Relaxin has a minor role in rat mammary gland growth and differentiation during pregnancy

Growth and differentiation of mammary gland is associated with numerous hor mones and a variety of cell-cell, cell-matrix interactions. This study addr essed the role of relaxin (Rlx) on these processes. Morphologic and biochem ical changes that occur throughout the second half of pregnancy are reporte d. Temporal patterns and spatial distributions of markers useful to evaluat e proliferation, secretion, and collagen remodeling were established. To ev aluate the role of Rlx, an ablation/replacement animal model was used. Cons idering Rlx secretion pattern, two periods were selected: d I I through d 1 3, and d 20 through d 23. In the stroma, the extracellular compartment show ed changes associated with the lack of Rlx. Collagen remodeling within the lobuloalveolar structure, measured by a significant increase in collagen bi refringence, decreased at d 12, d 21, and d 22. Parenchymal structures were less sensitive to the absence of Rlx than stroma. Epithelial cell prolifer ation was lower in Rlx-deficient rats only at d 12, and a-lactalbumin expre ssion decreased at d 21 and d 22. Both lobuloalveolar diameter and percenta ge of area occupied by these structures showed no changes. In the absence o f Rlx, some of the studied markers showed statistically significant differe nces in scattered days; these do not make clear trends. No differences were found on d 23 on any of the studied parameters suggesting that compensator y mechanisms might be activated to overcome the effects of the absence of R lx. Unlike the critical role of Rlx either in uterine cervix dilation or in nipple development during rat pregnancy, Rlx had a minor role in growth an d differentiation of rat mammary gland.