Objectives: To study collagen phagocytosis by human extravillous trophoblast.

Methods: First-trimester extravillous trophoblastic cell lines and primary trophoblast cell preparations were cultured in vitro with collagen-coated fluorescent latex beads and fluorescent-labeled collagen. Confocal microscopy was used to demonstrate internalization of collagen and beads. The effect of cytochalasin B, temperature, metabolic inhibitors, and cytokines was studied by culturing trophoblast cells with tritated collagen. Acridine orange was used to stain for lysosomal compartments, and histochemical methods were used to demonstrate acid phosphatase in trophoblast cells.

Results: Both cell lines and primary culture cells internalize collagen and beads. Confocal microscopy unequivocally localized the phagocytosed material to the intracellular compartment. Inhibition by cytochalasin B and culture at 4C of uptake of [³H] collagen suggested that the process was phagocytosis. Cytokines and growth factors did not affect phagocytosis. Lysosomal compartments and acid phosphatase appear to colocalize.

Conclusions: The continuous remodeling and turnover of collagen which occur in a wide variety of tissues under both physiologic and pathologic conditions are thought to be mediated by two pathways: one external (involving release of proteolytic enzymes), and the other internal (involving phagocytosis). Similar remodeling events are likely to occur during trophoblast invasion. Although current views emphasize the importance of the extracellular pathway, we postulate, on the basis of our findings, that both pathways are used, with the internal pathway probably being dominant. We hypothesize that the proteolytic enzymes (extracellular pathway) disrupt collagen matrices, thereby facilitating phagocytosis. It is teleologically sound to conceive of a dominant interacellular pathway, as it allows for more precise control of the process of invasion and is economical, as the products of collagen degradation can be used as energy sources or building blocks.