Alkaline phosphatase distribution in rat endometrial epithelium during early pregnancy: a scanning electron-microscopic study.

The participation of apical membranes of uterine epithelial cells in the process of blastocyst adhesion makes them an interesting object in the study of changes occurring during early pregnancy. In the study of these changes alkaline phosphatase (AIP), a typical brush border enzyme, was chosen for demonstration with the scanning electron microscope (SEM) by means of a backscatter detector. Thus the temporal and spatial pattern of enzyme activity on the uterine luminal surface was made visible with lead salt procedures. AIP activity was shown to be located on apical membranes and microvilli of endometrial epithelial cells with high activity on day 2 of pregnancy decreasing to virtually no activity on day 5. This decrease in overall AIP activity was shown to be asymmetrical with respect to the uterine cavity. It begins on the antimesometrial half of the uterine lining on day 2. A distribution pattern demarcating a presumptive implantation site along the uterine horn was not found. However, on day 5 of pregnancy, a characteristic pattern of surface folds was found, dividing the uterine horn into 'implantation segments'. In addition, SEM investigation revealed a marked variation of AIP activity from one individual cell to the next on day 2 of pregnancy resulting in a mosaic-like pattern. This pattern is lost with the decrease of AIP activity on day 5. Thus heterogeneity of uterine epithelial cells in AIP activity is apparently a feature of nonreceptive epithelium in contrast to the homogeneous epithelium on day 5. It is proposed that epithelial cell homogeneity could be a marker for uterine receptivity.