Release of Zona Pellucida.
Prior to implantation, lytic factors in the endometrial cavity are essential in degenerating the zona pellucida that surrounds the blastocyst, otherwise known as “hatching”.
The theory is that a molecule secreted from the blastocyst activates a lytic precursor, most likely plasminogen and is responsible for the degradation of the zona pellucida membrane. This is hypothesised as placing an unfertilised egg within the uterus under same conditions does not elicit this hatching.
Sometimes the zona pellucida is toughed and can therefore cause fertility problems, as implantation cannot occur when the zona pellucida is still intact. The procedure known as 'assisted zona hatching' (AZH) is performed, which involves a small hole made by micromanipulation to the zona pellucida to facilitate this hatching (see link).
Assisted hatching image: www.ivf.com/ivf_hatching.html