Expression and localization of inducible nitric oxide synthase in human non-pregnant and early pregnant endometrium

The aim of the present study was to investigate the expression and distribution patterns of inducible nitric oxide synthase (iNOS) in human non-pregnant and early pregnant endometrium using Northern blot analysis, reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry. Northern blot analysis revealed the expression of iNOS mRNA in human decidua and chorionic villi in the first trimester but not in the endometrium at any stage of the menstrual cycle. Nested RT-PCR, however, detected iNOS mRNA in human endometrium at all stages of the menstrual cycle. Immunohistochemical staining of the secretory endometrium using an anti-human iNOS polyclonal antibody revealed labelling specifically concentrated in glandular epithelial cells. Staining was absent in stromal cells. However, iNOS staining was positive in decidualized stromal cells in tissues obtained in the first trimester of pregnancy. Furthermore, extensive staining was observed in both syncytiotrophoblastic and cytotrophoblastic cells. The finding of a large amount of iNOS mRNA at the feto-maternal interface throughout the first trimester of pregnancy suggests that iNOS may play an important role in the maintenance of pregnancy.