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What is the process of embryo implantation?

The embryo starts to hatch on days 5 and 6 of development and continues to do so until it separates from its zona pellucida, the outer protein membrane that served as protection for it in the early days following fertilization. The zona pellucida thins as the embryo grows until it eventually disintegrates. Through a series of contractions, the embryo eventually escapes from it and starts interacting with the endometrium. Blastocyst that have already hatched can be transplanted to the uterus in patients undergoing IVF treatment, either because hatching has already taken place in culture or because aided hatching has been carried out. Egg implantation may be favored by assisted hatching.

Apposition

The embryo is immobile during this phase as it locates itself on the endometrial tissue and is oriented so that its internal cell mass faces the endometrium to allow for the proper formation of the placenta later on. The alleged pinopods are crucial in this situation: These are endometrial epithelial cells’ cytoplasmic projections, which provide interaction between the blastocyst and its environment. The presence of these pinopods, which only form during the implantation window and vanish around the 24th day of the cycle, is evidence that they are distinct morphological markers of endometrial receptivity.

Adhesion

The endometrial epithelium and the blastocyst trophectoderm are joined by the adhesion molecules integrins 1, 3, and 4, L-selectins, proteoglucans, fibronectins, etc. Around 7 days after fertilization, when the blastocyst already measures 300–400 m in diameter, this takes place. Although the process of implanting a blastocyst in the mother’s endometrium may appear straightforward at first glance, it is actually very complicated and not entirely understood.