1 Frontal section of the uterus anduterine tube, showing development7 days after fertilization.

Implantationof blastocyst

Uterinegland

Maternalarteriole

2 Implantation of the blastocyst, withsyncytiotrophoblast beginning toinvade the uterine wall (at about8–12 days).

Syncytiotrophoblastinvading

Cytotrophoblast

Cells thatbecomeembryo

Uterineendometrium

2

Uterineepithelium

Implantedblastocyst

Maternalarteriole

3 Intermediate stage of placentalformation (at about 14–20 days).As maternal blood vessels areencountered by thesyncytiotrophoblast, lacunae formand are filled with maternalblood.

Syncytiotrophoblast

Cytotrophoblast

Maternalarteriole wall

Arteriole walldigested bysyncytiotrophoblast

Lacuna filled withmaternal blood

Connecting stalkdeveloping embryo

Maternalarteriole

Syncytiotrophoblast

4 Cytotrophoblast cords surround thesyncytiotrophoblast and lacunae,and embryonic blood vessels enterthe cord (at about 1 month).

Cytotrophoblast

Lacuna filled withmaternal blood

Cytotrophoblastcord

Connecting stalk todeveloping embryo

Mesoderm

Embryonicvessels forming(contain embryonicblood)

PROCESS FIGURE 29.3 Implantation of the Blastocyst and Formation of the Placenta

As the blastocyst is implanted, syncytiotrophoblasts and cytotrophoblasts invade to form the placenta.

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