Biochemistry and molecular biology of trophoblast invasion.

Cytotrophoblastic cells (CTBs) from first-trimester placenta form columns of invasive CTBs. This invasive behavior is due to the ability of CTBs to secrete matrix metalloproteinases (MMPs), since tissue inhibitor of MMPs (TIMP) inhibits their invasiveness in the extracellular space. Although CTBs behave like metastic cells, in vivo they are only transiently invasive (first trimester), and their invasion is normally limited only to the endometrium and to the proximal third of the myometrium. This temporal and spatial regulation of trophoblast invasion is believed to be mediated in an autocrine way by trophoblastic factors and in a paracrine way by uterine factors. Several types of regulators have been investigated: hormones, extracellular matrix glycoproteins, and cytokines or growth factors. This review is not intended to be an exhaustive catalogue of potential regulators of trophoblast invasion but is aimed at summarizing the most important categories of factors affecting trophoblast-endometrium interactions.