Mice without a functional relaxin gene are unable to deliver milk to theirpups

We have used gene targeting to generate relaxin (rlx)-deficient mice. The m ajority (15 of 17) of homozygous (rlx(-/-)) mice are fertile and produce no rmal litters. However their mammary development is deficient; pups are unab le to suckle and die within 24 h of birth unless cross-fostered to a wild-t ype (rlx(+/+)) foster mother. The nipples of rlx(-/-) animals do not enlarg e significantly during pregnancy, and their histology retains the appearanc e of the virgin state. Breast parenchyma is somewhat underdeveloped at term even though milk is produced. Mammary ducts become grossly dilated in thes e animals. Heterozygous (rlx(+/-)) mice lactate normally. The interpubic ligament does not relax during pregnancy in rlx(-/-) mice. P lasma osmolality during late gestation was significantly higher (P < 0.001) in rlx(-/-) mice than in wild-type controls.