Keratins in the human trophoblast.

Besides microfilaments and microtubules, intermediate filaments are major components of the cytoskeleton. In epithelial cells intermediate filaments are formed by heterodimers of specific keratins, whose expression pattern highly depends on the type of epithelium and differentiation degree of the cell. During the process of blastocyst implantation and subsequent development of the human placenta a very specialized epithelium appears at the feto-maternal interface. Arising from the trophectoderm of the blastocyst, the epithelium-like layer surrounding the early embryoblast, different trophoblast subtypes differentiate. They either develop into polar cells fulfilling real epithelial functions, or apolar tumor-like cells invading the maternal uterine wall to adapt the maternal tissue to progressing pregnancy. Thus, the whole trophoblast population, with all its subtypes, can be considered as an epithelial compartment and hence expresses keratin filaments. However, differentiation of trophoblast into different phenotypes may be linked to remodeling of the cytoskeletal composition, depending on spatiotemporal requirements of the respective cells. Here, we focus on the keratin composition of different trophoblast subtypes, how these keratins are used in trophoblast research and what is known about placental keratins in pregnancy pathologies.