Normal Development of the Outflow Tract in the Rat

Abstract

The outflow tract (OFT) provides the structural components forming the ventriculoarterial connection. The prevailing concept that this junction "rotates" to acquire its definitive topography also requires a concept of "counterrotation" and is difficult to reconcile with cell-marking studies. Rats between 10 embryonic days (EDs) and 2 postnatal days were stained immunohistochemically and by in situ hybridization. DNA replication was determined by incorporation of bromodeoxyuridine and apoptosis by the annexin V binding and terminal deoxynucleotidyl transferase-mediated dUTP-X nick end labeling (TUNEL) assays. Starting at ED12, cardiomyocytes in the distal (truncal) part of the OFT begin to shed their myocardial phenotype without proceeding into apoptosis, suggesting transdifferentiation. Myocardial regression is most pronounced on the dextroposterior side and continues until after birth, as revealed by the disappearance of the myocardial cuff surrounding the coronary roots and semilunar sinuses and by the establishment of fibrous continuity between mitral and aortic semilunar valves. Fusion of the endocardial ridges of the truncus on late ED13 is accompanied by the organization of alpha-smooth muscle actin-and nonmuscle myosin heavy chain-positive myofibroblasts into a central whorl and the appearance of the semilunar valve anlagen at their definitive topographical position within the proximal portion of the truncus. After fusion of the proximal (conal) portion of the endocardial ridges, many of the resident myofibroblasts undergo apoptosis and are replaced by cardiomyocytes. The distal myocardial boundary of the OFT is not a stable landmark but moves proximally over the spiraling course of the aortic and pulmonary routes, so that the semilunar valves develop at their definitive topographic position. After septation, the distal boundary of the OFT continues to regress, particularly in its subaortic portion. The myocardializing conus septum, on the other hand, becomes largely incorporated into the right ventricle. These opposite developments account for the pronounced asymmetry of the subaortic and subpulmonary outlets in the formed heart.

Cardiac outflow tract: A review of some embryogenetic aspects of the conotruncal region of the heart

“…Indeed, the proximal conus absorption pulling down the distal conus and the truncus, as indicated by several authors (Goor et al, 1972;Pexieder, 1975;Watanabe et al, 2001), might produce, we think, some sort of sliding of the myocardial cuff down along the cardiac jelly, thereby allowing the aortic sac to envelop the intercalated truncal swellings and ultimately the developing sinuses of Valsalva. In this respect, it is also important to emphasize, as previously mentioned, that since no apoptosis was identified along the truncal myocardial wall (Ya et al, 1998a), as a consequence, the myocardial retraction is not to be referred to as a programmed cell death. Specifically, as far as the development of the sinuses of Valsalva is concerned, Anderson et al (2003) emphasize the fact that some sort of cavitation is supposed to occur within the truncal and the intercalated endocardial cushions.…”

“…Furthermore, Ya et al (1998a) emphasize the fact that no apoptosis was found at the truncal level; consequently, they excluded the fact that the truncal cardiomyocytes were replaced by a local invasion of mesodermic mesenchymal cells differentiating subsequently into smooth muscle cells. In contrast, Waldo et al (2005b) show that SHF pharyngeal mesoderm provides smooth muscle precursor cells to the caudal portion of the aortic sac.…”

“…In this respect, Ya et al (1998a) and more recently Anderson et al (2003) reemphasized the concept originally advanced by Patten (1964) and Arguello et al (1978) according to which, in the definitive heart, the smooth muscle tissue of the sinuses of Valsalva and even the intrapericardial portions of the great arteries are the result of a transdifferentiation of the myocardial mantel of the embryonic truncus arteriosus.…”

Versican proteolysis mediates myocardial regression during outflow tract development

“…Several processes have been proposed to account for the apparent shortening of the cardiac outlet: retraction of the distal myocardial rim toward the ventricle, perhaps including absorption of conal myocardium predominantly into the right ventricle (Anderson et al, 1974;Thompson and Fitzharris, 1979;Thompson et al, 1987); the loss of myocardium through apoptosis, prominent along the conal (proximal) myocardium (Watanabe et al, 1998;van den Hoff et al, 2000;Watanabe et al, 2001;Cheng et al, 2002;Kubalak et al, 2002;Sugishita et al, 2004), myocardialization of the proximal cushions (van den Hoff et al, 1999) and nonapoptotic loss of myocardium from the distal end (truncus) of the outlet through reorganization as arterial muscle (Arguello et al, 1978;Ya et al, 1998b;Yang et al, 2004). However, the underlying mechanisms for the replacement of the myocardial wall with smooth muscle are largely unknown and a topic of controversy.…”

Mutation in the Trapα/Ssr1 Gene, Encoding Translocon-Associated Protein α, Results in Outflow Tract Morphogenetic Defects

“…The endocardium undergoes EMT at the onset of endocardial cushion formation, when the endocardial cells invade the cardiac jelly, which is followed by migration of the NCC into the conotruncal ridges. Proliferation of the mesenchymal cells and cell apoptosis precede compaction and fusion of the ridges before myocardialization (35,41). While the essential role of NCC in the distal septation of the OFT is very well established, their participation in septation of the proximal OFT is still controversial even though the use of transgenic mice expressing markers in NCC has shown their presence in endocardial ridges (18,38).…”