<P> Lytic factors in the uterine cavity, as well as factors from the blastocyst itself are essential for this process . Mechanisms in the latter are indicated by that the zona pellucida remains intact if an unfertilized egg is placed in the uterus under the same conditions . A substance probably involved is plasmin . Plasminogen, the plasmin precursor, is found in the uterine cavity, and blastocyst factors contribute to its conversion to active plasmin . This hypothesis is supported by lytic effects in vitro by plasmin . Furthermore, plasmin inhibitors also inhibit the entire zona hatching in rat experiments . </P> <P> The very first, albeit loose, connection between the blastocyst and the endometrium is called the apposition . </P> <P> On the endometrium, the apposition is usually made where there is a small crypt in it, perhaps because it increases the area of contact with the rather spherical blastocyst . </P> <P> On the blastocyst, on the other hand, it occurs at a location where there has been enough lysis of the zona pellucida to have created a rupture to enable direct contact between the underlying trophoblast and the decidua of the endometrium . However, ultimately, the inner cell mass, inside the trophoblast layer, is aligned closest to the decidua . Nevertheless, the apposition on the blastocyst is not dependent on if it is on the same side of the blastocyst as the inner cell mass . Rather, the inner cell mass rotates inside the trophoblast to align to the apposition . In short, the entire surface of the blastocyst has a potential to form the apposition to the decidua . </P>

Where does implantation of the blastocyst take place