Sialic acids are involved in many cellular interactions. They can serve as an adhesion ligand or act as an inhibitor to cellular adhesion by charge repulsion or by masking potential ligands. Although sialic acids are implicated in the process of blastocyst implantation, their expression and regulation in uterine epithelium of mice have not been studied. The lectin, Sambucus nigra (SNA) specifically recognizes ��2,6-linked sialic acids, which are involved in cell recognition phenomena. It was used to probe frozen uterine sections from mice during days one through six of pregnancy. SNA staining was most intense at the apical surface of uterine epithelial cells on day one of pregnancy, decreased gradually through day four, and was undetectable by day five. The role of the steroid hormones, estrogen and progesterone, in regulating the expression of ��2,6-linked sialic acids was studied in uterine sections from mice during the estrous cycle and in ovariectomized mice given hormone replacement using SNA. SNA staining of these sections during the estrous cycle showed that the expression of ��2,6-linked sialic acids was stage dependent. Staining was most intense in uterine sections from mice in estrus, and was not detected in sections from mice in diestrus. In ovariectomized mice, staining was most intense in mice injected with estradiol alone, and no staining was evident in mice injected with progesterone alone. These results suggest that the expression of ��2,6-linked sialic acids decreases during the time of implantation and that estrogen stimulates and progesterone inhibits its expression. ��-Galactoside ��2,6-Sialyltransferase (��2,6-ST) is the enzyme that links sialic acids to Gal��1-4GlcNAc termini of N-linked oligosaccharides. In order to investigate the mechanism behind the hormonal regulation of ��2,6-linked sialic acids, the expression of ��2,6-ST was followed in uterine sections from mice during early pregnancy, during the estrous cycle, and in ovariectomized mice given hormone replacement. In-situ hybridization was performed using digoxigenin labeled RNA probes to characterize ��2,6-ST mRNA levels in uterine sections. Expression of ��2,6-ST protein was also measured in uterine sections with a polyclonal antibody against ��2,6-ST. The expression of ��2,6-ST mRNA and protein correlated well with the timing of the appearance of ��2,6-linked sialic acids. These results show that the expression of ��2,6-linked sialic acids on the surface of mouse uterine epithelium decreases at the time of implantation and furthermore, that this decrease is due to the regulation of ��2,6-ST by the steroid hormones. ��2,6-linked sialic acids may serve to inhibit cellular adhesion by creating a charge repulsion, or by masking potential binding sites. Removal of this inhibition may permit blastocyst implantation. / Graduation date: 2001