Nesting of the fertilized egg

Nesting of the egg

At the beginning of pregnancy, the newly formed embryo inserts itself into the uterine wall thanks to particular 'adhesive' molecules, the production of which 'clicks' after six days from fertilization. 

When the egg is ready for implant in the uterus, its outer surface is filled with a protein, to L-selectin and as long as the concentrations of carbohydrates and L-selectin remain high (that is, up to the 16th week) the embryo is able to 'nest' in the uterine wall.

Read also: Implant losses, what they look like and how long they last

1. Between fertilization and the formation of the blastocisti, about 5-6 days pass, the blastocyst implants itself in the uterus giving rise to the so-called nesting. If the egg came before its transformation into a blastocyst it could not implant. The blastocyst rotates on the uterine surface until the internal cell mass is on the mucosal side. This is the stage where thenesting true.
2. Il trophoblast it divides into 2 layers of cells, an internal one (cytotrophoblast) consisting of a row of cells, and the syncytium trophoblast (consisting of cells that have joined the various cytoplasm, but not the nuclei). The trophoblast syncytium begins to produce lytic enzymes that "digest" the uterine mucosa, and sinks into it branched roots, gradually deeper. The "roots" continue to evolve and partly to merge with each other. The portions of mucosa that remain imprisoned by the fusion of the roots are digested giving rise to the lacunae of the trophoblast syncytium, some of which communicate with each other.
3. At the end of the nesting, at the point where the blastocyst begins its penetration, a "plug" of fibrin is formed, not too strong. At the same time, the gaps are filled with the mother's blood which, if it is found to have high pressure, partially escapes from the fibrin plug. This takes about 28 days after fertilization, therefore any blood stains that many women find and that would suggest a new menstrual cycle is coming, actually represent the symptom of an ongoing pregnancy (otherwise called implantation losses).
4. In the meantime, the internal cell mass begins to cavitate, forming inside pockets full of amniotic fluid that easily expand, giving rise to 3 cell layers and constituting the first embryonic annex (ammios).
5. THEendoderma it begins to proliferate by sliding along the cytotrophoblast and, at the same time, another membrane (exocelomic) differs and develops until it reaches the endoderm. At this point we will have the primary yolk sac. The ammios defends the embryo from shocks. The primary yolk sac contains reserve material and has only a trophic function. It is not, however, particularly important in the human species.
6. At this point the extraembryonic mesoderma, i.e. a new cell layer similar to a very soft connective tissue, which tends to rise completely covering the two internal vesicles