The Chicken as a Model Organism to Study Heart Development

Abstract

Heart development is a complex process and begins with the long-range migration of cardiac progenitor cells during gastrulation. This culminates in the formation of a simple contractile tube with multiple layers, which undergoes remodeling into a four-chambered heart. During this morphogenesis, additional cell populations become incorporated. It is important to unravel the underlying genetic and cellular mechanisms to be able to identify the embryonic origin of diseases, including congenital malformations, which impair cardiac function and may affect life expectancy or quality. Owing to the evolutionary conservation of development, observations made in nonamniote and amniote vertebrate species allow us to extrapolate to human. This review will focus on the contributions made to a better understanding of heart development through studying avian embryos-mainly the chicken but also quail embryos. We will illustrate the classic and recent approaches used in the avian system, give an overview of the important discoveries made, and summarize the early stages of cardiac development up to the establishment of the four-chambered heart.

Figure 1.

Experimental approaches in avian embryos. Schematic overview of experimental approaches commonly used in avian species, namely grafting, microinjection, and electroporation. Grafting has, for example, identified a subpopulation of neural crest cells from the neural folds near somites 1–4 that contribute to the outflow tract. Various microinjection techniques have been described, including the delivery of anatagomirs into the myocardial wall to block microRNA function. This approach allows analysis of up to 3 days postinjection (∼HH27), which covers different phases of cardiac remodeling. Finally, chick embryo electroporation is widely used and allows the targeted delivery of plasmid or viral constructs designed to interfere with cellular processes. (M) mesencephalic, (C) cardiac, (V) vagal, (T) trunk.

Figure 2.

From heart fields to early looping. The major steps of early heart formation are illustrated here in cartoon form. The bilateral cardiogenic mesoderm comprises progenitors of the first heart field (green) and second heart field (red), which are organized mediolaterally (HH5). Together they contribute to outflow tract and right ventricle (red) and left ventricle and atria (green). During fusion, which is closely associated with foregut morphogenesis (see text), the mediolateral organization converts into an anteroposterior organization generating the early heart tube by HH10. At that stage, the heart is still linear and shortly after undergoes bulging and dextral (rightward) bending, which initiates the C-shape (HH11–HH13). Subsequent remodeling leads to compartmentalization of the heart.

Figure 3.

Formation of cushions and septa. Schematic illustrations of sectioned hearts that depict important steps of cardiac cushion maturation to septa and valves in the chicken embryo. By HH20, the heart consists of a primitive atrium and a primitive ventricle; endocardial cushions have formed in the outflow tract (OFT) and at the atrioventricular canal (AVC) junction. Mesenchymal cells have entered the cushions, originating from epithelial-to-mesenchymal transition (EMT) induced by the endocardium. By HH30, the dorsal endocardial cushion and ventral endocardial cushion have fused to form the septum intermedium (SIM), which is joined with the interatrial septum. More cushions have developed, namely, the left and right lateral cushions, which contribute to the formation of the tricuspid and mitral valves later in development. Growth of the ventricular septum (VS) toward the SIM has not finished by this stage but is completed by HH38 separating the ventricles. By then, all valves have matured. (C) conus, (CJ) cardiac jelly, neural crest cell (NCC), (DEC) dorsal endocardial cushion, (EC) endocardial cushions, (ENC) endocardium, (E) epicardium, (LA) left atrium, (LLC) left lateral cushion, (LV) left ventricle, (M) myocardium, (MV) mitral valve, (OFTV) outflow tract valves, (PA) primitive atrium, (PV) primitive ventricle, (RA) right atrium, (RLC) right lateral cushion, (RV) right ventricle, (T) trabeculae, (TV) tricuspid valve, (VEC) ventral endocardial cushion.

Figure 4.

Chick versus human heart. Shown are illustrations of the chick and human heart to depict their strong similarity and a few species-specific differences. Among these differences are a smaller right ventricle (RV) in the chick, different directions of the ascending aorta, right in chick and left in human, and a thicker myocardium in the chick to facilitate higher cardiac load together with higher heart rates. (LA) left atrium, (LV) left ventricle, (RA) right atrium.