Thevitelline arteriesconnect the yolk sac to the abdominal dorsal
aorta. They will form the arteries of the GI tract: celiac, superior mesen-
teric,andinferior mesenteric.
Blood islandsare the first sites of hematopoiesisand seed other
hematopoietic tissues.
The paired umbilical arteriesdevelop from the caudal end of the dor-
sal aorta and invade the mesoderm of the placenta. They carry deoxy-
genated blood from the fetus to the placenta.
Thecaval venous systemis derived mostly from the right anterior
and posterior cardinal veins.
Thevitelline veinsform the veins of the digestive system, including
theportal vein,and the terminal part of the inferior vena cava.
No components of the umbilical veinsremain patent after closure of
the ductus venosus.
DEVELOPMENT OF THE HEMATOPOIETIC SYSTEMOnset of hematopoiesisbegins with formation of blood islands in the wall
of the yolk sac(derived from the hypoblast) during week 3.
Pluripotent stem cells from the blood islands seed the other hematopoi-
etic sites. These are, in succession, the liver(week 5), spleen(week 5), and
bone marrow(month 6).
All components of hematopoietic organs are derived from mesoderm
except for the epithelium of the thymus,which is derived from endo-
dermof the third pharyngeal pouch.
DEVELOPMENT OF THE DIGESTIVE SYSTEMThe epithelium of the digestive tract and associated organs is formed by the
endodermal tube, whereas connective tissue and smooth muscle are
derived from splanchnic lateral plate mesoderm.The mesoderm induces
regional specialization in the endoderm.
The midgut endoderm is the last to fold into a tube and remains con-
nected to the yolk sac via the yolk stalk.
Formation of the mesodermal urorectal septumdivides the cloaca
into the urogenital sinusandprimitive rectum.
Cell proliferation results in closure of the endodermal tube lumen dur-
ing week 6. The lumen is reopened by recanalizationin week 8.
Failure to recanalize can result in stenosis,preventing the passage of
amniotic fluid swallowed by the fetus causingpolyhydramnios.
High-Yield Facts 9