placental villi Flashcards
when does development of the circulatory system begin?
on day 9 by the formation of trophoblastic lacunae within the syncytiotrophoblast. maternal capillaries are eroded and blood fills lacunar spaces.
why is circulatory system necessary?
for efficient exchange of gases, nutrients and waste.
how does extraembryonic mesoderm work in formation of villi?
extraebryonic mesoderm has developed underneath the cytotrophoblast layer and this mesoderm induces local regions of the cytotrophoblast to proliferate and extend towards the lacunae with a covering of syncytiotrophoblast.
what mediates localized cell proliferation often?
molecules such as fibroblast growth factor
what are initial extensions of cytotrophoblast into syncytiotrophoblast called?
primary villi (primary stem villi). each individual extension is a primary villus.
when do secondary villi form and how?
by day 16, extraembryonic mesoderm invades the core of primary villi
when do tertiary villi form and how?
at the end of the 3rd week, extraembryonic mesoderm within the villi and in the chorionic plate differentiates into circulating blood cells and the endothelial cells of blood vessels.
how is a functional uteroplacental circulation established?
the blood vessels anastomose (fuse) with embryonic vessels in the connecting stalk - two arteries and one vein within the umbilical cord. There is a also a newly beating embryonic heart.
what vessels exist in umbilical cord?
two arteries and one vein
what must gases, nutrients, water, waste, etc., INITIALLY cross?
3 cell layers of a mature villus
A. endothelium of the villous capillaries
B. Cytotrophoblast which becomes intermittent later in pregnancy
C. Syncytiotrophoblast
