Role of Insulin as a ‘Permissive’ Hormone in Mammary Gland Development

Abstract

THE growth and differentiation of the mammary gland is controlled by the synergistic action of insulin, prolactin and ovarian or corticosteroid hormones^1–4. The ovarian steroid induced alveolar development of the virgin mammary gland in vivo is mediated by activation of nucleic acid and protein synthesis and mitosis^5–9. On the other hand, numerous studies using organ cultures, mostly of fragments of mid-pregnant mammary gland, emphasised the role of insulin as the mitogenic hormone in mammary gland development^10–12. This variable nature of responsiveness of the mammary tissue to hormones in vivo and in organ culture has been discussed in recent reports^13,14. The proliferative mammary gland of pregnancy, mostly used in these organ culture studies^10–12, should contain a large proportion of cells in the division cycle at the time of excision; we considered that hormonal regulation of the fate of these dividing cells during incubation in vitro may influence the interpretation of the results obtained from the organ culture studies. In order to test this possibility, mid-pregnant mammary cells of Balb/c mice were labelled for 1 h with ³H-thymidine (³H-dT) in vivo, and the transit patterns of the labelled cells reaching mitosis were determined at different times in organ culture.