Colony stimulating factor 1 is required for mammary gland development during pregnancy.

The study of colony stimulating factor 1 (CSF-1), a homodimeric serum growth factor that regulates mononuclear phagocytes and is involved in maternal-fetal interactions during pregnancy, was dramatically enhanced by the observation that the recessive mutation osteopetrosis, op, is an inactivating mutation in the CSF-1-encoding gene. Homozygous mutant (op/op) mice completely lack CSF-1, are osteopetrotic consequent to a deficiency in osteoclasts, have severely reduced numbers of macrophages, and have reduced fertility evident at the pre- and postimplantation stages of pregnancy. We show here that op/op females have a lactational defect, and consequently, although some are able to produce offspring, few nurture any pups and none feeds a full litter. This lactational defect is due to incomplete mammary gland ductal growth during pregnancy, a precocious development of the lobulo-alveolar system, and despite expression of milk proteins, a failure to switch to a lactational state. These data show that CSF-1 has a role in the development of the mammary gland during pregnancy.