Follow the high pressure pathway from the
ventricles into the aortic system.
Start from the cardiac apex on D7 and check that
no heart structure is seen below this level in
Apex of left ventricle: spongy network of
endocardium; a small dorsal cavity (part of the
intra-embryonic (I-E) coelom which will become the
pericardial cavity - no pericardium yet);
mesenchymal jelly of body wall.
Tip of right ventricle; left ventricle; I-E coelom;
liver, with its ventral transverse border, the
Trabeculae of L and R ventricles; interventricular
septum; interembryonic coelom; sinus venosus caudal
tip of part of left atrium = L auricular
R auricular appendage. Endocardial jelly at dorsal
wall of interventricular septum.
Ventricles; incomplete interventricular septum;
dorsal endocardial jelly; communication of l
auricle and L atrium; R atrium; R auricle.
Aperture in interatrial septum, the ostium
(foramen) primum. L and R atrioventricular
Communication of L auricle with left atrium.
Transition from R ventricle to the outflow tract,
including the truncus arteriosus complete
interatrial septum (cf. C5).
Truncus arteriosus. Atria. Right venous valve in R
Return to C7 and proceed to C3, noting how left
ventricular blood has to pass obliquely across
right ventricle to exit from heart via the outflow
tract and truncus arteriosus.
C1 Truncus arteriosus shifting to midline.
Jelly and mantle of the truncus. Cranial end of R
venous valve. Note also extent of I-E coelom; thin
Attachment of truncus to ventral body wall and to
dorsal roof of intra embryonic coelom. Note the
anlage of the transverse pericardial sinus in B6,
caudal to the attachment of the truncus.
Entry of truncus arteriosus into aortic sac,
completely embedded in pharyngeal arch mesectoderm.
Note position of aortic sac in relation to pharynx
and pharyngeal arches. 4th pharyngeal arch artery
on left. Return to B5 and note the small "6th"
pharyngeal arch artery either side of the
Cranial end of aortic sac. 4th pharyngeal arch
arteries and emerging 3rd pharyngeal arch artery.
Note dark thyroid primordium ventral to origin of
3rd arch arteries.
(lst and 2nd pharyngeal arch arteries not seen. 3rd
arch arteries. Bilaterally, communication of 4th
arch artery (at sides of pharynx) with dorsal
3rd arch arteries. Dorsal aortae. (Superior
cardinal veins lateral to aortae).
On left side, communication of 3rd arch artery with
aorta (i.e. occurring cranial to the 4th arch
communication). From here on the arterial blood is
distributed through fine branches to vessels
outside the brain (pial plexus).
Follow the caudal path of the arterial
blood Noting the position of the dorsal aortae, go
back down the embryo from A7 to D3.
Tiny dorsal branches of paired dorsal aortae.
(Inferior cardinal veins lateral to aortae, also
with small dorsal branches). move to D6.
Dorsal aortae close together.
Fusion of dorsal aortae into a midline vessel.
Dorsal branches of single dorsal aorta.
Dorsal aorta giving rise to a ventral branch
(mesenteric artery), and branch to mesonephros (the
Dorsal aorta sending branch into mesonephros (to
Dorsal aorta curving around flexed trunk of embryo.
Note spinal cord cut twice.
Sections gradually passing dorsally beyond
curvature of single dorsal aorta.
Return to G4. We now will follow the aorta in
the lumbar region of the embryo as it passes into
the sacral region.
Single dorsal aorta ventral to sacral (smaller)
Bifurcation into paired dorsal aortae.
Lateral major branch of dorsal aorta becomes
umbilical artery: it reaches a crest in E5 (i.e.
not seen in E4) and then "descends" in the ventral
body wall to F2, where it enters the body stalk. We
will follow this complex twisting again when we
study the development of the hindgut.
Follow the umbilical and vitelline (yolk
sac) venous inflow to the heart. Start with F6.
Oblique section through both umbilical veins in
body wall. Follow them to E6.
Both umbilical veins with a large vitelline vein in
the central, mesenteric region.
Umbilical veins enter liver, along with vitelline.
Most venous blood drains through the liver region
into the sinus venosus, which nestles in the
cranial surface of the growing liver. This is the
site of the caudal attachment of the free heart to
the body mesenchyme.
Entry of venous blood into sinus venosus. Note
smaller left horn of sinus
venosus, and location of sinus venosus caudal to
Drainage of right horn of sinus venosus into right
We now trace the low pressure pathway of the
embryo's venous blood.
Identify the large veins lateral to dorsal aortae -
these are the superior or anterior cardinal veins
which drain the head region. Now go directly to
Identify the large veins lateral to dorsal aortae -
these are the inferior or posterior cardinal veins
which drain the lower part of the embryo.
Follow alternately, the course of the
superior cardinal veins caudally, and the course of
the inferior cardinal veins cranially until they
emerge about Dl -C5 into the common cardinal
Left common cardinal entering sinus venosus.
Right common cardinal entering sinus venosus.
Finally examine the atria in more detail. Note
there is only one interatrial septum, the septum
primum with its ostium primum. Note in C4, the
attachment of septum primum to the most cranial of
the endocardial cushions. Follow the high
pressure pathway from the ventricles into the
aortic system. Start from the cardiac apex on D7
and check that no heart structure is seen below
this level in E1.