Time lapse sequence (starts bottom centre then moves clockwise) of the formation of the inner mass in an early mouse embryo. Two photon microscopy was used to visualise live mouse embryos during preimplantation development, the time between fertilisation and implantation of the embryo into the mother's uterus (womb). Whole mouse embryos were digitally segmented and changes in each cell's shape and position over time was tracked in 4D. Here, a cell (grey) in an early 8-cell embryo (neighbouring cells are blue) undergoes division to form two daughter cells (yellow and red). Over time, one of the daughter cells (red cell) moves from the surface to the inside of the embryo. This process of internalisation takes approximately 4-5 hours and each snapshot in the time-lapse sequence is about 45 minutes apart. The early mouse embryo is approximately 90 micrometres in diameter.
Mammalian embryos start out as a small group of identical cells. At an early stage, during the transition from 8 to 16 cells, some cells take up an internal position within the embryo. These internal cells will go on to form all of the cells in the body while the outer cells go on to form other tissues such as the placenta. The internal cell adopts its position in the embryo through a special process of cell division.
