6.2 Implantation stages



Stages

Quiz

Quiz 10

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 (3), it comes into contact with the maternal uterine mucosa in that it embeds itself in the endometrium with its embryonic pole. Fig. 15 - Hatching  Legend

1
2
3

4
5		 Pellucid zone
Trophoblast (outer cell mass)
Hypoblast (part of the inner cell
mass)
Blastocyst cavity
Epiblast (part of the inner cell
mass)

Fig. 15
The blastocyst hatches out of the partially dissolved zona pellucida. One distinguishes between the trophoblast (the outer cell mass) and the embryoblast (inner cell layer with epi- and hypoblasts) along with the blastocyst cavity.


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 4.


Fig. 16 - Implantation window  Legend

A
B
C
D		 Menstruation
Proliferation
Secretion
Implantation window

Fig. 16
Menstruation cycle with the cyclic alterations of the endometrium. The "implantation window" that corresponds to the period of maximum receptivity is depicted here (D).


Quiz

Quiz 02


More info

Pellucid zone
During the whole time span from ovulation up to implantation, the oocyte is enveloped by the pellucid zone, the role of which changes. Initially it is united with the cells of the corona radiata during the transport of the oocyte within the fallopian tube. At the moment of fertilization it facilitates the acrosomal reaction of the sperm cells. Finally, after the cortical reaction has taken place, it undergoes physical and chemical changes. The pellucid zone possesses no HLA antigen and thus acts as an immunological barrier in relation to the mother. Another important role is the prevention of a premature implantation of the embryo in the tube region.



Adhesion of the blastocyst to the endometrium

After the apposition of the free blastocyst at the uterine epithelium (4) 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. Fig. 17 - Adhesion of the blastocyst  Legend

Fig. 17
Hatching of the blastocyst and adhesion on the endometrium. One sees the cells of the syncytiotrophoblast that grow in between the cells of the uterine epithelium.


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