Monoclonal antibodies specific for rat relaxin. VII. Passive immunization with monoclonal antibodies throughout the second half of pregnancy prevents development of normal mammary nipple morphology and function in rats.

Abstract

We recently demonstrated that relaxin-dependent development of the mammary nipples during the second half of pregnancy is required for pup survival during lactation in the rat. The two related objectives of this investigation were to 1) characterize the effects of endogenous relaxin on the histological modifications that normally occur in the mammary nipples, and 2) test the hypothesis that the cause of lactational failure in relaxin-deficient rats is attributable to failure of the nipples to grow and develop during the second half of pregnancy. Endogenous relaxin was neutralized by daily iv injection of a highly purified monoclonal antibody specific for rat relaxin (MCA1) to intact rats from days 12-22 of pregnancy. Mammary nipples were collected on day 22 of pregnancy and routinely prepared for light microscopy. Tissue cross-sections (6 microns) obtained from the midpoint of mammary nipples were stained with either Gomori's trichrome stain (to visualize collagen) or orcein (to visualize elastin). Nipple size as well as histological characteristics of nipple cross sections were determined by morphometric analysis. MCA1-treated rats were significantly different from controls with the following parameters: shorter length of the nipples; smaller cross-sectional areas of the entire nipple, lactiferous duct lumen, and blood vessels; greater percentage of the analysis field composed of collagen; lower percentage of the analysis field composed of amorphous ground substance; and longer elastin fibers. To test the hypothesis that the cause of lactational failure in relaxin-deficient rats is attributable to the failure of nipples to grow and develop, MCA1 and control rats were cesarean sectioned between 2100-2400 h on day 22 of pregnancy, and lactation was examined using normal foster pups from intact donor females. Unlike pups fostered to controls, pups fostered to MCA1-treated dams failed to grasp the nipples, stimulate PRL release, or have milk in their abdomens. This study demonstrates that endogenous relaxin promotes not only growth, but also modifications of the histological characteristics of the nipple that are consistent with relaxin's effects on the cervix and mammary glands. Additionally, this study provides evidence that lactational failure in relaxin-deficient rats is attributable to the small size and different histology of the mammary nipples, which results in the inability of the pups to attach to the nipple, stimulate PRL release, and obtain milk from the dams.