Schematically, three implantation stages can be distinguished:·
- Adplantation of the blastocyst on the endometrium
- Adhesion of the blastocyst to the endometrium
- Invasion of the trophoblast and embedding
Adplantation of the blastocyst on the uterine mucosa
When the blastocyst emerges from the pellucid zone on the 5th day, it comes into contact with the maternal uterine mucosa in that it embeds itself in the endometrium with its embryonic pole.
The adhesion can occur when beforehand the uterus has entered its secretory phase (luteinizing phase). This reception-ready phase of the endometrium lasts 4 days (20th -23rd day) and is usually termed the "implantation window". It follows around 6 days after the LH peak and is characterized by the appearance of small elevations at the apical pole of the epithelial endometrium cells. One of the tasks of these elevations consists in the absorption of the uterine fluid, which brings the blastocyst nearer to the endometrium and immobilizes it at the same time. In this stage the blastocyst can still be eliminated by being flushed out. There is also a hypothesis that the progesterone and the oestrogen are responsible for an oedema that already fills the flattened out uterine cavity. This is also supposed to contribute to the blastocyst being pressed against the uterine epithelium.
Adhesion of the blastocyst to the endometrium
After the apposition of the free blastocyst at the uterine epithelium the microvilli on the surface of the outermost trophoblast cells interact with the epithelial cells of the uterus. In this stage the blastocyst can no longer be eliminated by a simple flushing out. The adhesion of the blastocyst on the endometrium arises through cell surface glycoproteins, the specific mechanisms of which, though, are not yet well understood.
Invasion of the trophoblast and embedding
Thedifferentiates into two different cell masses, shortly before it comes into contact with the endometrium:
- the outer syncytiotrophoblast (ST)
- the inner cytotrophoblast (CT)
The cytotrophoblast, deep inside, consists in an inner irregular layer of ovoid, single- nucleus cells. This is also where intensive mitotic activity takes place.
In the periphery the syncytiotrophoblast forms a syncytium, i.e., a multi-nucleic layer without cell boundaries that arises from the fusion of cytotrophoblast cells. The syncytiotrophoblast produces lytic enzymes and secretes factors that cause apoptosis of the endometrial epithelial cells. The syncytiotrophoblast also crosses the basal lamina and penetrates into the stroma that lies below, eroding the wall of capillaries. With the implantation of the blastocyst in the endometrium the syncytiotrophoblast develops quickly and will entirely surround the embryo as soon as it has completely embedded itself in the endometrium.
The uterine mucosa reacts to the implantation by the decidual reaction. The syncytiotrophoblast cells phagocytize the apoptotic decidual cells of the endometrium and resorb the proteins, sugars and lipids that have been formed there. They also erode the canals of the endometrial glands and the capillaries of the stroma.
In the middle of the 2nd week extracellular vacuoles appear in the ST. They join together forming lacunae. Initially these lacunae are filled with tissue fluids and uterine secretions. Following the erosion of the maternal capillaries, their blood fills the lacunae that later develop further into intervillous spaces. The invasive growth of the ST ceases in the zona compacta of the endometrium. At around the 13th day the primitive utero-placental circulatory system arises.
At the end of the 2nd week, when implantation has ended, the embryonic bud consists schematically of two hemispheric cavities that lie on one another: the amniotic cavity (dorsal) and the umbilical vesicle (ventral).
The floor of the amniotic cavity is formed by the epiblast, and the roof of the umbilical vesicle by the hypoblast. These two layers, which lie on one another, form the embryo or the double-layered embryonic disc.