Nitric oxide synthase I immunoreactivity and NOS-associated NADPHd histochemistry in the visceral epithelial cells of the intraplacental mouse yolk sac.

In the course of our studies on the local blood flow modulation in the NMRI-mouse placenta we have focussed on regulatory pathways involving recently appreciated gaseous messenger molecules nitric oxide (NO) and carbon monoxide (CO), which are generated by NO synthase (NOS) and heme oxygenase (HO)-2, respectively. The distribution of NOS was investigated by immunohistochemistry using an antiserum to the neuronal isoform (NOS-I) and by NADPH diaphorase (NADPHd) histochemistry, supplemented with procedures (permanganate and formaldehyde method) serving to enhance the specificity of the enzyme histochemical method for NOS visualization. HO-2 was demonstrated immunohistochemically. In addition, cyclic guanosine monophosphate (cGMP)-forming soluble guanylate cyclase (sGC) and dehydrogenases generating the NOS co-substrate NADPH were analysed either by immunohistochemistry or enzyme histochemistry. NOS-I immunostaining was observed in the intraplacental visceral yolk sac epithelial cells but not in the placenta and extraplacental visceral epithelial yolk sac cells. Co-localization of NOS-I immunolabeling and NOS-associated NADPHd was exclusively found in the intraplacental visceral epithelial cells, while NADPHd activity not associated to NOS was present in other placental and extraplacental cells additionally analysed for control reasons. HO-2 and sGC immunoreactivity could not be detected in the placenta including the intraplacental visceral epithelial cells but were expressed in several extraplacental cells. Dehydrogenases producing the NOS co-substrate NADPH were present in the intraplacental visceral epithelium as well as in other placental and extraplacental cells. Since the intraplacental visceral epithelial yolk sac layer closely accompanies large fetal blood vessels entering the placental labyrinth from the chorionic plate it may be assumed that NO, generated by the NADPH-consuming NOS-I in the intraplacental yolk sac epithelium, acts to regulate the blood flow by relaxing smooth muscle cells in the wall of these fetal vessels. The lack of immunoreactivity to the NO-effector molecule sGC may be due to methodological reasons. The absence of the HO-2/CO system suggests its insignificant role as a potential gas signaling pathway in the vascular smooth muscle system of the intraplacental visceral yolk sac of mice.