Invasion

Embryo Invasion into Endometrium.

Rapid proliferation of the trophoblast occurs when it attaches to the endometrial lining and penetrates it. 

The trophoblast differentiates into two layers: the deep layer becomes the cytotrophoblast and the superficial layer becomes the syncytiotrophoblast, a multinucleate mass (without plasma membranes). 

Tumor necrosis factor-α (TNF-α) is secreted by protrusions from the syncytiotrophoblast and causes the digestion of endometrial decidual cells, as it interferes with cell adhesion molecules (cadherins and β-catenin).  These degenerating decidual cells provide nutrients, lipid and glycogen, for the developing embryo. 

The long protrusions grow between the uterine epithelial cells by digesting the extracellular matrix by secreting proteases.  They continue to do this till ultimately reaching the uterine stroma and making contact with the maternal blood supply.  These protrusions represent an undeveloped form of choroionic villi of the mature placenta.   

The endometrium reacts to this injury by growing over the trophoblast to enclose it. 

Image courtesy of Wikipedia Commons from Gray's Anatomy under public domain: en.wikipedia.org/wiki/Image:Gray36.png