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Chapter 9: MOLECULAR DEVELOPMENT OF THE HEART CONDUCTION SYSTEM DEVELOPMENT

The development of the conduction system has fascinated cardiovascular embryologists from the moment it became clear that a subpopulation of specialized myocytes is responsible for the regulation of the cardiac impulse in the heart.16! During the last decade, several studies have revealed new aspects regarding the development of the conduction system. Immunohistochemical studies have shown that the developing conduction system in humans and other vertebrates is characterized by the expression of a unique set of antigens and genes, some of which also are expressed in the nervous system, sometimes referred to as neuromuscular markers (Fig. 914) 125,162,163 Retroviral cell-targeting and -tracing methods have defined subpopulations of cardiomyocytes that differentiate into Purkinje's cells within the trabecular myocardium.125,164,165 Altered patterns of ventricular depolarization have been recognized in association with structural heart defects, such as the pattern of depolarization noted with endocardial cushion defects, and conduction abnormalities associated with atrial septum defects, as has been observed in patients with mutations in the Nkx2.5 and TBX5 (Holt Oram syndrome) genes.166

Vertebrate Hearts

Figure 9-14: Expression of neuromuscular markers in the developing vertebrate heart. Panel A shows a transverse section of a human heart at 6 weeks of development immunohistochemically stained for the presence of a carbohydrate moiety recognized by the monoclonal antibody GlN2 (see also Wessels et al.125). The section shown in panel B is from a rabbit embryo at 15 days of development and is immunohistochemically stained for the presence of neurofilaments (Wessels et al.139). RAVR = right atrioventricular ring bundle; His = bundle of His; LBB = left bundle

Figure 9-14: Expression of neuromuscular markers in the developing vertebrate heart. Panel A shows a transverse section of a human heart at 6 weeks of development immunohistochemically stained for the presence of a carbohydrate moiety recognized by the monoclonal antibody GlN2 (see also Wessels et al.125). The section shown in panel B is from a rabbit embryo at 15 days of development and is immunohistochemically stained for the presence of neurofilaments (Wessels et al.139). RAVR = right atrioventricular ring bundle; His = bundle of His; LBB = left bundle branch; RBB = right bundle branch.

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Essentials of Human Physiology

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