ORIGINAL PAPER Emin Tu¨rkay Korgun Æ Ciler Celik-Ozenci Nuray Acar Æ Sevil Cayli Æ Gernot Desoye Ramazan Demir Location of cell cycle regulators cyclin B1, cyclin A, PCNA, Ki67 and cell cycle inhibitors p21, p27 and p57 in human first trimester placenta and deciduas Accepted: 25 January 2006 / Published online: 21 February 2006 Ó Springer-Verlag 2006 Abstract Although placental development and implan- tation depend on the coordination of trophoblast pro- liferation, differentiation and invasion, little is known about the cell cycle regulators that govern the control of these events. The hypothesis that the coordinated expression of cell cycle progression and inhibition factors will determine whether cytotrophoblasts proliferate or undergo cell cycle arrest or cell cycle exit allowing sub- sequent differentiation was tested. The cell cycle pro- motors cyclin A, cyclin B1, PCNA, Ki67 and the cell cycle inhibitors p21, p27 and p57 were immunolocalized in tissue sections of first trimester pregnancies (weeks 6 and 9–12). Double staining with cytokeratin 7 allowed unambiguous identification of extravillous cytotropho- blast (EVT) in the decidua. Villous cytotrophoblasts were immunolabelled for Ki67 and cyclin A but only few were stained with anti-cyclin B1. The syncytiotropho- blast was devoid of immunoreactivity for any of the cell cycle progression factors. It expressed especially p21, whereas p27 and p57 were predominantly found in vil- lous cytotrophoblasts. PCNA, Ki67, cyclin A and cyclin B1 were immunolocalized in proximal and distal EVTs of anchoring villi and in EVT which had invaded the upper decidual segments. All EVTs strongly expressed p27 and p57, but not p21. These data clearly suggest different functions for p21, p27 and p57 in placental development with distinct roles for p21 and p57 in syncytiotrophoblast and EVT differentiation, respectively. p27 appears to be involved in both the processes. The results may also challenge the concept of differential mitotic activity in the proximal and distal parts of the first trimester cyto- trophoblast cell column, but more functional studies are clearly needed. The presence of p27 and p57 in EVT cells, which invade the deciduas deeply, may account for the loss of mitogenic potential of these cells. Keywords PCNA Æ Ki67 Æ Cyclin B1 Æ Cyclin A Æ CDK inhibitors Æ Early pregnancy Introduction Human placental development is established by tro- phoblast invasion into the uterine endometrium and its vasculature. The resulting changes will facilitate an in- crease in intervillous blood flow and, hence, the ex- change of nutrients and molecules between maternal and fetal blood. The transport as well as metabolic and endocrine functions of the placenta reside primarily in the floating villi covered by the syncytiotrophoblast, a tissue that results from terminal differentiation of underlying villous cytotrophoblasts and their sub- sequent fusion. Anchoring villi establish physical con- nection of the placenta with the decidua predominantly by a subpopulation of cytotrophoblasts known as ex- travillous cytotrophoblast (EVT). They accumulate at the tips of the anchoring villi and form cell columns. Both villous and extravillous cytotrophoblast sub- populations arise by proliferation and differentiation from stem cells located within the cytotrophoblast layer of the chorionic villi (Kaufmann and Castelluci 1997). On the basis of the immunostaining of the Ki67 antigen, a cell cycle regulator with yet unknown role, EVTs have been categorized as the proliferative phenotype, which is primarily located in the proximal part, and the invasive phenotype that is located mainly in the distal part of cell columns (Kaufmann and Castelluci 1997). Current understanding assumes that EVT can differentiate, thereby acquiring an invasive phenotype, which even- E. T. Korgun (&) Æ C. Celik-Ozenci Æ N. Acar Æ S. Cayli R. Demir Department of Histology and Embryology, Medical Faculty, Akdeniz University, 07070, Antalya, Turkey E-mail: korgun@akdeniz.edu.tr Tel.: +90-242-2274486 Fax: +90-242-2274486 G. Desoye Æ E. T. Korgun Clinic of Obstetrics and Gynecology, Medical University of Graz, 8036, Graz, Austria Histochem Cell Biol (2006) 125: 615–624 DOI 10.1007/s00418-006-0160-y