Morphogenesis of chordae tendineae. I: Scanning electron microscopy

Abstract

The formation of the chordae tendineae of the left atrioventricular valve in the chick embryo is described using scanning electron microscopy. These supportive structures for the valve cusps develop between days 6 and 13 of incubation. Elevations which represent the primitive papillary muscles form on the ventricular wall. These elevations bifurcate into thin, web-like folds which are attached to the primitive valve cusps. The folds are the primordia of the chordae tendineae. Linear ridges develop on the web between the cusp and papillary muscle. These ridges alternate with depressions. The depressions become perforate to create the individual chorda from the linear ridges. Multiple perforations form initially but they typically consolidate to create one large aperture between two chordae. Some interchordal connections of tissue do persist throughout the period studied. During the period of perforation, prominent rounded cells are typical of the endocardium between the chordae. These cells are similar at the scanning electron microscope level to those present in the formation of the foramina secunda of the atrial septum. Primary, secondary, and tertiary chordae tendineae appear to develop in the same manner. First order chordae (those attached at the free margin of a cusp) are not found in the chick embryo. The majority of the chordae are second order, which insert into the ventricular surface of the cusp a short distance from the free edge. These chordae typically have a horizontal banding or grooving along their length. Third order chordae which extend from the papillary muscle to the ventricular wall are also present. It is suggested that chordal development is a programmed cellular and hemodynamic event.

Formation and perforation of closing plates in the chick embryo

“…Instead, perforations through the closing plates and oral membrane appear to be brought about by cellular rearrangement and disassembly of intercellular junctions resulting in the separation of intact adjacent cells. This may be a characteristic feature of the union between basal epithelial surfaces since similar cellular reorientation has been demonstrated during formation of the foramina secunda in the interatrial septum (Morse and Hendrix, 1980), during development of chordae tendineae (Morse et al, 1984), and during disappearance of the dorsal mesocardium (Litke and Johnson, 1980) in the chick heart.…”

Lineage and Morphogenetic Analysis of the Cardiac Valves

“…It has been suggested that myocardium transdifferentiates into fibrous tissues during valvar formation, or mesenchymal cells into myocardium during the "muscularization" of the mesenchymal septal structures. 7,13,31 However, the pattern of labeling in the ␣MHC-Cre/R26R embryos was identical to the pattern of expression of Serca2 and complementary to the pattern of labeling in the Tie2-Cre/ R26R embryos. Therefore, we conclude that after the onset of recombination in the heart at E8 to E9, there is no transdifferentiation of either myocardial cells or endocardial cells and their derived mesenchyme.…”

“…2,3,7,[11][12][13] Using the recombination system, we have shown for the first time that the myocardial lineage does not contribute materially to the definitive fibrous tissues of the heart, including the valvar leaflets and cords of the tension apparatus. Our data indicate that these components are generated almost entirely from the cells derived from the endocardium, a finding also reported very recently by Lincoln et al, 32 although we cannot rule out the possibility that small numbers of cells of nonendocardial origin are present in these components.…”

“…7 The material contributions of the different cell types to the components of the valvar complex, nonetheless, remain controversial. To date, it has been suggested that these components are derived from the subepicardial mesenchyme of the atrioventricular groove, 11,12 the myocardium, 2,3,7,13 and the atrioventricular endocardial cushions. 6 -9 Further insight into these lineages is essential if we are to clarify the origin of the components of the valvar apparatus.…”

Papillary Muscles in the Heart Ventricles of the Mature Dromedary Camel (Camelus dromedaries) with Special References to the Chordae Tendineae: Gross and Microanatomy

“…Those structures are in direct contact with blood flow and actuate determining the adequate blood course, participating directly in the myocardium contractile function. The structure of left atrioventricular valve apparatus is composed by the mitral annulus, mitral valve leaflets, chordae tendineae and papillary muscles and has been studied elsewhere [15,16] concerning the diseases that attempt its integrity and function, including its morphogenesis [17,18].…”