The key question in Developmental Biology is how cells make decisions in order to generate tissues and organs in a coordinated manner.

If there is any problem in this cell decision making, the embryo will not grow, resulting in miscarriage even before the woman acknowledges the pregnancy. It has been calculated that 76% of miscarriages happen during this stage of embryo development.

Every animal starts as a single cell (the fertilized oocyte) which starts dividing, to form different groups of cells: each of these groups will form all the tissues present in the adult but also those required to support embryonic growth.

In the first decision, some cells separate from the rest to form the placenta.

In the second decision, cells decide to become part of the embryo proper, while others form the precursors of the yolk sac. In our lab, we are studying this second decision which happens between 3.5 and 4.5 days after fertilization in the mouse (figure 1).

The mouse is an ideal model to understand this process as there are clear similarities with humans and important discoveries were made initially in this model system, before undertaking similar studies in humans.

However, in order to reduce the number of animals used for experimentation, we also use mouse embryonic stem cells (mESCs) which have been modified to allow the study of this decision (figure 2).

The use of mESCs to understand embryonic development is a new approach which is allowing rapid progress in the early embryo development field.