Basic Biomedical Sciences I Case One Trigger One: Shape of My Heart

Basic Biomedical Sciences I Case One Trigger One: Shape of My Heart Block Four Module Two EMBRYOLOGY OF THE HEART AND BLOOD VESSELS

. The cardiovascular system is the first major system to function in the embryo.

. The primordial heart and vascular system appears in the middle of the third week

. The CVS is derived mainly from:

Splanchnic mesoderm which forms the primordium of the heart

Paraxial and lateral mesoderm near the otic placodes from which the internal ears develop

Neural crest cells from the region between the otic vesicles and the caudal limits of the third pair of somites

These sources will produce respectively:

The endocardium and the cardiac mesenchymal cells which produce the valvular tissue of the heart

The myocardium, including the conducting tissue of the heart, and the specific matrix proteins associated with the developing heart, i.e. the cardiac jelly

The aorticopulmonary septum and the media of the great vessels, and, possibly contributes to the conducting tissue of the heart.

. The heart begins to beat at twenty-two to twenty-three days

. Blood flow begins during the fourth week and can be visualized by Doppler ultrasonography.

. Three paired veins drain into the tubular heart of a four-week-old embryo:

Vitelline veins return poorly oxygenated blood from the yolk sac;

Umbilical veins carry well-oxygenated blood from the chorionic villi of the embryonic placenta; only the left umbilical vein persists.

Common cardinal veins

. return poorly oxygenated blood from the body of the embryo.

. Transformation of the umbilical veins may be summarized as follows:

The right umbilical vein and the caudal part of the left umbilical vein between the liver and the sinus venous degenerate.

The persistent caudal part of the left umbilical vein becomes the umbilical vein, which carries all the blood from the placenta to a large venous shunt - the ductus venosus - develops within the liver and connects the umbilical vein with the inferior vena cava.

The ductus venosus forms a bypass through the liver, enabling most of the blood from the placenta to pass directly to the heart without passing through the capillary networks of the liver.

. The cardinal veins constitute the main venous drainage system of the embryo. The anterior and posterior cardinal veins, the earliest veins to develop, drain cranial and caudal parts of the embryo, respectively. They join the common cardinal veins, which enter the sinus venosus.

. During the eighth week, the anterior cardinal veins are connected by an anastomosis, which shunts blood from the left to the right anterior cardinal vein. This anastomotic shunt becomes the left brachiocephalic vein when the caudal part of the left anterior cardinal vein degenerates. The superior vena cava forms from the right anterior cardinal vein and the right common cardinal vein.

. The posterior cardinal veins develop primarily as the vessels of the mesonephroi, interim kidneys, and largely disappear with these transitory kidneys. The only adult derivatives of these veins are the root of the azygos vein and common iliac veins. The subcardinal and supracardinal veins gradually develop and replace and supplement the posterior cardinal veins.

. The subcardinal veins appear first. They are connected with each other through the subcardinal anastomosis and with the posterior cardinal veins through the mesonephric sinusoids. The subcardinal veins form the stem of the left renal vein, the suprarenal veins, the gonadal veins, testicular and ovarian, and a segment of the inferior vena cava. The subcardinal veins become disrupted in the region of the kidneys. Cranial to this region, they are united by an anastomosis that is represented in the adult by the azygos and hemiazygos veins. Caudal to the kidneys, the left supracardinal vein degenerates; however, the right supracardinal vein becomes the inferior part of the inferior vena cava.

. The inferior vena cava forms during a series of changes in the primordial veins of the trunk that occur as blood, returning from the caudal part of the embryo, is shifted from the left to the right side of the body. The inferior vena cava is composed of four main segments:

A hepatic segment derived from the hepatic vein, proximal part of right vitelline vein, and hepatic sinusoids

A prerenal segment derived from the right subcardinal vein

A renal segment derived from the subcardinal-supracardinal anastomosis

A postrenal segment derived from the right supracardinal vein

. The superior vena cava is derived from the right anterior cardinal vein and the right common cardinal vein.

DERIVATIVES OF THE PHARYNGEAL ARCH ARTERIES

. The pharyngeal arch arteries arise from the aortic sac and terminate in the dorsal aorta. There are six pairs of aortic arches:

Derivatives of the first pair

Maxillary arteries - supply the ears, teeth, and muscles of the eye and face

External carotid arteries

Derivatives of the second pair

Stapedial arteries - small vessels that run through the ring of the stapes, a small ear bone

Derivatives of the third pair

Common carotid arteries - supply structures in the head.

Internal carotid arteries - supply the ears, orbits, brain and its meninges.

Derivatives of the fourth pair

Arch of the aorta - formed partly by the fourth aortic arch. The proximal part of the arch develops from the aortic sac and the distal part is derived from the left dorsal aorta.

Right subclavian artery - its proximal part is derived from the right fourth pharyngeal arch artery while the distal part is derived from the right dorsal aorta and right seventh intersegmental artery.

Derivatives of the fifth pair

In fifty percent of embryos the fifth pair are rudimentary vessels that soon degenerate, leaving no vascular derivatives. In other embryos, these arteries do not develop.

Derivatives of the sixth pair

The left sixth aortic arch develops as follows:

Proximal part of the left pulmonary artery - derived from the proximal part of the arch.

Ductus arteriosus - a prenatal shunt and derived from the distal part of the arch which passes from the left pulmonary artery to the dorsal aorta.

The right sixth aortic arch develop as follows:

Proximal part of the right pulmonary artery - derived from the proximal part of the arch.