Expression of polo-like kinase (PLK) in the mouse placenta and ovary

The polo-like kinase (PLK) is a mammalian serine/threonine kinase involved in cell cycle regulation. Much evidence for the role of PLK in the cell cycle has come from studies of cultured cells; however, little is known about its function or even expression in vivo. The present study examined the features of PLK expression in the mouse placenta and ovary. Immunohistochemical studies showed that PLK is highly expressed in the basement membrane of the endometrial gland, in some endothelial cells, in endometrium after embryo implantation, in trophoblastic tissue invading the decidua, in the ovarian stroma and in some lutein bodies. In contrast, PLK was not detectable by immunohistochemistry in endometrial stroma before decidualization, in decidua, in trophoblastic tissue not invading the decidua or in ovarian follicles. PLK expression seemed to be correlated with the expression of proliferation cellular nuclear antigen (PCNA) in many placental and ovarian cells, reflecting a role in cellular proliferation. Nevertheless, in ovarian stroma and lutein bodies where PCNA was not expressed, PLK was strongly expressed. This finding indicates that PLK may have some post mitotic functions in certain specialized cell types.