Embryonic sources of heart tunics
The wall of the formed heart consists of three main layers: the inner (endocardium), the middle muscular (myocardium), and the outer (epicardium). During histogenesis, these tunics develop from two completely different embryonic sources.
The inner lining of the heart, the endocardium, is of mesenchymal origin. Its development is largely analogous to the formation of the walls of regular blood vessels. At the same time, the myocardium and epicardium are formed from the visceral layer of the splanchnotome. In the early stages, this layer forms a specific thickening, the myoepicardial plate, which subsequently differentiates into the contractile tissue and the outer serous membrane.
Primary heart tube stage
The logic of the earliest stages of cardiac embryogenesis lies in the transition from paired structures to a single organ, which occurs due to the folding of the embryo's body.
- Formation of paired primordia. Before the embryo begins to fold, the mesenchyme gives rise to two separate endocardial tubes. They are surrounded by two myoepicardial plates derived from the splanchnotome.
- Lateral folding. As the body folds close, the right and left primordia begin to approach each other in the embryonic cervical region.
- Fusion. The paired tubes fuse to form a single structure whose wall already contains all three future tunics. The result of this stage is the appearance of the primary single-chamber heart model.
Morphogenesis and septation
Following the formation of the single tube, its rapid growth begins. Since the heart tube lengthens significantly faster than the surrounding tissues grow, it lacks space and is forced to bend. This forms the characteristic S-shaped loop.
At this stage, the future chambers of the heart are clearly defined:
- Posterior (atrial) segment — the area into which venous vessels empty.
- Anterior (ventricular) segment — the region that transitions into a single truncus arteriosus.
Following the bending, septation begins — the process of dividing the internal space into chambers. The atrial region is divided into two independent atria, and the ventricular region into two ventricles. Concurrently, the single truncus arteriosus is also divided by a septum, forming the aorta and pulmonary trunk.
Histogenesis of the valve apparatus
The heart valves form simultaneously with the formation of the internal septa of the organ. Their histological structure depends on their localization.
Atrioventricular valves initially form as folds, or duplicatures, of the endocardium. However, as development proceeds, connective tissue—sourced from the epicardium—begins to grow into these folds. The right atrioventricular valve has an interesting feature: muscular tissue of the myocardium also takes part in the formation of its cusps.
Valves of the outgoing vessels (aortic and pulmonary valves), which isolate the ventricles from the great arteries, have a different structure. The surface of their cusps facing the ventricular cavity is covered by endocardium. The side facing the vessel lumen (e.g., the aorta), on the other hand, consists of connective tissue from the fibrous ring and is covered by typical vascular endothelium.
Briefly: answers to topical questions
How does the formation of the interatrial septum and foramen ovale occur?
The formation of the interatrial septum is associated with the division of the atrium by a septum and proceeds in two stages:
- first, the septum primum is formed;
- parallel to it, the septum secundum is formed.
The foramen ovale persists between the atria and closes after birth. The interatrial septum includes the septum primum and septum secundum / fossa ovalis; the fossa ovalis is a very thin part of the septum.
What parts does the interventricular septum consist of during development?
The interventricular septum consists of two parts:
- the muscular part;
- the membranous part.
During development, the ventricular primordium is divided by the interventricular septum. At the stage of the incomplete interventricular septum, there is an opening in it; the closure of this opening occurs 6–7 weeks after birth.
From the left ventricle side, the interventricular septum is smooth, without trabeculae. The membranous part is very thin.
The endocardium develops from MESENCHYME (like vessels), while the myocardium and epicardium develop from the myoepicardial plate of the splanchnotome.
Class and exam questions
From which embryonic primordia do the heart tunics develop?
The endocardium develops from the mesenchyme. The myocardium and epicardium are formed from the visceral layer of the splanchnotome (the myoepicardial plate).
Why does the heart tube acquire an S-shape?
The single heart tube grows in length much faster than the surrounding structures of the embryo. Due to a lack of space, it is forced to bend, forming an S-shaped loop.
How do the atrioventricular valves form?
They develop as folds (duplicatures) of the endocardium, into which connective tissue from the epicardium subsequently grows. Myocardial tissue is also present in the right valve.
What else to study on this topic
- Histological structure of the endocardial layers
- Features of the development of the right atrioventricular valve
- Mechanisms of division of the truncus arteriosus
- Stages of formation of the interatrial and interventricular septa
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