Evolutionary approach to defects
Most of the defects under consideration are based on a severe disruption of normal embryogenesis. From a biological perspective, the development of the human circulatory system is closely linked to the phylogenetic stages that our ancestors underwent during evolution.
Two main mechanisms of such disorders are distinguished:
- Heterotopia — a change in the normal spatial position of an organ (e.g., disruption of the rotation of cardiac primordia).
- Heterochronia — a change in the timing of organ primordium formation or complete arrest of organ development at one of the intermediate stages. In this case, recapitulation is often observed — the repetition in human anatomy of structures characteristic of lower vertebrate classes (fish, amphibians, reptiles).
Heterotopias: positioning disorders
These ontogenetic disorders are associated with the organ forming but occupying an incorrect position in the body.
- Ectopia cordis (Ectopia cordis)
In this anomaly, the heart is located outside the thoracic cavity. This frequently occurs due to an associated sternal defect through which the organ protrudes outwardly. This is a striking example of severe spatial heterotopia.
- Dextrocardia
Normally, during embryogenesis, the cardiac tube undergoes a strictly defined rotation. If this rotation is disrupted, heterotopia occurs, in which the heart is located on the right side of the thorax. The organ assumes a mirror-image position relative to its normal anatomical location.
Heterochronias: chamber development arrest
With early arrest in the formation of cardiac structures (heterochronia), the organ may retain a primitive structure.
- Two-chambered heart
Development completely stops at the earliest stage (the two-chamber stage). Anatomically, this is manifested by the fact that only a single common vessel emerges from the heart — the truncus arteriosus (the vital division into the aorta and pulmonary trunk does not occur). Such a defect is a recapitulation of the circulatory system structure of fish.
- Three-chambered heart with a single atrium
During intrauterine development, the interatrial septum fails to form completely. As a result, a single atrial cavity is formed in the presence of two ventricles (sometimes one, if the defect is combined). This is also a classic example of heterochronia.
- Three-chambered heart with a single ventricle (Holmes heart)
Characterized by the complete absence of the interventricular septum. Anatomically, a single ventricle, a specific infundibular chamber, and the emerging pulmonary artery are identified. This condition corresponds to the heart structure of amphibians.
Cardiac septal defects
Even if the heart chambers are correctly formed, the process of septum formation between them may halt or complete incompletely.
Atrial septal defect (ASD) A fairly common pathology with a frequency of 1:1000 newborns. In this condition, a pathological communication persists between the atria. Depending on the mechanism of occurrence, defects of the primary and secondary interatrial septa are distinguished.
Ventricular septal defect (VSD) Manifests as incomplete closure of the septum between the right and left ventricles. From an evolutionary biology perspective, this developmental defect is considered a recapitulation — it directly corresponds to the heart structure of reptiles, in which the ventricle is normally not completely divided.
Briefly: answers to topical questions
What are the anatomical and embryological differences between a primary and a secondary atrial septal defect?
The anatomical and embryological differences between the defects lie in their localization and relationship to the stages of interatrial septum formation.
| Characteristic | Primary septal defect (Primum ASD) | Secondary septal defect (Secundum ASD) |
|---|---|---|
| Embryogenesis | Associated with the primary interatrial septum; a rare form is described as the persistence of the ostium primum | Associated with the secondary interatrial septum; normally, the secondary septum forms parallel to the primary one |
| Localization | Located in the lower part of the septum, adjacent to the atrioventricular valves | Located in the region of the fossa ovalis |
What hemodynamic disorders and directions of blood shunting are characteristic of a ventricular septal defect?
Ventricular septal defect is characterized by an initial left-to-right blood shunt, which may reverse to the opposite direction in the terminal stage.
- Primary shunt (left-to-right shunt) — shunting of blood from left to right due to pressure differences.
- Right heart overload — an increase in blood volume leads to compensatory hypertrophy of the right ventricle.
- Pulmonary hypertension (Pulmonary arterial hypertension) — occurs due to increased pulmonary blood flow, accompanied by vascular sclerosis.
- Reversal of shunt direction — when compensation is exhausted, right-to-left shunt reversal occurs, leading to cyanosis.
Which four anatomical anomalies comprise the tetralogy of Fallot?
The tetralogy of Fallot includes four classic anatomical components.
- Ventricular septal defect — a large (high) defect providing communication between the ventricles.
- Pulmonary stenosis — narrowing of the right ventricular outflow tract or the pulmonary valve orifice.
- Dextroposition of the aorta — displacement of the aorta to the right ("overriding aorta"), positioning it directly above the septal defect.
- Right ventricular hypertrophy — compensatory thickening of the myocardium in response to hemodynamic overload.
At which week of intrauterine development does the formation of the interventricular septum normally complete?
According to the cited sources, by the 8th week of intrauterine development, the heart is fully formed and has two atria and two ventricles.
The sources also indicate that at the three-chambered heart stage with an incomplete interventricular septum, there is an opening in the septum, and its closure occurs at 6–7 weeks after birth.
To easily remember the connection between defects and evolution, focus on the chambers: 2 chambers — fish, 3 chambers with a single ventricle — amphibians, incomplete septum in the ventricle — reptiles.
Class and exam questions
What is dextrocardia and why does it occur?
This is an ontogenetic disorder (heterotopia) in which the heart is located on the right side of the thorax. It arises due to a disruption in the normal rotation of the primitive cardiac tube during embryogenesis.
The heart structure of which animals resembles the «two-chambered heart» defect?
Developmental arrest at the two-chamber stage with the formation of a single truncus arteriosus is a recapitulation of fish heart structure.
What is the anatomical feature of the «Holmes heart»?
This is a three-chambered heart with the complete absence of the interventricular septum (a single ventricle) and the presence of an infundibular chamber, which corresponds to the amphibian stage of development.
What else to study on this topic
- Stages of normal formation of the primary and secondary interatrial septa
- The process of spiral division of the truncus arteriosus into the aorta and pulmonary trunk
- Chromosomal aberrations leading to severe heart defects
- Mechanism of cardiac looping in early stages of ontogenesis
- Hemodynamics in ventricular septal defect
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