Human Anatomy: Part 2 - K. A. Dyubenko, A. K. Kolomiytsev, Yu. B. Chaykovsky 2008

Specialized Section
Cardiovascular System, systema cardiovasculare – Venous System
Veins of the Systemic Circulation – Embryonic Development of the Cardiovascular System – Heart Development

In mammals and humans, The Heart develops from paired primordia that appear at the beginning of the 3rd week of development, when the embryo is still flat and spread out over the yolk sac (Fig. 93 A).

During this period, in the prospective cervical region of the embryo between the endoderm and the visceral leaves of the ventral mesoderm, mesenchymal Cell cords arise on the right and left sides; these develop lumens and transform into endothelial tubes. The areas of the visceral leaves of the mesoderm covering these tubes thicken and are called myoepicardial plates (Fig. 93). As the embryo separates from the yolk sac, the ventral body wall forms, and the gut appears, the two heart primordia approach each other and fuse. The heart primordium becomes unpaired and takes on a tubular shape—forming a simple tubular heart, cor tubuläre simplex (LNE) (Fig. 93)—which lies ventral to the pharyngeal gut. The inner lining of the heart, the endocardium, develops from the material of the endothelial tubes, while the myocardium and epicardium develop from the myoepicardial plates.

As the heart forms as an unpaired Structure, the right and left coelomic cavities merge, resulting in The formation of a single pericardial cavity.

The areas of the myoepicardial plate on the DORSAL SIDE OF the heart come close together but do not fuse, forming the dorsal mesocardium, which anchors the heart to the posterior wall of the pericardial cavity. At this stage of development, the heart occupies a median position and resembles the primitive tubular heart of amphioxus and fish.

Subsequent transformations of the heart tube are largely determined by its rapid longitudinal growth. Since its upper (cranial) section is fixed by the initial segments of the aorta and its lower (caudal) section by the major Veins, the middle section of the heart tube undergoes the greatest positional changes as the heart grows. This is facilitated by the disappearance of the dorsal mesocardium, which previously anchored the heart.

Due to accelerated growth, the heart takes on the appearance of an S-shaped bent loop.

By the end of the 4th week of development, several distinct regions can be distinguished in the cardiac loop, separated from one another by shallow constrictions. The most cranially located heart plate changes very little externally. It is called the truncus arteriosus, with the conus arteriosus located just below it. Below lies the bent middle part of the heart tube, which forms the primordia of the ventricles. The truncus arteriosus connects the ventricular primordium with the roots of the ventral aorta.

Below the ventricular region lies a slightly expanded part of the heart, which is the primordium of the right and left atria. The region of the heart between the atria and ventricles is somewhat narrowed and forms the atrioventricular (auricular) canal. Caudal to the atrial primordium, the sinus venosus later forms, into which the major veins empty. The sinus venosus is the last component to form during The Development of the two-chambered heart, because the fusion of the two heart primordia begins in their anterior regions and proceeds backward.

As heart growth continues, the venous region shifts more cranially and the arterial region more caudally, until both come to lie in a transverse plane.

In the ventricular primordium, an interventricular sulcus appears on both the anterior and posterior surfaces, running from top to bottom and dividing it into two halves. The first half continues upward into the truncus arteriosus.

As already noted, at this stage of development the mammalian heart resembles that of fish. It consists of an atrium, which receives venous Blood, and a ventricle, which pumps blood into the arterial vessels. With the Development of the Lungs, this Circulatory system changes even during the Embryonic period.

The Emergence of lungs close to the heart is accompanied by the division of the heart into two halves and the doubling of the circulatory loops. The establishment of a double Circulation is associated with the septation of those heart chambers (atrium, ventricle, truncus arteriosus) where the blood destined for the SYSTEMIC AND PULMONARY circuits previously mixed in a common stream.

The division of the heart into right and left halves begins as early as the end of the 1st month of Embryogenesis. The completion of this process, leading to the complete Separation of blood streams, occurs only after birth, when the embryonic Organism loses its connection with the Placenta—which served as the source of oxygen supply—and transitions to independent pulmonary Respiration.

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Fig. 93. Initial Stages of human embryonic heart development (schematic).

A – 3-somite stage (17th day of development). B – 4–6-somite stage (18th day of development). C – 7–9-somite stage (19–20th day of development)

Fig. 94. Frontal sections of a human embryonic heart (after B. M. Patten)

Areas marked with dots represent the tissue of the endocardial cushions. Muscles are indicated by diagonal hatching, and the epicardium by a solid black line. A – embryo 4–5 mm in length; A.1 – interatrial septum; 2 – atrioventricular canal; 3 – interventricular septum. B – embryo 6–7 mm. B.1 – septum spurium; 2 – ostium; 3 – interventricular septum; 4 – atrioventricular canal; 5 – interatrial septum. C – 1 – septum spurium; 2 – ostium II (open); 3 – septum I; 4 – ostium I (closed); 5 – atrioventricular canal cushion; 6 – interventricular foramen. D – 1 – septum II; 2 – septum I; 3 – ostium II; 4 – ostium I (closed); 5 – interventricular foramen (closes in 16–17 mm embryos); 6 – atrioventricular canal cushion; 7 – septum II (caudal part); 8 – septum I; 9 – septum spurium. E – 1 – crista terminalis; 2 – septum II; 3 – open foramen ovale; 4 – septum I (valvula foramen ovale); 5 – bundle of His; 6 – atrioventricular canal Valves; 7 – septum II; 8 – foramen ovale. F – 1 – crista terminalis; 2 – septum II; 3 – open foramen ovale; 4 – septum I (valvula foramen ovale); 5 – bundle of His; 6 – atrioventricular canal valves; 7 – septum II; 8 – foramen ovale.

Septa in each of the aforementioned heart regions develop independently. In the 4th week, a septum appears in the atrium, which at this time constitutes the most voluminous region of the heart.

A ridge (septum I) forms on the upper and posterior walls, growing in a vertical direction and dividing the atrium. The two halves of the atrium receive different venous trunks. The right chamber receives the aforementioned sinus venosus, into which the vitelline veins, umbilical veins, and Cuvierian ducts pour their blood. The left atrial chamber receives the unpaired pulmonary vein, which at this time is a small vessel receiving blood from four veins originating two by two from the rudimentary lungs.

Following the formation of septum I, the atrioventricular (auricular) canal divides into right and left channels. This is associated with the appearance of endocardial thickenings on its dorsal and ventral surfaces, which grow toward each other (Fig. 94).

By fusing, they initially divide the single atrioventricular canal into two (right and left) atrioventricular orifices.

A primary interatrial foramen remains between the lower edge of septum I and the growing atrioventricular cushions, and it gradually closes. As the growth of septum I continues and it fuses with the endocardial cushions, the primary interatrial foramen closes, while the cranial part of septum I perforates to form a new (secondary) interatrial foramen. Formed in the cranial region, septum I thereby ensures The transport of blood into the left atrium from the right, which receives blood via the major venous trunks.

Following the Formation of the secondary interatrial foramen to the right of septum I, the secondary interatrial septum (septum II) arises (Fig. 94).

It also takes on a crescent shape and does not become solid as it grows. Its growth gradually slows down, leaving an opening known as the foramen ovale, through which blood flows from the right atrium into the left. The remaining lower part of septum I begins to function as a one-way valve that closes the foramen ovale from the left atrium side (Fig. 94).

Almost simultaneously with the appearance of the primary interatrial septum, the interventricular septum begins to form. Initially, its muscular portion develops, growing from bottom to top toward the cushions of the atrioventricular canal. Between its upper edge and the septum of the atrioventricular canal lies the interventricular foramen, which persists throughout life in reptiles (except crocodiles). Once the interventricular foramen closes, the heart is divided into four chambers, disregarding the foramen ovale connecting the right and left atria. In mammals, this opening closes due to the proliferation of Connective Tissue from both the interventricular septum and the atrioventricular canal septum, as well as the formation of longitudinal folds in the conus arteriosus. The latter continue into folds that divide the arterial trunk into two vessels: the aorta, originating from the left ventricle, and the pulmonary trunk, which carries blood out of the right ventricle. The process of dividing the arterial trunk resembles the division of the atrioventricular canal—it begins at the top (between the roots of the fourth and sixth aortic arches) and then extends downward toward the ventricles.

Connective tissue protrusions form at the orifices of the atrioventricular canals opening into the ventricles, later developing into the cusps of the definitive valves. On their ventricular surface, Cytology/cytology/34.html">Cardiac Muscle tissue is initially present and connects to the trabeculae of the ventricular wall. Subsequently, the valves become thinner, The cardiac muscle within them disappears, and the muscular trabeculae previously attached to the valve cusps are replaced by fibrous cords. Those PARTS OF THE muscular trabeculae that connect to the ventricular myocardium are preserved and transform into papillary muscles.

The valves of the aorta and pulmonary trunk (which form at the level where the conus arteriosus transitions into the arterial trunk) develop As a result of the thickening of the endocardial connective tissue within the Regions of the folds that divide the arterial trunk.

During the development of the aorta and pulmonary trunk, the aortic orifice shifts to the right and posteriorly relative to the opening of the pulmonary trunk, which is a result of the heart's rotation along its longitudinal axis.

As these cardiac changes occur, the heart shifts from the cervical region of the embryo into the thoracic cavity and assumes its definitive position.

Fig. 95 A, B. Schematic representation of aortic arch development.

A - Ventral and dorsal aortas. Primary aortic arches (I–VI) connecting the aortas. B - Reduction of aortic arches (I, II, V) and formation of pericardia and major Arteries