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Origin of mesenchymal tissue in the septum primum: a structural and ultrastructural study
H Arrechedera1, M Alvarez, M Strauss
1Intituto Venezolano de Cardiologia, Caracas, Venezuela.
Insights
The study reveals that endocardial cells activate and transform into mesenchymal cells, forming the septum primum in chick embryos. This process is crucial for normal heart development and preventing atrial communications.
Area of Science:
- Developmental Biology
- Embryology
- Histochemistry
Background:
- Formation of the septum primum is essential for proper heart septation.
- The cellular origins of the septum primum's mesenchymal tissue have been debated.
- Understanding this process is key to addressing congenital heart defects.
Purpose of the Study:
- To investigate the origin and development of septum primum mesenchymal tissue in chick embryos.
- To elucidate the role of endocardial cells in forming this critical cardiac structure.
- To explore potential links between developmental errors and persistent atrial communications.
Main Methods:
- Structural, ultrastructural, and histochemical analysis of chick embryos.
- Light microscopy observations of embryonic mouse hearts.
- Tannic acid staining to visualize extracellular matrix components.
Main Results:
- Septum primum mesenchymal tissue originates from activated endocardial cells between days 3 and 5.
- Endocardial cells exhibit increased migratory, secretory, and mitotic activity, then delaminate and translocate.
- Accumulation of extracellular matrix components, like proteoglycans, precedes and accompanies mesenchymal tissue formation.
- Similar processes were observed in embryonic mouse hearts, suggesting conserved mechanisms.
Conclusions:
- The endocardium is the primary source of septum primum mesenchymal tissue.
- Endocardial cell activation and extracellular matrix accumulation are key events in septum primum formation.
- Disruptions in this process may lead to persistent atrial communications, a type of congenital heart defect.
Abstract:
A structural, ultrastructural and histochemical study in chick embryos indicates that the septum primum mesenchymal tissue originate between 3 and 5 days of development and that their origin may be related to an activation of endocardial cells that cover the septum primum. By day 3, endocardial cells display migratory appendages, cell hypertrophy and an increase in secretory and mitotic activity. In later stages (day 4) hypertrophic endocardial cells undergoing division seem to delaminate and translocate toward the subendocardial space to give rise to free mesenchymal-type cells. These results suggest that the endocardium makes up the bulk of the septum primum mesenchymal tissue as has been demonstrated during mesenchymal tissue formation in the atrioventricular canal and outflow tract. Before and during mesenchymal tissue formation an accumulation of extracellular matrix components like proteoglycans can be visualized using tannic acid. These extracellular components might be related to the promotion of cellular events described during endocardial activation. The fusion of the septum primum with the atrioventricular (AV) endocardial cushions which would obliterate the foramen primum, occurs between mesenchymal tissues. Therefore, any alteration in the normal development of these mesenchymal tissues could be related to pathological cases of persistent atrial communications. Light microscopy preliminary observations of embryonic mouse heart indicate that septum primum mesenchymal tissue formation occurs similarly between mouse and chick embryos.