Natural killer cells are the predominant population of leukocytes present in midluteal phase endometrium and in decidua throughout the first trimester Oohnson, 1999). By term, however, relatively few dNK cells are present in decidua. In first-trimester decidua, dNK cells lie close to extravillous trophoblasts, and there they purportedly serve to regulate invasion. These dNKs have a distinct phenotype characterized by a high surface density of CD56 or neural cell adhesion molecules (Manaster, 2008; Mofett-King, 2002). Their infiltration is increased by progesterone and by stromal cell production of IL-15 and decidual prolactin (Dunn, 2002; Gubbay, 2002). Although dNK cells have the capacity for cytotoxicity, they are not cytotoxic toward fetal trophoblasts. Their cytotoxic potential is prevented by molecular cues from decidual macrophages. In addition, the expression of specific HLA molecules protects against dNK killing. Also, dNK cells function to restrict trophoblast invasiveness to protect the mother.
Of other cell types, decidual macro phages are distinct from proinflammatory M 1 or antiinlammatory M2 macro phages. Decidual macro phages express the complement receptor COlIc at high or low levels: CD11cHI and CD11cLO. These cells function to regulate adaptive T cell responses; control dNK diferentiation, activation, and cytotoxicity; and produce antiinlammatory cytokines such as IL-10 to ensure fetal tolerance and inhibition of harmful immune responses.
Dendritic cells are cells that present antigens to T cells. hey play an important role in the development of a receptive endometrium for implantation.
Maternal T cels, as part of the adaptive immune response, increase in number and function after encounter with a speciic antigen. These cells subsequently retain the ability to respond rapidly in a subsequent encounter with the same antigen. Specific populations of Treg cells persist and can protect against aberrant immune responses. During pregnancy, there is a systemic expansion of maternal Treg cell populations. These cells are FOXP3+ cells with defined fetal speciicity. They are immunosuppressive and playia role in fetal tolerance.
At term, the amnion is a tough and tenacious but pliable membrane. his innermost avascular fetal membrane is contiguous with amnionic fluid and plays a role of incredible importance in human pregnancy. he amnion provides almost all tensile strength of the fetal membranes. Thus, its resilience to rupture is vitally important to successful pregnancy outcome. Indeed, preterm rupture of fetal membranes is a major cause of preterm delivery (Chap. 42, p. 819).
FIGURE 5-16 Photomicrograph of fetal membranes. From left to right: AE = amnion epithelium; AM = amnion mesenchyme; S zona spongiosa; eM = chorionic mesenchyme; TR = trophoblast; 0= decidua. (Used with permission from Dr. Judith R. Head.)
Bourne (1962) described five separate amnion layers. he inner surface, which is bathed by amnionic luid, is an uninterrupted, single layer of cuboidal epithelium (Fig. 5-16). his epithelium is attached irmly to a distinct basement membrane that is connected to an acellular compact layer composed primarily of interstitial collagens. On the outer side of the compact layer, there is a row of ibroblast-like mesenchymal cells, which are widely dispersed at term. here also are a few fetal macrophages in the amnion. he outermost amnion layer is the relatively acellular zona spongiosa, which is contiguous with the second fetal membrane, the chorion laeve. he human amnion lacks smooth muscle cells, nerves, lymphatics, and importantly, blood vessels.