Cardiogenic plate — the future heart develops in splanchnic mesoderm anterior to the embryo
• blood islands, vesicles of blood cells surrounded by endothelium, appear first in splanchnic mesoderm
• angiogenesis (vessel formation) involves fusion of blood islands, budding, and vascular growth
• blood vessels coalesce to form a U-shaped vessel in the cardiogenic plate;
it becomes heart & paired ventral aortae
• associated with head process formation and progression from a flat to a tubular embryo:
— the cardiogenic plate and ventral aortae are re-positioned ventral to the pharynx, and
— bilateral ventral aortae merge along the midline to form a single elongate endocardial tube

Endocardial tube — splanchnic mesoderm surrounding the endocardial tube forms cardiac muscle
• the tube receives blood from veins and pumps blood through arotic arches to paired dorsal aortae

• while pumping, the tube differentiates into five regions (in order, from output to input):
1] truncus arteriosus – becomes ascending aorta and pulmonary trunk of the adult
2] bulbus cordis – an enlargement that becomes right ventricle, including conus arteriosus
3] ventricle – an enlargement that becomes the left ventricle
4] atrium – a compartment that becomes adult right and left auricles, and
5] sinus venosus – a paired region into which veins drain; the right becomes incorporated
into the adult atrial wall, the left becomes coronary sinus

Morphogenesis into a four-chambered heart — while the heart continues to pump blood
The following morphological transformations are proceeding simultaneously:
• the endocardial tube grows in length and thus loops on itself
this places the endocardial bulbus cordis beside the endocardial ventricle, and
the atrium on top of the ventricle, and the truncus arteriosus atop the bulbus cordis

• venous blood returning to the heart shifts to the right
the right sinus venosus enlarges and becomes the wall of the adult right atrium
the smaller left sinus venosus is incorporated into the wall of the right atrium as coronary sinus

• the endocardial atrium enlarges and the common opening between atria & ventricles
is partitioned into two openings
paired endocardial cushions grow inward from the wall of the common opening
the cushions meet, dividing the common opening into right and left AV (atrioventricular) openings

• right and left ventricles are formed by external growth and selective internal excavation:
the endocardial ventricle becomes adult left ventricle and the bulbus cordis becomes right ventricle
the interventricular septum is sculptured by selective excavation of surrounding tissue, as are other
ventricular features, including papillary muscles, chordae tendineae & AV valves
the original canal between the ventricle and bulbus cordis remains as an interventricular foramen;
the foramen is closed by membranous growths from endocardial cushions and from the
spiral septum that divides the truncus arteriosus (growth failure leaves an interventricular
septal defect that impairs cardiac function)

• right and left atria are formed by partitioning the common atrium into two chambers:
the partition must retain an opening to allow blood flow from right to left in utero
the process of partitioning right and left atria involves two septae and three formina:
- Septum One grows from the dorsal wall of the atrium
- Foramen One, the original opening, is closed as Septum One grows to completion
- Foramen Two develops as cell death produces fenestration dorsally in Septum One
- Septum Two grows on the right side, establishing a Foramen Ovale when it stops growing
- Foramen Ovale allows blood to flow from the right atrium, between the two septae,
through Foramen Two, and into the left atrium. Septum One acts as a valve closing
the opening when pressure in the left atrium exceeds that in the right atrium.

• the aorta and pulmonary trunk are formed by partitioning truncus arteriosus (directed by neural crest)
spiral ridges grow inward creating a spiral septum that divides the truncus arteriosus in two
as a result the aorta and pulmonary trunk spiral around one another in the adult, and
the aorta connects to the left ventricle & the pulmonary trunk connects to the right ventricle
aortic and pulmonary semilunar valves are scultured by wall growth and selective cells death

Tetralogy of Fallot —a congenital anomally related to defective partitioning of the truncus arteriosus
four cardiac defects are described:
1] Ventricular septal defect: the interventricular foramen is not close by membrane growth
from endocardial cushions
2] Stenosus of pulmonary trunk: unequal partitioning of the truncus arteriosus produces a
narrow pulmonary trunk
3] Enlarged aorta that receives blood from the right ventricle as well as the left,
because of unequal partitioning :
4] right ventricle hypertrophy: this is a physiological consequence of exposure to high pressure
generated by the left ventricle


Dorsal & Ventral Aortae — right and left dorsal aortas develop early in the embryo; ultimately,
bilateral aortas fuse to create the single adult aorta
• paired ventral aortas develop in association with the cardiogenic plate and shift
ventral to the pharynx along with the heart
• bilateral fusion of ventral aortas produces the endocardial tube and the adult brachiocephalic trunk
• dorsal and ventral aortas are connected by a series of aortic arches that run within pharyngeal arches
• each of the paired dorsal aortas gives off:
- intersegmental aa. that become vertebral, subclavian, intercostal, lumbar, etc. arteries in the adult
- vitelline a. which supplies the yolk sac; the right vitelline a. becomes adult cranial mesenteric a.
- umbilical a. which supplie the allantois & placenta; iliac arteries are umbilical a. branches

Aortic Arch Derivatives — the third, fourth & sixth aortic arches contribute to adult vessels
• vessels cranial to the heart are derived from aortic arches which undergo branching, elongation,
and degeneration in the process of forming adult vessels (malformation of the vessels
can result in compression of the esophagus)
• bilaterally, the third aortic arch becomes common carotid a. and internal carotid a.,
the external carotid a. arises as a branch of the third arch; degeneration of dorsal aorta between
arches 3 & 4 frees the third arch to move forward into the head
• the left fourth aortic arch becomes the adult arch of the aorta, freed to shift caudally
by the degeneration between arches 3 & 4
• the right fourth aortic arch plus the 7th intersegmental a. becomes the adult right subclavian a.,
freed to supply the limb by degeneration of the dorsal aorta caudal to the subclavian a.
• bilaterally, each sixth aortic arch gives rise to a pulmonary a. as a branch from the arch
on the right side the connection of the sixth arch to the dorsal aorta degenerates
on the left side the connection persists as the Ductus Arteriosus


Veins — embryonic veins proliferate, anastomose , and selectively degenerate to form adult veins
• bilaterally, three veins drain into the sinus venosus of the embryonic heart:
1] vitelline v., drains the yolk sac
2] umbilical v., drains the allantois and placenta
3] common cardinal v., formed by convergence of cranial & caudal cardinal vv., drains the embryo
• Cranial Vena Cava: formed when the left cranial cardinal v. anastomoses with the right (shifting
blood return to the right); the external jugular v. and the subclavian v. arise as buds from
the cranial cardinal v. which becomes internal jugular v.
• Caudal Vena Cava: patched together from selective anastomosis and degeneration of caudal
cardinal vv. and the subcardinal and supracardinal branches of caudal cardinal vv.
• Azygos v.: arises from segments of supracardinal and caudal cardinal vv.
- the right azygos v. drains into the right atrium (right sinus venosus)
- the left azygos vein, when present, drains into the coronary sinus (left sinus venosus)
• Pulmonary veins: arise as buds of the left atrium, right and left buds give rise to lobar veins
which become incorporated into the wall of the adult left atrium


Portal Vein and Ductus Venosus — the portal vein conveys blood from the intestine to the liver
• segments of bilateral vitelline veins anastomose and enlarge to form the adult portal vein
• vitelline veins also form sinusoids within the liver
• the left umbilical vein conveys blood from the placenta to the liver (the right umbilical vein atrophies)
• the Ductus Venosus, a venous shunt, forms within the liver to convey placental blood in the left
umbilical vein directly to hepatic veins and the caudal vena cava, bypassing liver sinusoids
• a portosystemic shunt, due to a persistent ductus venosus or other venous malformation,
leads to disease by allowing blood from the intestine to by-pass the liver


Three in-utero accommodations — the following adaptations allow a fetus to
survive within the uterus and live outside the uterus:
1] ductus venosus: allows the large volume of blood returning from the placenta to by-pass the
liver sinusoids bottleneck; postnatally, the ductus venosus gradually closes over a period of days
2] ductus arteriosus: conveys blood from the pulmonary trunk to the aorta, because lungs cannot
accommodate much blood; the elevated O2 concentration associate with breathing triggers
constriction of the ductus arteriosus which ultimately transforms to a fibrous ligamentum arteriosum
3] foramen ovale: allows venous blood returning from the body to enter the left atrium and ventricle,
since little returns from the lungs; the foramen closes and eventually seals when blood from
the lungs increases and equalizes pressure in the two atria