10.1 Early development and implantation

• Early development
• Implantation
  • The trophoblast
  • The embryoblast
    

The trophoblast

During implantation the trophoblast differentiates itself into two layers:

• the syncytiotrophoblast (ST)
• the cytotrophoblast (CT)

The syncytiotrophoblast forms an external layer without intercellular boundaries (syncytium) and its cells form cords infiltrating the endometrium. The syncythiothrophoblast derives from the fusion of cytotrophoblast cells. Through apoptosis (programmed cell death) of uterine stromal cells spaces are created through which the blastocyst penetrates further into the endometrium.
The cytotrophoblast, on the other hand, consists of an irregular layer of ovoid, mono-nucleated cells and lies directly below the syncytiotrophoblast.

Fig. 3
Free blastocyst (after the dissolution of the zona pellucida) in the phase of its adhesion to the uterine wall (6th to 7th day after ovulation). The trophoblast cells of the embryonic pole differentiate and multiply and form the invasive syncytio-
trophoblast. The abembryonic pole consists of cytotrophoblast cells.
Fig. 4
Bilaminar embryonic disk (hypoblast and epiblast) after 8 days. The amniotic cavity appears in the epiblast. The ST continues its invasive activity into the maternal tissue.

In the middle of the 2nd week irregular spaces appear in the syncythiothrophoblast. They then come together to form lacunae. These lacunae are originally filled with tissue fluids and uterine secretions.

Fig. 5, Fig. 6
Complete implantation of the embryo in the endometrium and the covering of the implantation location by a fibrin plug. The amniotic cavity enlarges and a layer of cells (amnioblasts) separates them now from the CT.
The hypoblast cells also begin to multiply and partly coat the interior of the blastocyst cavity 5b.
Irregular spaces appear in the syncythio-
throphoblast and join together into lacunae.

The ST grows more and more into the uterine wall and erodes the vessel walls (capillaries, venules). The blood that circulates in them is delivered into the lacunae 5c.

Fig. 7, Fig. 8
The destructive activity of the ST reaches the capillaries of the endometrium. Maternal blood flows into the lacunae. The ST envelops the maternal capillaries, enlarges its network of lacunae and forms an arterial in- and venous outflow system.