Inhibin/activin subunits alpha, beta-A and beta-B are differentially expressed in normal human endometrium throughout the menstrual cycle.

Inhibins are dimeric glycoproteins composed of an alpha (alpha) subunit and one of two possible beta (beta-) subunits (betaA or betaB). The aims of this study were to assess the frequency and tissue distribution patterns of the inhibin subunits in normal human endometrium. Samples from human endometrium from proliferative phase (PP; n=32), early secretory phase (ES; n=10) and late secretory phase (LS; n=12) were obtained. Immunohistochemistry, immunofluorescence and a statistical analysis were performed. All three inhibin subunits were expressed by normal endometrium by immunohistochemistry and immunofluorescence. Inhibin-alpha was primarily detected in glandular epithelial cells, while inhibin-beta subunits were additionally localised in stromal tissue. Inhibin-alpha staining reaction increased significantly between PP and ES (P<0.05), PP and LS (P<0.01), and ES and LS (P<0.02). Inhibin-betaA and -betaB were significant higher in LS than PP (P<0.05) and LS than ES (P<0.05). All three inhibin subunits were expressed by human endometrium varying across the menstrual cycle. This suggests substantial functions in human implantation of inhibin-alpha subunit, while stromal expression of the beta subunits could be important in the paracrine signalling for adequate endometrial maturation. The distinct expression in human endometrial tissue suggests a synthesis of inhibins into the lumen and a predominant secretion of activins into the stroma.