DEVELOPMENT OF THE HEART

DEVELOPMENT OF THE HEART

The first sign of heart development is the formation of cardiogenic cords.  These are canalized to form two endocardial tubes that fuse to make one heart tube.  Before we look at the development of the heart in detail, we will look at the development of the pericardial cavity.

Pericardial Cavity: 

Coelomic sacs in the adult are closed, independent, mesothelial-lined sacs of the trunk, and under normal (non-pathological) circumstances, they are empty except for a very small amount of lubricating fluid that allows the surfaces to move against one another without friction.  This is important because dissection of those organs that develop adjacent to coelomic sacs (lungs, heart, abdominal viscera) might give you the illusion that these organs develop within the sacs.  The organs are actually pushed into the lining of the related sac in a way that can be remembered by the image of pushing a fist into an inflated balloon.   The fist represents the organ and the balloon represents the coelomic sac.  Notice that the coelomic sac (balloon) is empty.

All coelomic sacs can be described as a cavity lined by a mesothelial membrane called a serosa.  It is the internal serosal surface that produces a serous fluid and enables opposing serous membranes to slide across one another.  The coelomic sac that surrounds the heart during its development is known as the pericardial sac.

A mesentery consists of two intact, opposed serous membranes that support a thoracic or abdominal organ.  Mesenteries are usually named for the organ that they suspend.  The mesentery relevant to heart development is the dorsal mesocardium (meso = mesentery, cardium = heart).  The heart pushes into the dorsal wall of the pericardial sac during development.  As the heart pushes into this empty sac, it becomes surrounded to such a degree that two layers of the dorsal tissue become opposed and are given the name, the dorsal mesocardium.  The dorsal mesocardium suspends the heart for a time but soon breaks down leaving the heart suspended at its cranial and caudal surfaces but not at the back.  The gap which persists where the dorsal mesocardium once was is called the transverse sinus.  You will explore this space in gross dissection laboratory.  The heart tissue is still encased by the visceral layer of pericardium and the parietal layer surrounds it.  All that has disintegrated is the dorsal mesocardium.  More will be said about the details of the anatomy of the adult pericardial sac in the lecture on the heart.

Early Development of the Heart:

Endocardial tubes (Day 19):  These thin-walled,

endothelial tubes develop from condensations of splanchnopleuric mesoderm in the cardiogenic region of the trilaminar germ disc.  The cardiogenic region is cranial to the neural plate. 

 Embryonic folding (begins Day 20):  Lateral and cephalic folding of the trilaminar germ disc over the course of several days brings the endocardial tubes together and tucks them ventrally in the thoracic region at the base of the yolk sac.  This process also brings the septum transversum into its adult position inferior to the heart.

Primary heart tube (Day 21):  The endocardial

tubes fuse together into a primary heart tube through which blood eventually flows in a cranial direction.  The mesenchyme surrounding the tube condenses to form the myoepicardial mantle (the future myocardium).  Gelatinous connective tissue called cardiac jelly separates this mantle from the endothelial heart tube (the

future endocardium).  In addition, a series of constrictions (sulci) divide the heart tube into sections: sinus venosus, into which the common cardinal veins, the umbilical veins and the vitelline veins drain; the primitive common atrium; the primitive ventricle; and the bulbus cordis, through which blood flows to the paired dorsal aortae.  The heart is beating at Day 22.  Contractions are of myocardic origin and are likened to peristalsis.  Several processes that are not restricted to the heart proper continue to occur during this time that have a great impact on heart development: embryonic head folding, embryonic lateral folding, and elongation of the heart tube in an increasingly restricted space.

Looping of heart tube

(Days 23-28):  The primitive atrium loops up behind and above the primitive ventricle and behind and to the left of the bulbus cordis.  Blood begins to circulate through the embryo by Day 24.  The looping process brings the primitive areas of the heart into the proper spatial relationship for development of the adult heart.

Circulation at 22 Days:

Three paired veins drain into the heart of the embryo during the fourth week:

Vitelline veins drain blood from the yolk sac; their formation has associations with formation of the liver and the portal system.

Umbilical veins bring oxygenated blood from the chorion (early placenta).  There are two at the start.  The right umbilical vein degenerates and disappears.  The left umbilical vein persists, carrying all blood from the placenta to the fetus.  The umbilical vein degenerates after it is cut at birth and is viewed in the adult cadaver as ligamentum teres.

Common cardinal veins return blood to the heart from the body of the embryo.  The derivation of the adult structures from the cardinal veins are complex but produce:  the left brachiocephalic v., the azygous v., the common iliac v., the left renal v., the suprarenal v., the hemiazygous v. and the IVC.

Three sets of paired arteries are evident in the embryo.  The first two carry blood to portions of the developing organism while the third carries blood away from the organism to the placenta.

The intersegmental arteries form 30-35 branches off the dorsal aortae.  Many of these are transient structures, but some persist to form the vertebral a., the intercostal a., and the common iliac a.  See Larsen (pp. 193-204) for a good discussion of arterial development.

The vitelline arteries pass to the yolk sac and later to the primitive gut which forms from the yolk sac.  Three of the vitelline arteries remain that provide blood to the gut in the adult.  These include: the celiac artery, the superior mesenteric artery and the inferior mesenteric artery.  The vitelline system becomes the portal system of the adult.  You will become intimately acquainted with this system during your dissection of the gut.

The umbilical arteries carry oxygen-depleted blood to the placenta.  The proximal parts of these arteries become the internal iliac arteries and the superior vesical arteries but the distal parts break down after birth and become the medial umbilical ligaments.

Formation of Heart Chambers:

Right atrium:  As the heart continues to grow, the right side of the sinus venosus is incorporated into the right side of the primitive atrium.  The primitive atrial wall is pushed ventrally, eventually becoming the right auricle.  In the adult heart, the right auricle contains pectinate muscle derived from the primitive atrium, whereas the sinus venarum is derived from the sinus venosus and therefore smooth-walled.

            Left atrium:  The left side of the primitive atrium sprouts a pulmonary vein which branches and sends two veins toward each of the developing lungs.  The trunk of this pulmonary vein is incorporated into the left side of the primitive atrium, forming the smooth wall of the adult left atrium.  The left side of the primitive atrium is pushed forward and eventually becomes the trabeculated left auricle.

            Interatrial septum:  The interatrial septum in the adult heart is formed by the combination of two embryonic septa:  the septum primum and the septum secundum.  The septum primum grows along the midsagittal plane like a waxing moon toward the atrioventricular canal.  This septum separates the two atria, except for a temporary space at the posteroinferior edge of the septum primum called the foramen primum which permits the right-to-left shunt of fetal blood.  At the superior edge of the septum primum clefts begin to form that provide a second opening, the foramen secundum.  The foramen secundum provides an alternate right-to-left shunt, and the foramen primum closes.  During this process a second, partial septa, the septum secundum, also begins to grow down from the roof of the right atrium, just lateral to the septum primum.  The septum secundum is thicker and more muscular than the membranous septum primum and permits the right-to-left shunt of blood through a space called the foramen ovale.  The septum secundum eventually fuses inferiorly, but foramen ovale remains patent enabling the shunting of blood from the right to  left side of  the heart.  Fetal blood is shunted from the right atrium to the left atrium through two staggered openings: the foramen ovale and the foramen secundum.  After birth, when pulmonary circulation increases blood pressure in the left atrium, the membranous septum primum is pressed against the septum secundum.  They eventually fuse and block the right-to-left shunt of blood. 

            Interventricular septum:  The left ventricle is formed primarily from the primitive ventricle.  The right ventricle is formed primarily from the bulbus cordis.  The superior portion of the bulbus cordis becomes the conus arteriosus and the truncus arteriosus.  A muscular interventricular septum begins to grow superiorly from the ventricular floor between the presumptive right and left ventricles. This septum stops short of the atrioventricular canal, leaving a space called the interventricular foramen which permits blood from both ventricles to exit via the conus arteriosus.  Within the conus arteriosus, spiral aorticopulmonary septae form, dividing the conus in half and extending inferiorly to fuse with and complete the interventricular septum.  These septae continue superiorly into the truncus arteriosus, creating outflow tracks from the right and left ventricles that are the vestigial pulmonary trunk and aorta.