After initial cell division, the embryo moves to the womb where it embeds. This process is called implantation. Implantation and subsequent development of the placenta enables the embryo to obtain nutrients and oxygen from the mother. The embryo also passes carbon dioxide and other waste products to the mother via the placenta.

(above) Embryo (shown in yellow) implanting in the wall of mother’s womb Image © Wellcome Images

If an embryo does not implant, the lining of the womb is shed. Most women know this process as their period. Embryo implantation triggers a hormone called hCG (human chorionic gonadotrophin) which stops this process. Image © Human blastocyst,  Gillian Morriss-Kay, University of Oxford		Just before the embryo implants it is known as a blastocyst and is composed of an inner cell mass encircled by an outer cell layer of longer thinner cells (picture). While these outer cells contribute to the placenta, the inner cell mass will give rise to the embryo and is the source of embryonic stem (ES) cells. These cells are pluripotent, which means they can give rise to all cell types in the body – and they are the basis of many recent advances in regenerative medicine. Find out more: ISSCR What are embryonic stem cells? International Society for Stem Cell Research (ISSCR) Scottish Stem Cell Network Key labs that work on stem cells: Austin Smith Janet Rossant Rudi Jaenisch Shinya Yamanaka Key labs that work on implantation: University of Edinburgh Centre for Reproductive Biology Recent reviews: Recurrent implantation failure: the role of the endometrium Wnt signaling in implantation

Implantation dress: Silk jersey, silk chiffon The white part of the dress has been embroidered to show the embryo implanting into the uterus wall (black). Drawing 1997 Copyright Helen Storey