The surfacr of syncytiotrophoblast covering primary villi in the early human placenta may be smooth, lacy, exhibit a brush border with highly developed straight or branched microvilli, or display projections. These projections may have microvilli and they may contain granules, vacuoles and mitochondria. By contrast, syncytium of the basal or junctional zone has longer and straighter microvilli and numerous projections often extend as long, recurving streamers which penetrate into and engulf the cytolyzed detritus of decidual cells. These streamers contain granules, vacuoles and microvilli. Granular detritus is evident between the microvilli, adherent to their surfaces and in the bays and superficial vacuoles at or just beneath the surface plasma membrane, indicating that phagocytosis and pinocytosis occur in the basal trophoblast.
In occasional areas in the intervillous space and regularly in the basal junctional zone, clots composed of fibrin fibrils occur. Associated with them are amorphous masses of similar electron density. These represent different degrees of maternal blood clotting, possibly associated with clotting agents released after damage to fetal cells.
Fibrin clots and associated leucocytes adhere to the surface of the syncytium, penetrate into it and between the Langhans' cells, occasionally replacing the trophoblastic layers entirely. In such areas, transudates of maternal blood have direct access to the connective tissues of the fetal villus.