Which layer of the blastocyst actively invades the endometrium?

During the implantation of the blastocyst into the uterine wall, the outer layer of cells called the trophoblast differentiates into two distinct layers: an inner cytotrophoblast and an outer syncytiotrophoblast.

The syncytiotrophoblast is a multinucleated mass of cells without distinct cell boundaries. It secretes proteolytic enzymes that break down the extracellular matrix of the endometrial tissue, allowing it to actively invade the endometrium and establish a connection with the maternal blood vessels for nutrient exchange.

The inner cell mass develops into the embryo proper, while the zona pellucida degenerates prior to implantation to allow the blastocyst to attach to the uterine lining.

Answer: Syncytiotrophoblast