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BLOOD VESSELS
HEART TUBES

     Human embryos possess number of blood vessels which do not exist in adult humans that are typical of other vertebrates such as aortic arches, a ductus arteriosus, anterior cardinal veins, posterior cardinal veins, supracardinal veins, subcardinal veins, sacrocardinal veins, and paired superior vena cavae.  These blood vessels exist in human embryos and may even persist in adults as developmental abnormalities.  The dorsal aortae are the first blood vessels formed, beginning their development as the heart forms (Sadler, p. 180).

     In Amphioxus and the primitive vertebrate condition, a single ventral blood vessel produces a series of aortic arches to carry deoxygenated blood to the pharyngeal arches (and their gills) for gas exchange.  These arches then bring the oxygenated blood to the dorsal aorta (or, primitively, a pair of dorsal aortae).  In Amphioxus, there are a large number of aortic arches. 
AORTIC ARCHES
Jawed vertebrates typically have only 6 pharyngeal arches, at least as embryos.  In the jawed fish the pharyngeal arches do not form pouches for the gills as in the hagfish below  (Romer)
HAGFISH
In the picture below, the efferent portion of the aortic arches fuse with the dorsal aorta in the shark.
ARCHES IN SHARK
     The embryonic human circulatory system includes prominent aortic arches which service the pharyngeal arches.  Rhombencephalic neural crest cells migrate to arches and function in development of the blood vessels of the pharyngeal arches.  Thus, each of the aortic arches is unique in their origin, development, and predisposition towards anomalies. Specific anomalies in humans affect only certain regions of adult vessels derived from a certain aortic arches, such as the ductus arteriosus (6th arch) and segment B of the aortic arch and proximal right subclavian artery (4th arch) (Berwerff, 1999). 
EMBRYONIC ARCHES EMBRYONIC ARCHES
EMBRYONIC ARCHES

     In human embryos, the first aortic arch has already disappeared by the time the embryo reaches 4 mm (at which point the 6th aortic arch has yet to form) although one portion persists as the maxillary artery.  The second aortic arch disappears by the time the embryo has reached 10 mm although remnants contribute to the hyoid and stapedial arteries.  The third aortic arch forms the common carotid arteries and the initial portion of the internal carotid arteries (the external carotid arteries sprout from this arch).  The left branch of the fourth aortic arch forms part of the arch of the aorta while the right branch forms the initial portion of the right subclavian artery.  The fifth aortic arch is never well developed.  Fifth aortic arch may persist as a shunt from the systemic to the pulmonary circulation (Freedom, 1989; Sadler).

     The sixth aortic arch forms the pulmonary arteries.  While the right branch loses its connection to the dorsal aorta, this connection persists on the left as the ductus arteriosus (Sadler, p.208).  In lungfish and amphibians, the pulmonary artery develops as a branch of aortic arch VI which also services gills before emptying into the dorsal aorta through the ductus arteriosus. The ductus arteriosus is retained even in adult salamanders which presents the anatomical dilemma of emptying deoxygenated blood into systemic arteries.  While the ductus arteriosus closes and degenerates in most infant reptiles, it remains in some turtles (Chelonia) and in tuataras.  In mammals, including humans, the ductus arteriosus sometimes persists (Bergwerff, 1999b).  The gene HOXB5 may be involved in development of ductus arteriosus (Bergwerff, 1999b).

A patent ductus arteriosus is one of the most common defects of the great vessels of the heart (Sadler, p. 212).

     A number of blood vessels are thus composite vessels derived from several embryonic vessels.  The right subclavian, for example, is composed proximally by the fourth aortic arch and distally from the right dorsal aorta and 7th intersegmental artery.  The internal carotid is formed by the 3rd aortic arch and the cranial portion of the dorsal aorta (Sadler, p. 208). 

     In some people, the 4th aortic arch completely degenerates and the right subclavian artery develops only from the right dorsal aorta and 7th intersegmental artery.  This may result in difficulties in breathing and swallowing.  In some people, the right dorsal aorta persists to form a vascular ring and a double aorta which may also result in problems with breathing and swallowing. (Sadler, p. 213-4).

 

OTHER VESSELS

A large number of additional abnormalities are known in humans, including a double aortic arch. 

VESSEL ABNORMALITIES

In reptiles, the left and right systemic vessels (aortas) leaving the heart fuse with the fourth embryonic aortic arches, left and right, to form a pair of aortic arches which fuse in the abdomen to form the descending aorta.  In reptiles, the right fourth aortic arch larger than the left and the single common carotid originates from the right arch.  In birds, the left fourth aortic arch degenerates during development and the right branch of the fourth aortic arch composes the aorta.  In mammals, the right branch degenerates (its only remnant being the base of the right subclavian artery) and the left branch of the fourth aortic arch composes the aorta (Torrey, 1979).

Frog

FROG

TURTLE

TURTLE

     In the 7th week, the changes in the aortic arches cause the arterial blood supply to the face changes from the internal carotid to the external carotid (Shell 241).

     The vitelline arteries become the celiac, superior mesenteric, and inferior mesenteric arteries (Sadler, p. 212).

EMBRYONIC ARTERIES

     The coronary arteries which service the heart usually arise as the first branch from the aorta, but other anomalous origins of coronary arteries are also known in humans.  Coronary arteries can arise from the pulmonary artery, the pulmonary trunk, an anomalous site on aorta, from a ventricular cavity, and from a systemic blood vessel such as the innominate, subclavian, mammary, carotid, or descending aorta (Angelini, 1989; Cronk, 1951).

 

VEINS

EMBRYONIC VEINS

     By the end of the 5th week there are three major pairs of veins: the vitelline, umbilical, and cardinal veins, none of which are retained in adults.  The left vitelline vein and left umbilical vein degenerate when the embryo is 5-7 mm and the left cardinal vein degenerates when the embryo is 60 mm (about 10 weeks) (Sadler, p.185).  The anterior and posterior cardinal veins drain into the common cardinal vein (Sadler, p. 215-6).  In the fifth through seventh weeks, the subcardinal veins drain the kidneys and the sacrocardinal veins drain the hind limbs.  The supracardinal veins develop to drain the body wall and substitute for the posterior cardinal veins (Sadler, p. 218-9).

     While there are left and right halves of the cardinal venous system, branches (anastomoses) develop between the left and right veins.  These anatomoses will give rise to the vena cava system in which there is a single drainage system (derived from the right portion of cardinal system) into which remnants of the left cardinal system will drain.  One anastomosis develops into the left brachiocephalic vein which now drains into the superior vena cava (on the right).  The left common cardinal vein degenerates, persisting only in adults as the left superior intercostal veins.

EMBRYONIC VEINS

     The anastomosis between the subcardinal veins develops into the left renal vein.  The left subcardinal vein degenerates except for the portion which becomes the left gonadal vein.  The anastomosis between the sacro-cardinal veins becomes the left common iliac vein.  The right posterior cardinal vein degenerates except for a portion which contributes to the azygos vein in adults (Sadler, p. 218-9).

    The paired cardinal veins are replaced by the vena cava.  The inferior vena cava is a composite blood vessel derived from a hepatic segment (from the vitelline vein), a renal segment (from the right subcardinal vein), and a sacro-cardinal segment (Sadler, p. 218-9).

     In some people, there is a double inferior vena cava starting in the lumbar region because the left sacrocardinal vein is still connected to the left subcardinal vein.  Some people lack an inferior vena cava because the right subcardinal vein (what should be the renal segment of the inferior vena cava) does not fuse to the hepatic segment but instead enters the superior vena cava by way of the supracardinal azygos) vein.  Other abnormalities include a double superior vena cava and a vena cava which is derived from the left cardinal system rather than the right (Sadler, p. 219-220).

VENA CAVA ABNORMALITIES
Although a double superior vena cava is an abnormality in humans, it is normal in many animals, such as the opossum illustrated below.
OPOSSUM

 

Early embryos do not have much of a neck region.  As the heart and lungs descend, the trachea, esophagus, and cervical blood vessels lengthen.  The heart was originally located in the cervical region (Sadler, p. 239, 211).

The following image is of abnormal veins in a cat.

veins

 

The following image is of the descending aorta in a developing pig.

PIG AORTA

RED BLOOD CELLS

Nonmammalian vertebrates possess nucleated red blood cells.  Although the red blood cells of adult mammals lack nuclei, the first red blood cells synthesized in the embryo are nucleated.  The nucleated erythrocytes of pig embryos are pictured below.

NUCLEATED RED BLOOD CELLS NUCLEATED RED BLOOD CELLS