Association of monocytes and neutrophils with early events of blastocyst implantation in mice

The distribution and activation of monocytes and neutrophils in the mouse uterus were examined early in the process of blastocyst implantation. Implantation regions, detected by increased capillary permeability at sites of blastocyst apposition to the uterine luminal epithelium, could be distinguished at about 21:00 h on day 4 of pregnancy (day 1 = day of vaginal plug), and were also examined at 01:00, 05:00 and 09:00 h on day 5. Serial sections containing the implanting blastocyst (implantation sites), and random sections of interimplantation regions were examined by immunohistochemistry using interleukin-1β as a marker for monocytes–macrophages and lactoferrin as a marker for neutrophils in the uterine stroma. At implantation sites, interleukin-1β-positive cells were transiently abundant within endometrial capillaries. In interimplantation regions only a few interleukin-1β-positive cells were dispersed at the myometrial–stromal junction. Northern blot hybridization to RNAs from implantation and interimplantation regions showed that the abundance of interleukin-1β and interleukin-1α mRNAs was much lower than that found in the uterus during acute inflammation. However, these cytokine mRNAs were more abundant in implantation regions. On the evening of day 4 and the early morning of day 5, lactoferrin-positive neutrophils were detected juxtaposed to the basolateral surface of the antimesometrial epithelial cells surrounding the implanting blastocyst. They were primarily at the myometrial–stromal junction in interimplantation regions. Metallothionein gene expression was examined as a marker for uterine responses to inflammatory reactions. In situ hybridization showed high metallothionein mRNA specifically in antimesometrial epithelial cells underlying the implanting blastocyst and in deeper stromal cells in the implantation region at 05:00 h on day 5. These data suggest that monocytes and neutrophils are transiently abundant near implantation sites, and neutrophils become closely associated with the luminal epithelium. Although monocytes are not highly active in cytokine gene expression, neutrophils and monocytes may play a role in a localized inflammatory response during the initiation of blastocyst implantation.