The impact of altered haemodynamics on the development of the epicardium

During embryo development the heart is the first functioning organ. Although quiescent in the adult heart, the epicardium is essential during development to form a normal functioning heart. Epicardial derived cells contribute to the heart as it develops, including fibroblasts and vascular smooth muscle cells. Previous studies have shown that a heartbeat is required for epicardium formation. Further, preliminary studies from our laboratory have shown that the development of the epicardium is aberrant when the haemodynamics are altered. This study aims to investigate how the epicardium and some of its derived cell lineages respond to altered haemodynamics in the developing embryo. Since the aetiology of many congenital heart diseases (CHDs) is unknown, we suggest that an alteration in the heart’s haemodynamics might provide an explanatory basis for some of them. In order to change the heart’s haemodynamics, outflow tract (OFT) banding using a double overhang knot was performed on Hamburger and Hamilton (HH) stage 21 chick embryos, with harvesting at different developmental stages. Upon alteration of haemodynamics, the epicardium exhibited abnormal morphology and minor bleeding at HH29 using morphological analysis. This phenotype was exacerbated at HH35 with severe changes in the structure of the extracellular matrix (ECM) and the coronary vasculature. A number of genes tied to ECM production were also differentially expressed in HH29 and HH35 OTB hearts including, collagen I and collagen XII. At HH35, there was also downregulation in a number of vascular genes. It is the first time that the importance of the epicardium is shown regarding CHDs that are caused by altered haemodynamics. In this study, the epicardium was found to be severely impacted by OFT banding. The altered phenotype also showed signs of becoming embryonically lethal as development ensued. More studies should be conducted regarding the effects of haemodynamics on the epicardium with respect to ECM and coronary vessel maturation

Characterisation of the developing heart in a pressure overloaded model utilising RNA sequencing to direct functional analysis

Cardiogenesis is influenced by both environmental and genetic factors, with blood flow playing a critical role in cardiac remodelling. Perturbation of any of these factors could lead to abnormal heart development and hence the formation of congenital heart defects. Although abnormal blood flow has been associated with a number of heart defects, the effects of abnormal pressure load on the developing heart gene expression profile have to date not clearly been defined. To determine the heart transcriptional response to haemodynamic alteration during development, outflow tract (OFT) banding was employed in the chick embryo at Hamburger and Hamilton stage (HH) 21. Stereological and expression studies, including the use of global expression analysis by RNA sequencing with an optimised procedure for effective globin depletion, were subsequently performed on HH29 OFT‐banded hearts and compared with sham control hearts, with further targeted expression investigations at HH35. The OFT‐banded hearts were found to have an abnormal morphology with a rounded appearance and left‐sided dilation in comparison with controls. Internal analysis showed they typically had a ventricular septal defect and reductions in the myocardial wall and trabeculae, with an increase in the lumen on the left side of the heart. There was also a significant reduction in apoptosis. The differentially expressed genes were found to be predominately involved in contraction, metabolism, apoptosis and neural development, suggesting a cardioprotective mechanism had been induced. Therefore, altered haemodynamics during development leads to left‐sided dilation and differential expression of genes that may be associated with stress and maintaining cardiac output

Altered haemodynamics cause aberrations in the epicardium

During embryo development, the heart is the first functioning organ. Although quiescent in the adult, the epicardium is essential during development to form a normal four‐chambered heart. Epicardial‐derived cells contribute to the heart as it develops with fibroblasts and vascular smooth muscle cells. Previous studies have shown that a heartbeat is required for epicardium formation, but no study to our knowledge has shown the effects of haemodynamic changes on the epicardium. Since the aetiologies of many congenital heart defects are unknown, we suggest that an alteration in the heart's haemodynamics might provide an explanatory basis for some of them. To change the heart's haemodynamics, outflow tract (OFT) banding using a double overhang knot was performed on HH21 chick embryos, with harvesting at different developmental stages. The epicardium of the heart was phenotypically and functionally characterised using a range of techniques. Upon alteration of haemodynamics, the epicardium exhibited abnormal morphology at HH29, even though migration of epicardial cells along the surface of the heart was found to be normal between HH24 and HH28. The abnormal epicardial phenotype was exacerbated at HH35 with severe changes in the structure of the extracellular matrix (ECM). A number of genes tied to ECM production were also differentially expressed in HH29 OFT‐banded hearts, including DDR2 and collagen XII. At HH35, the differential expression of these genes was even greater with additional downregulation of collagen I and TCF21. In this study, the epicardium was found to be severely impacted by altered haemodynamics upon OFT banding. The increased volume of the epicardium at HH29, upon OFT‐banding, and the expression changes of ECM markers were the first indicative signs of aberrations in epicardial architecture; by HH35, the phenotype had progressed. The decrease in epicardial thickness at HH35 suggests an increase in tension, with a force acting perpendicular to the surface of the epicardium. Although the developing epicardium and the blood flowing through the heart are separated by the endocardium and myocardium, the data presented here demonstrate that altering the blood flow affects the structure and molecular expression of the epicardial layer. Due to the intrinsic role the epicardium in cardiogenesis, defects in epicardial formation could have a role in the formation of a wide range of congenital heart defects

The impact of altered haemodynamics on the development of the epicardium

During embryo development the heart is the first functioning organ. Although quiescent in the adult heart, the epicardium is essential during development to form a normal functioning heart. Epicardial derived cells contribute to the heart as it develops, including fibroblasts and vascular smooth muscle cells. Previous studies have shown that a heartbeat is required for epicardium formation. Further, preliminary studies from our laboratory have shown that the development of the epicardium is aberrant when the haemodynamics are altered. This study aims to investigate how the epicardium and some of its derived cell lineages respond to altered haemodynamics in the developing embryo. Since the aetiology of many congenital heart diseases (CHDs) is unknown, we suggest that an alteration in the heart’s haemodynamics might provide an explanatory basis for some of them. In order to change the heart’s haemodynamics, outflow tract (OFT) banding using a double overhang knot was performed on Hamburger and Hamilton (HH) stage 21 chick embryos, with harvesting at different developmental stages. Upon alteration of haemodynamics, the epicardium exhibited abnormal morphology and minor bleeding at HH29 using morphological analysis. This phenotype was exacerbated at HH35 with severe changes in the structure of the extracellular matrix (ECM) and the coronary vasculature. A number of genes tied to ECM production were also differentially expressed in HH29 and HH35 OTB hearts including, collagen I and collagen XII. At HH35, there was also downregulation in a number of vascular genes. It is the first time that the importance of the epicardium is shown regarding CHDs that are caused by altered haemodynamics. In this study, the epicardium was found to be severely impacted by OFT banding. The altered phenotype also showed signs of becoming embryonically lethal as development ensued. More studies should be conducted regarding the effects of haemodynamics on the epicardium with respect to ECM and coronary vessel maturation