Heart development
Heart development, also known as cardiogenesis, refers to the prenatal development of the heart. This begins with the formation of two endocardial tubes which merge to form the tubular heart, also called the primitive heart tube. The heart is the first functional organ in vertebrate embryos.
This article is about a general overview of heart development. For protein signalling in heart development, see Protein signalling in heart development.Heart development
The tubular heart quickly differentiates into the truncus arteriosus, bulbus cordis, primitive ventricle, primitive atrium, and the sinus venosus. The truncus arteriosus splits into the ascending aorta and the pulmonary trunk. The bulbus cordis forms part of the ventricles. The sinus venosus connects to the fetal circulation.
The heart tube elongates on the right side, looping and becoming the first visual sign of left-right asymmetry of the body. Septa form within the atria and ventricles to separate the left and right sides of the heart.[3]
Heart folding and turning[edit]
The heart tube continues stretching and by day 23, in a process called morphogenesis, cardiac looping begins. The cephalic portion curves in a frontal clockwise direction. The atrial portion starts moving in a cephalically and then moves to the left from its original position. This curved shape approaches the heart and finishes its growth on day 28. The conduit forms the atrial and ventricular junctions which connect the common atrium and the common ventricle in the early embryo. The arterial bulb forms the trabecular portion of the right ventricle. A cone will form the infundibula blood of both ventricles. The arterial trunk and the roots will form the proximal portion of the aorta and the pulmonary artery. The junction between the ventricle and the arterial bulb will be called the primary intra-ventricular hole. The tube is divided into cardiac regions along its craniocaudal axis: the primitive ventricle, called primitive left ventricle, and the trabecular proximal arterial bulb, called the primitive right ventricle.[10] This time no septum is present in heart.
A functional explanation for the enigmatic turning of the heart and bowels is lacking, but one theory does give an explanation for the evolution and development of this phenomenon. According to this axial twist theory, this is due to a twist in the body of all vertebrates that occurs in the early embryo. The twist turns the anterior head (with the face and cerebrum) clockwise and the rest of the exterior body anticlockwise, such that the vertebrate body is symmetric on the outside. Since there is no evolutionary pressure on the heart and inner organs for bilateral symmetry, these body parts are excluded from the twisting and remain asymmetric.[11]
Heart chambers[edit]
Sinus venosus[edit]
In the middle of the fourth week, the sinus venosus receives venous blood from the poles of right and left sinus. Each pole receives blood from three major veins: the vitelline vein, the umbilical vein and the common cardinal vein. The sinus opening moves clockwise. This movement is caused mainly by the left to right shunt of blood, which occurs in the venous system during the fourth and fifth week of development.[12]
When the left common cardinal vein disappears in the tenth week only the oblique vein of the left atrium and the coronary sinus remain. The right pole joins the right atrium to form the wall portion of the right atrium. The right and left venous valves fuse and form a peak known as the septum spurium. At the beginning, these valves are large, but over time the left venous valve and the septum spurium fuse with the developing atrial septum. The upper right venous valve disappears, while the bottom venous valve evolves into the inferior valve of the vena cava and the coronary sinus valve.[12]
Heart wall[edit]
The main walls of the heart are formed between day 27 and 37 of the development of the early embryo. The growth consists of two tissue masses actively growing that approach one another until they merge and split light into two separate conduits. Tissue masses called endocardial cushions develop into atrioventricular and conotruncal regions. In these places, the cushions will help in the formation of auricular septum, ventricular conduits, atrio-ventricular valves and aortic and pulmonary channels.[13]
Valves and outflow tracts[edit]
Truncus septum formation and arterial cone[edit]
The arterial cone is closed by the infundibular cushions. The trunk cones are closed by the forming of an infundibulotroncal septum, which is made from a straight proximal portion and distal spiral portion. Then, the narrowest portion of the aorta is in the left and dorsal portion. The distal portion of the aorta is pushed forward to the right. The proximal pulmonary artery is right and ventral, and the distal portion of the pulmonary artery is in the left dorsal portion.[13]
Pacemaker and conduction system[edit]
The rhythmic electrical depolarization waves that trigger myocardial contraction is myogenic, which means that they begin in the heart muscle spontaneously and are then responsible for transmitting signals from cell to cell. Myocytes that were obtained in the primitive heart tube, start beating as they connect together by their walls in a syncytium. Myocytes initiate rhythmic electrical activity, before the fusion of the endocardial tubes. The heartbeat begins in the region of the pacemaker which has a spontaneous depolarization time faster than the rest of myocardium.[17]
The primitive ventricle acts as initial pacemaker. But this pacemaker activity is actually made by a group of cells that derive from the sinoatrial right venous sinus. These cells form an ovoid sinoatrial node (SAN), on the left venous valve. After the development of the SAN, the superior endocardial cushions begin to form a pacemaker also known as the atrioventricular node. With the development of the SAN, a band of specialized conducting cells start to form creating the bundle of His that sends a branch to the right ventricle and one to the left ventricle. Most conduction pathways originate from the cardiogenic mesoderm but the sinus node may be derived from the neural crest.[17]
The human embryonic heart displays cardiac activity approximately 21 days after fertilization, or five weeks after the last normal menstrual period (LMP), which is the date normally used to date pregnancy in the medical community. The electrical depolarizations that trigger cardiac myocytes to contract arise spontaneously within the myocyte itself. The heartbeat is initiated in the pacemaker regions and spreads to the rest of the heart through a conduction pathway. Pacemaker cells develop in the primitive atrium and the sinus venosus to form the sinoatrial node and the atrioventricular node respectively. Conductive cells develop the bundle of His and carry the depolarization into the lower heart. Cardiac activity is visible beginning at approximately 5 weeks of pregnancy.
The human heart begins beating at a rate near the mother's, about 75-80 beats per minute (BPM). The embryonic heart rate (EHR) then accelerates linearly for the first month of beating, peaking at 165-185 BPM during the early 7th week, (early 9th week after the LMP). This acceleration is approximately 3.3 BPM per day, or about 10 BPM every three days, an increase of 100 BPM in the first month.[18]
After peaking at about 9.2 weeks after the LMP, it decelerates to about 150 BPM (+/-25 BPM) during the 15th week after the LMP. After the 15th week the deceleration slows reaching an average rate of about 145 (+/-25 BPM) BPM at term.
Human embryo, 38 mm, 8–9 weeks–anterior view, heart is visible.
