Placenta and vascular development

Placenta and vascular development

1. Vessel overview

   • The outflow tract of the heart includes the conus and truncus arteriosus.
   • The dorsal aorta supplies the head and body wall. It also gives off vitelline (yolk) sac arteries and umbilical arteries.
   • Veins
     • cardinal veins (anterior and posterior) drain the head and body wall
     • vitelline veins drain the yolk sac
     • umbilical veins bring blood from the placenta. (only 1 that ever becomes functional)
     • All three sets of veins on each side flow into the right and left horns of the sinus venosus.

2. Vessel formation

   • Vasculogenesis
     • vessels form in situ by coalescence and differentiation of mesoderm cells into endothelial tubes
   • Angiogenesis
     • is how smaller vessels form
     • New vessels sprout from existing ones
     • Many of the vessels formed by angiogenesis develop as new organ systems differentiate and need to be supplied.
   • Molecular signals for vessel formation involve vascular endothelial growth factors (VEGFs) and their receptors.

3. Aortic arches

   • Main thing with aortic arches is to know that they exist and that some of the numbered arches (e.g. 3rd, 4th and 6th) are responsible for forming some of the major vessels you have already studied
   • There are five well-developed aortic and pharyngeal arches, and each pharyngeal arch contains an aortic arch.
   • The aortic arches are numbered I-VI, but the fifth either never forms or is very rudimentary.
   • The arches are never all present simultaneously. The most cranial ones form and then differentiate as more caudal ones are forming.
   • The aortic arches differentiate into vessels supplying the head and neck and upper limbs.
   • In forming the definitive pattern of arteries in this region, some parts of the arches and dorsal aorta undergo programmed cell death as the vessels regress.
   • Major vessels are formed by the third (carotid system), fourth (subclavian [right] and arch of the aorta [left]), and sixth (pulmonary arteries and ductus arteriosus) aortic arches.

4. Placenta

   • The syncytiotrophoblast forms the invasive layer for implantation and will erode uterine tissue – including blood vessels.
   • Invasion by syncytiotrophoblasts and formation of lacunae
     • Day 9-12
   • Villi
     • Day 13, villi are visible. These consist of a core cytotrophoblasts covered with syncytiotrophoblasts.
   • Foetal component of placenta = trophoblasts and extraembryonic mesoderm.
   • Maternal component = uterine endometrium.
   • Maternal blood reaches the placenta by spiral arteries that penetrate the cytotrophoblast shell and empty into intervillous spaces. Maternal veins drain these spaces.
   • Structure