The effect of pregnancy and lactation on the mesometrium of the rat.

Weight and collagen content of the mesometrium, that part of the reproductive tract which lies between the uterus and the body wall, were measured in the rat, throughout pregnancy and post-partum. Both the weight and the collagen content increased significantly during pregnancy and decreased rapidly after parturition, the collagen content halving in around four days. The changes were almost entirely in the part of the mesometrium between the main longitudinal blood vessels and the uterus. At any given stage of pregnancy, the larger the uterus, the larger were both the weight and collagen content of the mesometrium. In rats pregnant in only one uterine horn the mesometrium associated with the barren horn showed no changes in weight or collagen content during pregnancy. Removal of fetuses from one uterine horn reversed the growth of the mesometrium associated with that horn, but not the growth of the mesometrium associated with the other (pregnant) horn. Cutting the attachment to the body wall prevented mesometrial growth during pregnancy although the tissue remained attached to the uterus, with its blood supply and the metrial gland intact. These data suggest that mesometrial growth in pregnancy is stimulated by stretch associated with the pregnant uterine horn.