The time-course of uterine growth, cell proliferation, and microvascular development was evaluated during the first 72 h after implanting estradiol-17 beta (E(2)) into ovariectomized (OVX) ewes. Uterine fresh weight increased 2.3-fold by 24 h and increased further (3.3-fold) by 48 h. The majority (similar to 75%) of this growth response was associated with tissue growth rather than a change in the tissue dry weight:fresh weight ratio. Both uterine cell number (DNA content) and cell size (RNA:DNA ratio) increased from 0 to 24 h (1.8-fold and 1.7-fold, respectively). Cell proliferation also increased dramatically between 8 h and 24 h after E(2) implantation. Endometrial microvascular volume density (percentage of tissue volume occupied by microvessels) increased similar to 1.8-fold by 24 h and then remained constant or declined slightly through 72 h. The total endometrial microvascular volume, however, increased similar to 5-fold from 0 to 24 h and increased further by 72 h. Thus, treatment of OVX ewes with E(2) caused a dramatic increase in uterine fresh and dry weights by 24 h, due primarily to hyperplasia and hypertrophy, with only a relatively small change in tissue dry weight:fresh weight ratio. This dramatic uterine growth was associated with a profound increase in endometrial microvascular volume.
