Myocardialization of the Cardiac Outflow Tract

Abstract

During development, the single-circuited cardiac tube transforms into a double-circuited four-chambered heart by a complex process of remodeling, differential growth, and septation. In this process the endocardial cushion tissues of the atrioventricular junction and outflow tract (OFT) play a crucial role as they contribute to the mesenchymal components of the developing septa and valves in the developing heart. After fusion, the endocardial ridges in the proximal portion of the OFT initially form a mesenchymal outlet septum. In the adult heart, however, this outlet septum is basically a muscular structure. Hence, the mesenchyme of the proximal outlet septum has to be replaced by cardiomyocytes. We have dubbed this process "myocardialization." Our immunohistochemical analysis of staged chicken hearts demonstrates that myocardialization takes place by ingrowth of existing myocardium into the mesenchymal outlet septum. Compared to other events in cardiac septation, it is a relatively late process, being initialized around stage H/H28 and being basically completed around stage H/H38. To unravel the molecular mechanisms that are responsible for the induction and regulation of myocardialization, an in vitro culture system in which myocardialization could be mimicked and manipulated was developed. Using this in vitro myocardialization assay it was observed that under the standard culture conditions (i) whole OFT explants from stage H/H20 and younger did not spontaneously myocardialize the collagen matrix, (ii) explants from stage H/H21 and older spontaneously formed extensive myocardial networks, (iii) the myocardium of the OFT could be induced to myocardialize and was therefore "myocardialization-competent" at all stages tested (H/H16-30), (iv) myocardialization was induced by factors produced by, most likely, the nonmyocardial component of the outflow tract, (v) at none of the embryonic stages analyzed was ventricular myocardium myocardialization-competent, and finally, (vi) ventricular myocardium did not produce factors capable of supporting myocardialization.

Dynamic patterns of apoptosis in the developing chicken heart

“…The presence of PCD has been documented previously in developing cardiac tissues by our laboratory (Watanabe et al, 1998) and others (for reviews, see Pexieder, 1975;Fisher et al, 2000;Poelmann et al, 2000;van den Hoff et al, 2000).…”

“…Select LTR-stained and formalinfixed intact embryos were stained for MF20 (van den Hoff et al, 1999). Embryos with their hearts exposed were incubated at 4°C overnight in 1:4 DMSO:methanol solution, rehydrated in a graded alcohol series of 95%, 70%, and 50% methanol for 1 hr each at room temperature, incubated in PBS with 10% normal goat serum (NGS) for 1 hr to block nonspecific staining, incubated with a 1:200 dilution of MF20 overnight at 4°C, washed three times for 30 min in PBS with 10% NGS, incubated for 3 hr at room temperature with a goat anti-mouse IgG antibody labeled with Alexa Fluor 488 (Molecular Probes), washed with PBS, and observed with a fluorescence stereomicroscope or by confocal laser scanning microscopy.…”

The epicardium and epicardially derived cells (EPDCs) as cardiac stem cells

“…In this respect it is tempting to speculate that accessory AV bundles as seen in patients with Wolff-Parkinson-White syndrome could be the result of uncontrolled epicardial-tomyocardial differentiation, rather than a result of myocardial invasion of the mesenchyme as a result of ingrowth of existing myocardium (see e.g. Van den Hoff et al, 1999).…”

Probing chromatin landscape reveals roles of endocardial TBX20 in septation

“…Subsequently, cardiomyocytes that flank cushion mesenchyme protrude into cushion mesenchyme. This process, called myocardialization, involves polarized cell movements of OFT cardiomyocytes (18,19). In control embryos at E13.5, sarcomeric α-actinin-expressing (Actn2-expressing) cardiomyocytes invaded the OFT septum, and cells protruding from the left and right sides met at the base of the proximal OFT septum ( Figure 3B).…”