Cardiovascular System I: Structural and Functional Development of the Heart and Mediastinum

Institutional Framework and Educational Goals

The academic session titled "Cardiovascular System I: Structural and Functional Development of the Heart and Mediastinum" is part of the Structure and Function of the Human Body course within the Medicine program at Universidad Norbert Wiener. The institutional mission emphasizes transforming lives by training innovative, ethical, and global leaders. The vision is to be a benchmark university for prestige, international projection, and professional success. The core values guiding this education include integrity, which involves ethical action even when unobserved; responsibility, being conscious of the impact of one's actions; respect, treating others with dignity; and self-improvement, which rejects mediocrity and conformism. The specific goal of this session is for students to explain the development, structure, and function of the cardiovascular system and mediastinum using real clinical cases.

General Principles of Cardiovascular Development

The cardiovascular system is the first system to start functioning in the human embryo. Development begins during the $3^{ra}\,semana$ of gestation. This early development is an essential adaptation mechanism because the rapidly growing embryo can no longer satisfy its nutritional and oxygen needs solely through diffusion. Therefore, the establishment of a circulatory system is vital for continued growth. The process begins during gastrulation when bilateral and symmetric cardiac areas are formed in the mesoderm. These areas consist of cells that are histologically similar to other mesodermal cells but are destined to differentiate into cardiac cells. These cells are located on both sides of the primitive line at the level of the Hensen node (primitive node).

Formation of Cardiogenic Fields and the Heart Tube

The development involves two primary populations of cells. The Primary Cardiogenic Field (CCP) is formed when cells from the cardiac areas migrate cranially and converge in front of the precordal plate and the buccopharyngeal membrane. This results in the formation of a horseshoe-shaped cardiogenic plate. The CCP is responsible for forming the atria, the left ventricle, and the majority of the right ventricle. Adjacent to this, in the splanchnic mesoderm, lies the Secondary Cardiogenic Field (CCS), which contributes to the formation of the remainder of the right ventricle as well as the conus and truncus arteriosus. As these fields are established, the pharyngeal endoderm forms blood islands through vasculogenesis, and the cardiac region becomes surrounded by the intraembryonic coelom, which will become the pericardial cavity.

Transformation into a single heart tube occurs through two types of folding. Transverse folding brings the endocardial tubes and myocardial primordia closer together until they fuse into a single myoendocardial tube or primitive cardiac tube. Simultaneously, longitudinal or cephalocaudal folding occurs. Initially, the cardiogenic region is anterior to the oropharyngeal membrane and the neural plate in the cervical region. The closure of the neural tube and the formation of brain vesicles displace the heart tube to its definitive thoracic position, anterior to the esophagus. The resulting unique heart tube is lined internally by endothelium, surrounded by an external myocardial layer, and separated by a layer of connective tissue known as cardiac jelly. This tube receives venous drainage at its caudal pole and pumps blood out through the first aortic arch into the dorsal aorta at its cranial pole.

Cardiac Looping and Positional Anomalies

Between days $23$ and $28$ of development, the heart tube undergoes a process of elongation and bending known as cardiac looping. The cephalic portion of the tube curves in a ventral, caudal, and rightward direction. Meanwhile, the caudal portion moves dorsocranially and toward the left. This process establishes the major regional divisions of the heart. By the end of this process on day $28$, the heart originally consisting of one ventricle and one atrium communicated by the atrioventricular canal is reorganized. A significant clinical anomaly related to this process is Dextrocardia, which is the most frequent positional defect of the heart. It occurs when the heart loops toward the left instead of the right, resulting in a mirror-image orientation where the apex of the heart is located on the right side of the thorax.

Septation of the Heart Chambers

The formation of cardiac septa occurs primarily between days $27$ and $37$ of development. One of the main mechanisms involves the growth of endocardial cushions in the atrioventricular and tronconal regions. In the common atrium, the septum primum grows from the roof toward the endocardial cushions, leaving an initial opening called the ostium primum. Before the ostium primum closes, apoptosis creates a second opening, the ostium secundum. Subsequently, a septum secundum grows to the right of the septum primum but does not reach the cushions, creating a flap-like valve over the ostium secundum known as the foramen oval (or oval window). This allows blood to flow from the right atrium to the left atrium during fetal life.

Ventricular septation begins at the end of the fourth week. The muscular portion of the interventricular septum grows from the lower part of the bulboventricular sulcus toward the endocardial cushions. This growth pauses in the $7^{ma}\,semana$, leaving an interventricular foramen. The closure of this foramen to form the membranous portion of the septum occurs through contributions from the endocardial cushions, the aortopulmonary septum, and the mesenchyme of the muscular septum. This communication must close between the $6^{ta}$ and $7^{ma}\,semana$. Atrioventricular valves (mitral and tricuspid) develop between the $5^{ta}$ and $8^a\,semana$ as the superior, inferior, and lateral endocardial cushions fuse and reorganize into fibrous tissue attached to papillary muscles via chordae tendineae.

Septation of the Outflow Tract and Conduction System

In the $5^{ta}\,semana$, ridges appear in the truncus arteriosus and conus arterialis. The superior right truncal ridge grows distally and to the left, while the inferior left truncal ridge grows distally and to the right. These ridges fuse to form the spiral aortopulmonary septum, which divides the truncus into the aorta and the pulmonary artery. The conus is similarly divided into anterolateral and posteromedial channels. Regarding the electrical system, all myocardial cells initially possess pacemaker activity. By $21\,\text{dias}$, activity begins to localize. Cells around the atrioventricular canal form the atrioventricular (AV) node, while cells in the venous sinus form the sinoatrial (SA) node. The conduction system eventually includes the SA node, AV node, bundle branches, and Purkinje fibers, all of which are myocardial in origin, except for the sympathetic and parasympathetic nerve fibers that innervate them.

Anatomy and Physiology of the Adult Heart

The adult heart is a four-chambered muscular pump located in the middle mediastinum of the thorax, enclosed within the pericardial sac (consisting of parietal and visceral layers). Its wall is composed of three layers: the epicardium (outer layer), the myocardium (cardiac muscle), and the endocardium (inner endothelium). The heart consists of the right atrium, left atrium, right ventricle, and left ventricle, separated by atrial and ventricular septa. Blood flow is regulated by four valves: the tricuspid (right AV), mitral or bicuspid (left AV), pulmonary, and aortic valves. Irrigation is provided by the right coronary artery (supplying the SA node, marginal right, posterior interventricular, and AV node) and the left coronary artery (supplying the SA node, anterior interventricular, circumflex, and left marginal). Innervation is managed by the autonomic cardiac plexus, containing both sympathetic and parasympathetic fibers.

The cardiac cycle is a sequence of alternating contraction (systole) and relaxation (diastole). During systole, chambers contract to pump blood to the lungs and periphery; during diastole, they relax and fill with blood from the veins. These events are monitored via auscultation, where $R1$ represents the closure of the AV valves and $R2$ represents the closure of the aortic and pulmonary valves along with the reopening of the AV valves. In fetal life, circulation involves the placenta and the ductus arteriosus, with minimal pulmonary circulation as the lungs are filled with amniotic fluid.

The Mediastinum and Large Vessels

The mediastinum is the extrapleural anatomical compartment located between the lungs in the center of the thorax. Its boundaries consist of the superior thoracic aperture, the diaphragm inferiorly, the sternum anteriorly, and the thoracic vertebrae posteriorly. It is divided into the Superior Mediastinum and the Inferior Mediastinum (which is further subdivided into Anterior, Middle, and Posterior). The Superior Mediastinum contains the thymus (in children), trachea, esophagus, aortic arch and its branches, brachiocephalic venous trunks, superior vena cava, and lymphatic trunks. The Inferior-Anterior Mediastinum contains the thymus remnants, internal mammary vessels, and lymph nodes. The Inferior-Middle Mediastinum contains the heart, pericardium, ascending aorta, pulmonary vessels, trachea, bronchi, phrenic nerve, vago nerve, and brachiocephalic vessels. The Inferior-Posterior Mediastinum contains the esophagus, descending aorta, azygos and hemiarygos veins, sympathetic chain, thoracic duct, and the vagus nerve.

Congenital Heart Diseases

Congenital heart defects are the most frequent birth defects, occurring in approximately $1:125$ live births ($8:1,000$). These result from failures in the development of septa, cavities, or vessels. Causes include monogenic inheritance (e.g., Marfan, Noonan, DiGeorge), chromosomal anomalies (Trisomy $21$, $18$, $13$, and $45,X$), or exposure to teratogens like Rubella. Ectopia Cordis is a severe condition where the heart is exposed on the thoracic surface due to a failure in the fusion of lateral folds. Tetralogy of Fallot is a classic complex consisting of four elements: interventricular communication (VSD), pulmonary artery stenosis, overriding/dextroposition of the aorta, and right ventricular hypertrophy. This is the most frequent cause of cyanotic heart disease. Other defects include Transposition of the Great Vessels, where the aorta arises from the right ventricle and the pulmonary artery from the left; Interatrial Communication (CIA), often involving a patent foramen ovale; and Interventricular Communication (CIV), which involves perforations in the membranous or muscular septum. Patent Ductus Arteriosus and Coarctation of the Aorta (common in Turner Syndrome) are also significant clinical findings.

Questions & Discussion

The session concludes with an integrative activity using the "Snowball Methodology" focusing on the Tetralogy of Fallot. Students are tasked with integrating knowledge of embryonic development to diagnose and argument the alterations in a newborn case. Key questions for reflection include: What has been learned about the structural development and function of the heart and mediastinum? Which events in heart development are considered most interesting? Are there concepts regarding the development of the heart and mediastinum that require more depth? How will this activity help in future medical practice? Finally, how can student learning be improved? Evaluation criteria for this activity include active participation, quality of ideas and solutions, and the integration of knowledge specifically pertaining to the case study of the newborn with a frequency of $1:3,000$.