Pregnant rats were exposed to hypoxia (equivalent to 11% oxygen) from gestation day 6.5 to day 13.5, panels B and E and sacrificed on day 13.5. Gestationally matched pair-fed rats exposed to ambient conditions were used as controls (panels A and D). Placentation sites were sectioned and stained for ACTA2, identifying smooth muscle-associated blood vessels in the uterine mesometrial compartment (panels A and B) or for perforin (PRF1), a marker of NK cells (panels D and E). Chromogen: AEC; counterstain: hematoxylin; scale bars = 0.5 mm. Panel C, quantification of the ratio of blood vessel cross-sectional area to uterine mesometrial compartment cross-sectional area (BV:UMC) in pair-fed normoxic controls (n=7) and hypoxia exposed (n=7) placentation sites. Values are means ± the standard error of each mean. Please note that hypoxia exposure increased the BV:UMC cross-sectional area (asterisk; P<0.0006). Abbreviations: PF-N, pair-fed normoxia; HYP, maternal hypoxia.