Nkx2.5 cell-autonomous gene function is required for the postnatal formation of the peripheral ventricular conduction system.

The ventricular conduction system is responsible for rapid propagation of electrical activity to coordinate ventricular contraction. To investigate the role of the transcription factor Nkx2.5 in the morphogenesis of the ventricular conduction system, we crossed Nkx2.5(+/-) mice with Cx40(eGFP/+) mice in which eGFP expression permits visualization of the His-Purkinje conduction system. Major anatomical and functional disturbances were detected in the His-Purkinje system of adult Nkx2.5(+/-)/Cx40(eGFP/+) mice, including hypoplasia of eGFP-positive Purkinje fibers and the disorganization of the Purkinje fiber network in the ventricular apex. Although the action potential properties of the individual eGFP-positive cells were normal, the deficiency of Purkinje fibers in Nkx2.5 haploinsufficient mice was associated with abnormalities of ventricular electrical activation, including slowed and decremented conduction along the left bundle branch. During embryonic development, eGFP expression in the ventricular trabeculae of Nkx2.5(+/-) hearts was qualitatively normal, with a measurable deficiency in eGFP-positive cells being observed only after birth. Chimeric analyses showed that maximal Nkx2.5 levels are required cell-autonomously. Reduced Nkx2.5 levels are associated with a delay in cell cycle withdrawal in surrounding GFP-negative myocytes. Our results suggest that the formation of the peripheral conduction system is time- and dose-dependent on the transcription factor Nkx2.5 that is cell-autonomously required for the postnatal differentiation of Purkinje fibers.

His-Purkinje lineages and development.

The heartbeat is initiated and coordinated by a multi-component set of specialized muscle tissues collectively referred to as the pacemaking and conduction system. Over the last few years, impetus has gathered into unravelling the cellular and molecular processes that regulate differentiation and integration of this essential cardiac network. One focus of our collective work has been the developmental history of cells comprising His-Purkinje tissues of the conduction system. This interest in part arose from studies of the expression of connexins in periarterial Purkinje fibres of the chick heart. Using lineage-tracing strategies, including those based on replication-defective retroviruses and adenoviruses, it has been shown that conduction cells are derived from multipotent, cardiomyogenic progenitors in the tubular heart. Moreover, heterogeneity within myocardial clones has indicated that the elaboration of the conduction system in the chick embryo occurs by progressive, localized recruitment from within this pool of cardiomyogenic cells. Cell birth dating has revealed that inductive conscription of cells to central elements of the conduction system (e.g. the His bundle) precedes recruitment to the peripheral components of the network (i.e. subendocardial and periarterial Purkinje fibres). Birth dating studies in rodents suggest an analogous recruitment process is occurring in this species. In addition to summarizing earlier work, this chapter provides information on ongoing studies of cell-cell signalling and transcriptional mechanisms that may regulate the development of His-Purkinje tissues.

Differential developmental effects of acute hypoxia on the rabbit atrioventricular conduction axis.

The differential developmental effects of hypoxia on antegrade fast and slow and retrograde conduction through the atrioventricular junction are unknown. This study describes the effects of hypoxia on fast and slow antegrade atrioventricular node, infra-Hisian and retrograde conduction in immature and mature hearts during premature pacing protocols in excise, perfused adult and neonatal rabbits. The results are: (1) antegrade conduction delay through the atrioventricular node is the same developmentally, but delay through the His-Purkinje system is greater in adults; (2) hypoxia reduces the extra delay in the His-Purkinje system in adults; (3) fast atrioventricular node conduction is more sensitive to hypoxia in neonates than in adults, and slow atrioventricular node conduction is more sensitive to hypoxia in adults than in neonates, and (4) retrograde atrioventricular node conduction is more resistant to hypoxia in neonates than in adults.

Desarrollo del tronco del haz de His en embriones humanos / Development of truncus of the bundle of His in human embryos

El corazón humano comienza a latir a los 21 más menos 1 día durante la 4ta. semana de gestación. El latido es miogénico y el corazón en desarrollo comienza a latir al mismo tiempo que se va desarrollando y antes de existir una vía circulatoria completa. La actividad se inicia en la porción bulboventricular. Las características del sistema conductivo que se presentan en el adulto, no se han diferenciado totalmente a la 9a. semana del desarrollo, pero el primordio del nódulo sinusal y del haz de His son perfectamente reconocibles. Este primordio presenta algunos elementos derivados de células ectomesenquimáticas. se estudiaron nueve embriones (3 de siete, 3 de ocho y 3 de nueve semanas de desarrollo), cortados seriadamente en sección transversal y teñidos con hematoxilina eosina. En los embriones de siete semanas no se encontraron indicios evidentes del tronco del haz de His. Los embriones de ocho y nueve semanas mostraron tejido perteneciente al primordio del has de his. Estas imágenes se pueden observar en las láminas presentadas

Normal variations and pathologic changes in structure of the cardiac conduction system and their functional significance.

The rarity with which the cardiac conduction system is carefully examined in cases of sudden unexpected cardiac death is deplorable. Whatever the reasons for this failure, it is clearly one of the major lost opportunities for improving our understanding of a major national heart problem. To illustrate some of the morphologic changes found in such cases, two categories are discussed: normal variations (which may themselves promote electrical instability) and changes due to disease. During normal postnatal morphogenesis of the atrioventricular (AV) node and His bundle, there are a variety of derangements that may cause malfunction of those critical structures. In addition to large vessel coronary disease and platelet disorders, narrowing of small coronary arteries may also lead to ischemic damage in the conduction system. Certain intracardiac tumors and systemic diseases of an infiltrative or inflammatory nature may also involve elements of the conduction system, leading to arrhythmias or conduction disturbances with their clinical counterparts of syncope and sudden death. How any of these and many related anatomic changes may participate in the pathogenesis of electrical instability of the heart deserves much more careful study, but an essential requirement will be the wider practice of making careful clinicopathologic correlations.

The developing atrioventricular bundle and its branches in relation to small induced ventricular septal defects in the heart of chick embryos.

The form and position of the developing atrioventricular bundle and its branches has been studied in the hearts of domestic fowl embryos operated upon to induce delayed closure of the interventricular foramen. The operation was performed at 3 days of incubation when the outflow tract of the tubular heart was temporarily distorted by placing a nylon rod beneath the tract for 48 hours. The hearts from operated embryos were sectioned serially and examined with the light microscope at 11 days of incubation; normal hearts of the same incubation age were available for comparison. Small ventricular septal defects were found in a proportion of the operated hearts, and the form and position of developing conducting tissue with relation to the defects was of particular significance. The atrioventricular bundle and the septal component of the ring branch were represented by discrete fasciculi, but the right and left limbs of the bundle were disposed as a drape of tissue overlying the crest of the developing muscular septum, and situated beneath both the ventricular septal defect and cushion tissue of the septal truncus ridge. These observations of the developing conducting tissue in operated hearts are compatible with the concept that the bundle and branches develop normally from part of a precursor tissue distributed as a complete subendocardial sleeve in the tubular heart. They also imply that in the normal adult avian heart the positions of the atrioventricular bundle and its branches denote the junctional zones of the components forming the ventricular septum.

Observations on the development of the human atrioventricular node and bundle.

This light microscopic study of the cardiac junctional tissues was based on 27 human embryos, fetuses and postnatal hearts. Evidence was presented that superficial and deep portions of the postnatal AV node were derived from two cellular primordia in the posterior wall of the common atrium at the 6-mm stage. The small right primordia was associated with the right venous valve and give rise to the loosely organized superficial AV node that extended posteriorly to the coronary sinus ostium. A larger left primordia formed the more compact deep subdivision of the AV node located against the anulus fibrosus. In most postnatal hearts the two subdivisions are partially or completely fused to form the adult AV node. Failure of the nodal primordia to fuse during cardiogenesis may result in two separate nodal cell aggregates above the anulus. The present observations provide a rational explanation for the two AV nodal masses described in the literature and an additional specimen that is illustrated in this communication. An AV bundle was first identified in a 13-mm embryo and appeared to be derived from large clear cells of the posterior AV canal. At 25 mm the bundle formed a broad band across the top of the IV septum and continued into both ventricles. At this stage multiple cell strands penetrated the endocardial cushion to connect the AV bundle to the two nodal primordia. Failure of normal fusion between the AV node primordia and AV bundle can result in a variety of junctional anomalies including congenital heart block.

[Incorporation of 3H-thymidine into the cells of the heart conduction system (sinoatrial and atrioventricular nodes, bundle of His) during cardiogenesis in mice]. / Vkliuchenie 3H-timidina v kletki provodiashchei sistemy serdtsa (sinoatrial'nyi i atrioventrikuliarnyi uzly, puchok Gisa) v protsesse kardiogeneza myshi.

The number of DNA synthesizing myocytes in the conductive system, as compared with the working myocardium, was determined by means of light autoradiography. At the same time the number of mitosing cells was counted. The indices of labelled nuclei in the sinoatrial and atrioventricular nodes of the embryos after the single and repeated 3H-thymidine injections were significantly lower than in the working ventricle and atrium. During the postnatal development (3-11 days) only in the sinoatrial node the index of labelled nuclei was significantly lower than in the working ventricle. The proliferative activity in all the myocardium parts fell markedly by the 13th day of development. Following 10 3H-thymidine injections with 12 hrs intervals, the indices of labelled nuclei varied from 0.03 to 0.67% in the working ventricle and atrium and from 0.13 to 1.0% in the conductive system of adult mice. During the embryogenesis the index of labelled nuclei in the sinoatrial node was significantly higher than in the atrioventricular one and during the postnatal development-vice versa.

The human atrioventricular junctional area. A morphological study of the A-V node and bundle.

A study has been conducted into the morphological arrangement of the atrioventricular junctional area of human heart. The area was investigated in infantile, child, young adult, middle-aged, and old nodes. Although marked individual and ageing variations were observed, a general pattern of nodal architecture could be distinguished. The junctional area was therefore divided into four areas: (1) a transitional zone, (2) the compact node, (3) the penetrating bundle, and (4) the branching bundle. The transitional zone was intermediate between atrial myocardium and compact nodal specialized myocardium, but itself exhibited specialized characteristics. An important connection of the zone was to the myocardium of the left side of the interatrial septum. The compact node was itself composed of two segments which approached each other anteriorly from the mitral and tricuspid extremities of the septal anulus fibrosus. The junction of compact node and penetrating bundle could not be distinguished using cytological criteria. It was arbitrarily defined as the last point to make contact with transitional cells. Tissue distal to this was considered as penetrating bundle. A bypass tract was defined as any fiber contacting the bundle distal to this point, but such tracts were not observed in normal hearts. The branching bundle originated at the point of bifurcation of the penetrating bundle.