Surface of amnion. Scanning electron micrograph (SEM) of the epithelium of the amnion (the sac that protects the embryo), showing secretory epithelial columnar cells. These secretory cells are covered by a vast number of microvilli and secrete amniotic fluid. Amniotic fluid is the watery liquid that surrounds and cushions a growing embryo within the amnion. After the embryo has become a foetus (at 8 weeks), the amniotic fluid enables it to move freely without the walls of the uterus being too tight against its body.