Periderm week 11

Around week 10-11, the periderm cells have increased volume and a central bleb develops. They are still densely covered by microvilli.

Central bleb forms

This scanning electron micrograph shows the amniotic fluid surface of the periderm. Note how the cells are tightly packed, like cobblestones. The surface appears rough because of the microvilli. Some cells have larger whitish blebs appearing on the surface (you may need to zoom in maximally to appreciate this).