J. A. Los

Isomyosin expression patterns in tubular stages of chicken heart development: a 3-D immunohistochemical analysis

SummaryThe 3-D distribution of atrial and ventricular isomyosins is analysed in tubular chicken hearts (stage 12+ to 17 (H/H)) using antibodies specific for adult chicken atrial and ventricular myosin heavy chains, respectively.At stage 12+ (H/H) all myocytes express the atrial isomyosin; furthermore, all myocytes except those originally situated in the dorsolateral wall of the sinu-atrium coexpress the ventricular isomyosin as well. Moreover, it appears that recently incorporated myocardial cells at both ends of the heart tube start with a coexpression of both isomyosins. From stage 14 (H/H) onwards a regional loss of expression of one of either isomyosins is observed in the atrial and ventricular compartment. In this way the single isomyosin expression types that are characteristic for the adult working myocardium of the atria and ventricles arise. So, the isomyosin expression patterns are, unexpectedly, hardly useful to discriminate the different heart parts of the tubular heart. The ventricle, defined by its adult type of isomyosin expression, is even not detectable before stage 14 (H/H). Interestingly, interconnected coexpression areas, which may be precursor conductive tissues, are still present at stage 17 (H/H) in the outflow tract, the ventricular trabeculae, the atrio-ventricular transitional zone and in the sinuatrium.The pattern of isomyosin coexpression was found to correlate with a peristaltoid contraction and a slow conduction velocity, whereas single expression areas correlate with a synchronous contraction and a relatively fast conduction velocity.The possible implications of the changing isomyosin pattern for the differentiation of the tubular myocardium, in particular in relation to the development of the conductive tissues, will be discussed.

Isomyosin expression in developing chicken atria: a marker for the development of conductive tissue?

SummaryIsomyosin expression patterns in embryonic chicken atria during the first two weeks of development were analyzed immunohistochemically.In the 3-days embryonic chicken heart (HH 19–20), strong coexpression of both isomyosins can be found as band-like zones at the lateral sides of the sinoatrial junction. The zones converge on the bottom of the atrium and continue as a band around the atrioventricular canal.In the 5-days heart (HH 27–28) the coexpression area encompasses the entire sinoatrial junction and extends into parts of the sinus venosus and into the dorsocaudal atrial wall.In the 7-days heart (HH 32–33) the relative extension of coexpression areas reaches its maximum. Coexpression is also found in a ring-like band in the ventral (bottom) wall of the atria peripheral to the ring-like band in the atrioventricular junction. The latter band has now become continuous with the coexpression area in the bottom of the interatrial septum. Caudally coexpression extends behind the atrioventricular cushions towards the interventricular septum and cranially coexpression of the atrioventricular junction has become continuus with that of the ring around the ouflow tract (cf Sanders et al.1986). In the second week of incubation a decrease of coexpression is observed.The isomyosin expression pattern described in this study has put forward additional arguments that the conductive tissue originates from areas that continue to express both isomyosins relatively late in development.

The local expression of adult chicken heart myosins during development

SummaryImmunofluorescence studies were performed on serial sections of three days embryonic chicken hearts using antibodies specific for adult atrial and ventricular myosin heavy chains respectively.The anti-ventricular myosin serum reacted with the entire myocardium showing a decreasing intensity going from the truncus arteriosus to the atrial part; however, the antiatrial myosin serum reacted weakly with the myocardium of the atrial part.Two other interesting observations were made, i) the anti-atrial myosin serum reacted with non-myocardial cells the cardiac jelly, ii) both antisera reacted with a thin myocardial layer, extending from the ventral wall of the atrial part via the medio-dorsal wall of the atrio-ventricular canal to the dorsal wall of the ventricular part

The local expression of adult chicken heart myosins during development. II. Ventricular conducting tissue.

SummaryThe development of the ventricular conducting tissue of the embryonic chicken heart has been studied using a previous finding that morphologically recognizable atrial conducting tissue coexpresses the atrial and the ventricular myosin isoforms. It is found that, by these criteria, at 9 days part of the ventricular conduction system consists of a myocardial ring located around the infundibula of the aorta and truncus pulmonalis. Part of this ring is formed by the retro-aortic root branch. The ring continues via the septal branch into the atrioventricular bundle and its branches, that all express both myosin isoforms. The retroaortic root branch could be traced back as a part of the myocardial wall of the truncus arteriosus at the 4 days embryonic stage.At the 16th day of development, the septal branch, atrioventricular bundle and left and right bundle branches no longer express the atrial isomyosin, but two bundles originating from the septal branch still express both isomyosins, one being the retro-aortic root branch, the other being only immunologically recognizable and directed to the ventral side of the truncus pulmonalis; this latter we call the pulmonary root branch. Both bundles are remnants of the myocardial ring.

The conducting tissue in the adult chicken atria

Summary1.A three-dimensional reconstruction from serial sections of adult chicken heart was made to verify whether Purkinje cells, that can be recognized by a number of wellknown histological criteria, form specialized tracts in the adult chicken atria. This reconstruction revealed a loosely arranged network of Purkinje cells connecting the two atria. This network has not been described before. No tracts could be detected between the sinoatrial and the atrioventricular nodes.2.These atrial Purkinje cells express the atrial and ventricular myosin isoform, as determined by the use of monoclonal antibodies that were prepared against atrial and ventricular myosin isoform, respetively.3.Some atrial myocytes that are topographically closely related to the Purkinje cells and that cannot be distinguished from the surrounding myocytes with conventional histological criteria, express, apart from the atrial myosin isoform, also the ventricular myosin isoform.4.The similar expression pattern of these two cell types and their close topographical relationship suggest the presence of a more elaborate system specialized in conduction than the well-known conductive system found with conventional histological techniques.

The fusion of the endocardial cushions in the heart of the chick embryo

SummaryIn an investigation concerning the so-called “fusion” of the atrioventricular endocardial cushions in the heart of the chick embryo the following facts were established:1.The endocardial cushions in the chick embryo do really fuse. However, the region of fusion is less extensive than is generally accepted.2.No regressive cellular changes were observed in the fusing endothelia with light or electron microscopy. On the contrary, mitotic activity and possibly even erythroblastic transformation were observed in the endothelium as well as in the subendothelial mesenchyme.3.Under the light microscope, the process of fusion is characterized by interdigitation of the endothelial cells. In later stages the endothelial character of these cells disappears in such a way as to give rise to one single mesenchymal cushion mass.4.Under the electron microscope the fusing endothelial cells form extremely long tonguelike cytoplasmic processes which protrude freely into the lumen where they may fuse with similar processes from other endothelial cells of the same side. In this way small compartments become segregated from the lumen. the plasma membrane of the processes may also fuse with the plasma membrane of the cell from which they originate. Alternating persistence and disappearance of the plasma membrane gives rise to many membrane-lined intracellular vesicles.5.The numerous myelin figures found in and between the endothelial surfaces appear to represent the blind and empty membranous linings stripped off from cytoplasmic processes. The intracellular myelin figures are probably incorporated into the cells in the same way as described above. Apart from this form of endocytosis, signs of conventional micropinocytotic activity are present.6.The density of endoplasmic reticulum suggests intensified protein synthetic activity, in which membrane-bound as well as free ribosomes and polysomes seem to participate. Moreover, increased mitochondrial density in the cells in the fusion region is evident.7.Specialized junctional complexes between the plasma membranes in the form of desmosome-like junctions are relatively scarce.8.Fusion of cytoplasmic tongues of endothelial cells with adjacent endothelial cells of the same side or of the opposite side is responsible for the exchange of large quantities of cytoplasma between endothelial cells and, as a consequence, for displacement of cell boundaries without loss of cell individuality.

The vascular system in the orbit: spatial relationships

The relationships between blood vessels and orbital connective tissue, which forms an interindividually constant, bilateral symmetrical system of connective tissue septa bordering adipose tissue compartments, are characteristic and constant but essentially different for arteries and veins. The arteries run within the adipose tissue compartments only incidentally contacting the septa where they perforate them. The veins, in contrast to this, run within the septa, thus making up the walls of the adipose tissue compartments together with the septa. Resulting from this difference in histological relation to the connective tissue, arteries and veins show a different three-dimensional pattern. The arteries form a radiating system, whereas the veins form an anastomosing complex of venous rings which are arranged circularly inside as well as outside the muscle cone. The microvascular system has its own adaptation to the connective tissue system. It is predominantly confined to the adipose tissue compartments.

The position of the left and right ventricular outlets during septation

SummaryA comparative study was made of the relative position of the outflow tracts of chicken and rat hearts with respect to the ventricles during septation. For this purpose the position of the left and right ventricular outlet including the aortic and pulmonary valve primordia and the left and right ventricle were established with respect to the midsagittal plane of the embryo, using reconstructions of serial sections of chicken (stage 28–30) and rat (stage 28–30) embryos. In the chicken embryo no rotation of the outflow tract occurs, i.e. the position of the aortic and pulmonary valve primordia with respect to the left and right ventricle remains the same. In the rat embryo a clockwise rotation of the aortic and pulmonary valve primordia with respect to the ventricles does occur. This is in fact a detorsion. The left and right ventricle and the left ventricular outlet do not show change in position with regard to the midsagittal plane. The left ventricular outlet always straddles the interventricular septum, both lying in the midsagittal plane. These interspecies differences in the degree of detorsion of the outflow channels before septation may explain the differences in the extent of the region of contact between the endocardial outflow tract ridges.