<unk> s (from Greek : to feed; and : germinator) are cells that form the outer layer of a blastocyst. They are present four days post-fertilization in humans. They provide nutrients to the embryo and develop into a large part of the placenta. They form during the first stage of pregnancy and are the first cells to differentiate from the fertilized egg to become extraembryonic structures and do not directly contribute to the embryo. After gastrulation, the<unk>  is contiguous with the ectoderm of the embryo and is referred to as the trophectoderm. After the first differentiation, the cells in the human embryo lose their totipotency and are no longer totipotent stem cells because they cannot form a<unk> . They are now pluripotent stem cells.
Trophoblast
Hemangioblast
Nurse cell
Clue cell
Elliptocyte
Echinocyte
Lymphoblast
Neuroblast