Chronology and morphological changes
Development begins with the formation of the cardiac loop, followed by the sequential division of the organ into chambers. Frontal sections of the developing embryo allow tracking the formation of the main tissues:
- Epicardium is laid down along the outer contour;
- Muscular layer gradually thickens and differentiates;
- Endocardial cushions (clusters of specialized tissue) play a major role in chamber separation, starting from the atrioventricular canal.
The process correlates with the growth of the embryo itself. Morphological rearrangements are recorded at an embryo length of 5 mm to 100 mm (and up to the moment of birth), passing through checkpoints: 6–7 mm, 8–9 mm, 12–15 mm, and 25–30 mm.
Stages of chamber formation
The process can be divided into four consecutive stages of structural complexification:
- Two-chambered heart. Due to uneven growth of regions, the primary heart tube bends. At this point, it consists of four differentiated segments: the sinus venosus, primary atrium, ventricular rudiment, and truncus arteriosus.
- Three-chambered heart. Septation (division) of the atria begins. First, the primary interatrial septum forms, and parallel to it, the secondary septum. Complete fusion does not occur before birth: the foramen ovale remains between the right and left atria.
- Three-chambered heart with an incomplete septum. The muscular septum begins to grow in the common ventricle, but it is not solid. A defect (opening) persists within it, the final closure of which normally occurs only at 6–7 weeks after birth.
- Four-chambered heart. The final form characteristic of mammals and humans. The atria and ventricles are completely separated into right and left parts.
Great vessel development and hemodynamics
In the mature four-chambered heart, blood flows are strictly isolated. The left atrium and left ventricle pump arterial blood, whereas the right atrium and right ventricle work exclusively with venous blood.
The great vessels originating from the ventricles are formed by the longitudinal division of the primary common truncus arteriosus (truncus arteriosus). As a result of this division, the left aortic arch originates from the left ventricle, and the pulmonary trunk arises from the right ventricle.
Genetic regulation of septation
The transition from a primitive three-chambered structure to a fully functional four-chambered organ depends on the activity of the Tbx5 gene, which is expressed in the ventricular rudiment.
- If gene expression occurs uniformly throughout the tissue of the primordium, septation does not complete, and the heart remains three-chambered.
- Normally in humans, the gene is expressed only on the left side of the developing ventricle, which triggers the correct cascade of reactions and leads to the formation of a fully functional four-chambered heart.
Briefly: answers to topical questions
From which germ layers do the endocardium, myocardium, and epicardium develop?
The heart tunics are formed from two different embryonic primordia.
- Endocardium develops from mesenchyme, similarly to the formation of blood vessel walls.
- Myocardium is formed from the visceral layer of the splanchnotome (its specialized region—the myoepicardial plate).
- Epicardium also originates from the visceral layer of the splanchnotome (the myoepicardial plate).
Which congenital malformations are associated with a mutation or altered expression of the Tbx5 gene?
Impaired expression of the Tbx5 gene is associated with the formation of a three-chambered heart: uniform expression throughout the ventricular primordium results in a three-chambered heart, whereas expression restricted to the left side results in a four-chambered heart.
Mutation of the TBX5 gene is also cited in sources for Holt–Oram syndrome; its localization is 12q24.1.
What hemodynamic factors lead to the physiological closure of the foramen ovale after birth?
Physiological closure of the foramen ovale after birth is due to a change in the pressure gradient between the atria. Initiation of pulmonary circulation causes an increase in blood flow to the lungs, which leads to an increased return of blood to the left atrium and a subsequent rise in its pressure. Elevated pressure in the left atrium prevents blood from passing from the right atrium, causing functional closure of the foramen ovale by the valve flap (normally occurs by the 3rd month of life).
Which embryonic structures participate in the formation of the heart valve apparatus?
Derivatives of the endocardium and epicardium participate in the formation of the heart valve apparatus.
- Endocardial cushions (endocardial thickenings) — these form the atrioventricular valves and flaps.
- Endocardial folds — form the cusps of the atrioventricular valves as duplicatures.
- Epicardial connective tissue — grows into the endocardial duplicatures of the cusps during their development.
Remembering how Tbx5 works is simple: "Left means four-chambered." If the gene is active only on the left, 4 chambers form. If the gene is active everywhere (uniformly), 3 chambers remain.
Class and exam questions
When does differentiation of the cardiac tube segments occur?
This process actively takes place on days 23–25 of embryogenesis.
Does the foramen ovale close before birth?
No, during intrauterine development, the foramen ovale between the atria remains open and closes only after birth.
When does the interventricular septum finally close?
According to the developmental timeline, complete closure of the interventricular septal defect is completed by 6–7 weeks after birth.
What do the aorta and pulmonary trunk derive from?
They form from the primordial truncus arteriosus during its longitudinal division.
What else to study on this topic
- Mechanisms of endocardial cushion formation
- Histogenesis of the epicardium and myocardium at early stages
- Pathogenesis of atrial septal defects
- Cardiac looping: biomechanics of the process
- Role of the sinus venosus in the primordium of the conduction system
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