Transport of horseradish peroxidase across monkey trophoblastic epithelium in coated and uncoated vesicles

This study used membranous chorion of the macaque monkey placenta to examine uptake and processing of exogenous proteins. Tissue was incubated with either cationic or anionic horseradish peroxidase. Incubation time was varied between 5–25 min to follow the endocytic pathways. In spite of some differences in binding, uptake and processing of the isozymes was similar. In the presence of tracers at 37°C both horseradish peroxidases were taken up in large (150–175) nm diameter coated vesicles. In addition, coated tubules 300–400 nm in length and 50–100 nm in diameter were seen in the apical cytoplasm. Often, the tubules bud off small (85–105 nm diameter) protein‐filled coated vesicles which traversed the cytoplasm and fused with the basal‐lateral plasma membrane. In other cases, the tubules or vesicles lost their clathrin coats and fused to form larger endocytic vesicles which later fused with phagolysosomes. After longer incubation, larger uncoated vesicles (endosomes) were seen releasing their contents at the basal‐lateral membrane. These results suggest that multiple transport pathways exist in this epithelium. The first, involving only coated structures, may function to sort and concentrate specific ligands important for embryonic development. The second, involving the formation and translocation of large uncoated vesicles to the basal‐lateral membrane, may also provide nutrients to the embryo. A third pathway directs the protein to phagolysosomes where it is presumably degraded. Degradation products could be used by the cells of the membranous chorion or may provide nutrients to the embryo.