Pregnancy in humans and rodents is associated with dramatic changes in leukocyte populations within the uterus. In these species, recruitment of leukocytes, mostly natural killer (NK) lymphocytes, accompanies decidualization of endometrial stroma even in the absence of pregnancy. In the pig, a nondecidualizing species, the predominant lymphocytes in the pregnant uterus are T and/or NK cells, but their distribution relative to embryonic attachment sites has not been reported. The objective of this study was to compare the abundance of leukocytes in porcine endometrium in contact with trophoblast with that between attachment sites during the early postattachment period. Uteri were recovered on Days 15–17 (n = 4), 18 and 19 (n = 4), 21 and 22 (n = 5), and 25–27 (n = 2) of gestation and from cycling pigs during the luteal phase (Day 15; n = 3). Leukocytes were identified in uterus obtained at versus between attachment sites using an antibody reactive with all leukocytes (CD44). In all pregnant animals, leukocytes were diffusely scattered throughout the endometrial stroma but were rare or absent in the luminal epithelium. Leukocyte density was ∼3-fold greater in endometrium in contact with conceptuses than in endometrium between attachment sites throughout the early postattachment period. Leukocyte density during the luteal phase was similar to that between attachment sites, suggesting that leukocyte recruitment was a localized response to the embryo. The ability of an individual porcine conceptus to recruit maternal leukocytes to the adjacent stroma may be a vital step in early placental development and embryo survival.
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