PLACENTA

• The mulit-functional organ that ensures nutrient and gas exchange between maternal and offspring circulation.
• Weeks 2-8
  • Trophoblast differentiation: during implantation, the trophoblast gives rise to the cytotrophoblast and synctiotrophoblast; these layers are pivotal in establishing the fetal side of the placenta.
  • In week 2, the extra-embryonic mesodermal layers arise.
  • The first trimester is relatively hypoxic; though the synctiotrophoblast makes contact with maternal sinusoids, it also plugs maternal vessels to protect the embryo from high levels of oxygen in maternal blood, which is thought to be toxic to early developmental processes.

EVENTS OF WEEK 2

• Includes the final stages of implantation.
  • Fibrin coagulum seals endometrium at implantation site.

Embryonic Tissues of Day 9

• The outermost multi-nucleated synctiotrophoblast, which features spaces, called lacunae.
• Deep to this, is the cytotrophoblast; it comprises a single cell layer.

Recall that the cytotrophoblast and synctiotrophoblast are derived from the trophoblast layer of the blastocyst (more specifically, the trophoblast gave rise to cytotrophoblast cells, some of which then differentiated to form the synctiotrophoblast).

• Bilaminar embryonic disc:
  • The epiblast, which has ectodermal origins, and comprises columnar cells.
  • The hypoblast, which comprises cuboidal cells; because of its endodermal origins, it is also referred to as the primitive endoderm.
• Exocoelomic membrane and cavity
  • Cells from the hypoblast migrate to form the exocoelomic membrane (aka, Heuser’s membrane), which encloses the exocoelomic cavity (aka, primary yolk sac).

Embryonic Tissues of Day 12

• Synctiotrophoblast lacunae make contact with maternal sinusoids.
• Extraembryonic mesoderm separates the exocoelomic membrane and cytotrophoblast.
  • It is thought that this mesodermal tissue originates from the hypoblast, and, perhaps, the trophoblast.
  • The extraembryonic mesoderm closest to the cytotrophoblast comprises the somatic mesoderm.
  • The extraembryonic mesoderm surrounding the exocoelomic membrane is the splanchic mesoderm.
  • Spaces form within the mesoderm; these spaces become continuous to form a true separation between the splanchnic and somatic mesodermal layers.

Embryonic Tissues of Day 13

• Endometrium is healed, fibrin coagulum gone.
• Synctiotrophoblast is greatly expanded into the uterine wall and formed extensive connections with the maternal blood supply.
• The cytotrophoblast begins to transform: it gives rise to primary villi, which extend into the synctiotrophoblast.

As we’ll see, these villi become the functional sites of gas exchange between the mother and fetus.

• Extraembryonic mesoderm:
  • The outer somatic layer of extraembryonic mesoderm is adjacent to the cytotrophoblast.
  • The inner splanchnic layer surrounds the embryo proper.
  • The space between them is the chorionic cavity.
  • Mesodermal connecting stalk joins the two mesodermal layers and suspends the embryo within the chorionic cavity.

This connection will later become the umbilical cord, which carries the umbilical vessels.

• Bilaminar embryo:
  • Epiblast surrounds the amniotic cavity.
  • Primary yolk sac has regressed; the secondary yolk sac now faces the hypoblast.

End of Week 3

By the end of week three, dramatic transformations re-organize these tissues.

• Cells of the cytotrophoblast layer migrated to form the outer cytotrophoblast shell.
• Synctiotrophoblast lacunae are filled with blood from the maternal sinusoids; notice that cytotrophoblast cells surround them.
• Somatic extraembryonic mesoderm has infiltrated the cytotrophoblastic primary villi, and begins to develop embryonic vasculature -> Tertiary Villi
• Thus, the blood-filled lacunae now fill the intervillous spaces(spaces between the tertiary villi).
• Somatic mesoderm gives rise to the chorionic plate, which surrounds the chorionic cavity.

Be aware that we’ve skipped some transitional events that occurred earlier during week 3 (for example, the transition from primary to secondary and secondary to tertiary villi).

Vascular Remodeling & Tertiary Villi

During weeks 4-8, the overall organization of embryonic tissues remains as it is at the end of week 3.
However, extensive vascular remodeling lays the groundwork for the mature placenta.

• Chorionic plate gives rise to the mesodermal cores and blood vessels of the tertiary villi.
• Cytotrophoblast covers the mesodermal core of the tertiary villi, and, also, forms the outer cytotrophoblast shell; cellular columns connect outer shell and tertiary villi.
• Synctiotrophoblast lines the cytotrophoblast of the villi.
• Intervillous spaces are continuous with maternal blood vessels, and lined by synctiotrophoblast.
• As we learn elsewhere, these villi are the sites of gas, nutrient, and waste exchange between the maternal and fetal environments.