Midkine (MK) expression in extraembryonic tissues, amniotic fluid, and cerebrospinal fluid during mouse embryogenesis

Midkine (MK) and pleiotrophin (PTN) constitute a new family of heparin-binding growth factors. Extraembryonic membranes and the placenta of the mouse expressed MK mRNA at 11.5 days gestation. While the MK mRNA level in extraembryonic membranes decreased during embryogenesis, that in the placenta remained unchanged. Immunohistochemical studies showed that Mk was located in the yolk sac and in the amnion at 11.5 days gestation. PTN mRNA expression was weak in extraembryonic membranes and was scarcely detectable in the placenta. Western blot analysis revealed the presence of MK in amniotic fluid and cerebrospinal fluid, in amounts of more than 1 microgram/ml, raising the possibility that MK delivered by these fluids participates in the regulation of organogenesis. Transport of MK from the site of its synthesis appears to also occur in the adult kidney, since MK mRNA and the MK protein are localized in different regions of the kidney.