cultured for one to five days in chemicallyโdefined medium containing various combinations of insulin, corticosterone, and prolactin. The changes in RNA content and synthesis effected by the several hormone combinations were assessed after one, two, three, and five days of culture. Histological examination was also made to determine explant viability and secretory development.
Insulin was responsible for the initial stimulation of RNA synthesis, but its activity fell after the first day. Prolactin and insulin effected maximal accumulation of RNA on the second day, but the influence of these two hormones also declined by the third day unless corticosterone was also present. The hormones in the sequential order in which they manifest their activity on mammary RNA thus appear to be: insulin, prolactin, corticosterone.
The nonโparallel patterns of uridine incorporation and RNA accumulation suggest that the major fraction of hormoneโstimulated RNA is synthesized during the first two days and can be maintained by the appropriate hormonal milieu. The histological responses show that although insulin alone has an early effect on RNA synthesis, the subsequent secretory differentiation of the explants requires the further presence of prolactin and corticosterone.