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Blastocyst embryo

Agnieszka Jedrusik and Magdalena Zernicka-Goetz, Gurdon Institute, Cambridge

Blastocyst embryo

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Three-dimensional reconstruction of a mouse embryo at the blastocyst stage. The embryo was labelled with green fluorescent protein to mark the membrane of the cells and scanned using a confocal microscope. Multiple virtual sections through the embryo were recorded every 0.4 micrometres and reconstructed using 3D computer software.

What is a blastocyst?

A blastocyst is the developmental stage just before the embryo implants into the uterus. After fertilisation, the embryo starts to divide and cell numbers double until they form a compact ball of cells called a morula, after which the blastocyst is formed. In the mouse, the blastocyst is observed 3-4 days after fertilisation (5-6 days in humans). At this stage, the cells begin to differentiate into two types: the innermost cells (shown in red) are those that will go on to form the fetus and make the body of the mouse. The cells shown in white make up the trophectoderm, which will go on to form the supporting cells in the extraembryonic tissue (including the placenta).

Why are the red cells important?

The cells shown in red are called the inner cell mass and will give rise to the embryo. These cells are pluripotent, meaning they have the potential to form every cell type in the body. This quality makes them very important in the developing embryo and lots of research is attempting to understand more about what makes these early embryonic cells so special.

Stem cell research attempts to unravel the complex network of genes and signals that can turn a cell with no defined developmental fate into one that has strict instructions to differentiate into a particular cell type. Understanding this can help advancement of medical techniques, including stem cell therapy to repair or replace damaged cells.