Hatching of the blastocyst

Hatching blastocyst.

Hatching blastocyst.
Fully hatched blastocyst. The embryo is ready for implantation at this stage.

Fully hatched blastocyst. The embryo is ready for implantation at this stage.

The blastocyst is still surrounded by the zona pelucida. However, as the blastocyst expands, the zona pelucida becomes thinner due a coordinated pressure from blastocoels expansion and enzymatic digestion. Finally the zona pelucida is ruptured and the blastocyst starts to hatch. This is a vital step for the successful implantation of the embryo on the endometrium.

It is believed that blastocyst hatching may be a possible mechanism of twinning. If this embryo splits in two during the process of hatching, it is possible that the two resulting embryos will replace the missing cells and develop as two identical but separate organisms (mono-oogenic twins, that have the same sex and are a genetic clone of each other).

Hatched blastocyst. The embryo has completely escaped the zona pelucida and is now ready for implantation (Eugonia archive).

Hatched blastocyst. The embryo has completely escaped the zona pelucida and is now ready for implantation (Eugonia archive).