Extraembryonic cells of trophoblastic shell surrounding embryo, outside the cytotrophoblast layer, involved with implantation of the blastocyst by eroding extracellular matrix surrounding maternal endometrial cells at site of implantation, also contribute to villi. (dark staining, multinucleated). [ http://embryology.med.unsw.edu.au/Notes/placenta5.htm http://en.wikipedia.org/wiki/Syncytiotrophoblast ]