Abstract
Pregnancy induces critical physiological adaptations to support embryonic development and fetal survival. This study compared endometrial and placental phenotypic and histomorphometric characteristics of Piau and Commercial sows at two gestational ages (25 and 35 days). Twelve sows (six Piau and six Commercial) were evaluated in a randomized design, with samples collected from three regions of the right uterine horn of each animal. Histomorphometric analyses were performed using microscopy and ImageJ software. Statistical analyses employed linear mixed-effects models, with Shapiro-Wilk and Levene's tests applied to assess normality and homogeneity of variances, respectively. At 25 days of gestation, Commercial sows showed greater uterine and ovarian weights, a higher number of corpora lutea, and longer uterine horn horns, reflecting genetic selection for reproductive efficiency. Conversely, Piau sows exhibited more advanced embryonic development at this stage, with fetuses of greater size. At 35 days, the phenotypic superiority of Commercial sows persisted, while Piau fetuses maintained greater weight and length, indicating distinct temporal growth dynamics. Histomorphometric analyses at 25 days revealed that Commercial sows had increased placental connective tissue deposition and thicker endometrial epithelium, whereas Piau sows presented larger placental vascular area, as well as enhanced endometrial vascularization and glandular density across all uterine regions. At 35 days, no significant differences were observed in placental vascular area and endometrial vascularization; however, subtle trends in connective tissue development suggested ongoing placental differentiation. These findings highlight distinct reproductive strategies between Piau and Commercial sows, with potential implications for embryonic development and gestational success. Altogether, the results confirm that genetic background influences uterine and placental morphology during early gestation.