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 cyc
le has come from studies of cultured cells; however, little is known about
its function or even expression in vivo. The present study examined the fea
tures of PLK expression in the mouse placenta and ovary. Immunohistochemica
l studies showed that PLK is highly expressed in the basement membrane of t
he endometrial gland, in some endothelial cells, in endometrium after embry
o implantation, in trophoblastic tissue invading the decidua, in the ovaria
n stroma and in some lutein bodies. In contrast, PLK was not detectable by
immunohistochemistry in endometrial stroma before decidualization, in decid
ua, in trophoblastic tissue not invading the decidua or in ovarian follicle
s. PLK expression seemed to be correlated with the expression of proliferat
ion cellular nuclear antigen (PCNA) in many placental and ovarian cells, re
flecting 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 ce
rtain specialized cell types.