Anat Embryol (1987) 176:515-523 Anatomy and Embryology 9 Springer-Verlag 1987 Isomyosin expression in developing chicken atria: a marker for the development of conductive tissue ? Imelda J.M. de Groot, Edward Sanders, Saskia D. Visser, Wouter H. Lamers, Frits de Jong, Johannes A. Los, and Antoon F.M. Moorman Department of Anatomy and Embryology, University of Amsterdam, Amsterdam, The Netherlands Summary. Isomyosin expression patterns in embryonic chicken atria during the first two weeks of development were analyzed immunohistochemically, In the 3-days embryonic chicken heart (HH19-20), 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 con- tinue as a band around the atrioventricular canal. In the 5-days heart (HH27-28) 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 (HH32-33) 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 be- hind the atrioventricular cushions towards the interventri- cular septum and cranially coexpression of the atrioventri- cular junction has become continous with that of the ring around the outflow 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. Key words: Chicken heart Conduction system Immuno- histochemistry - Myosin isoforms - Development Introduction Studies on the development of the conductive tissue have usually focussed on the sinoatrial (SA) node and atrioven- tricular (AV) node and bundle. It is generally accepted that these structures can be morphologically detected in the sec- ond week of incubation of chicken embryos (Prakash et al. 1960; Kim et al. 1980; Vassall-Adams 1982). These studies are based on the cytological architecture Offprint requests to: A.F.M. Moorman, Department of Anatomy and Embryology, University of Amsterdam, Meibergdreef15, 1105 AZ Amsterdam, The Netherlands of these structures. Much less is known about the origin of the conductive tissue. Patten (1956) regarded the whole embryonic myocardium as primordial conducting tissue. He postulated that certain cells differentiate into working myo- cardium, whereas the remainder of the primordial myocar- dium would develop into the conduction system. Others have ascribed the origin of the conducting system to special- ized ring-tissue, that would be localized between the chambers of the primitive heart-tube. After rearrangements, parts of these rings are supposed to give rise to the definitive conduction system (Wenink 1976; Kim et al. 1980). One of the explanations for these possibly divergent opinions is the lack of specific staining procedures. For that reason we have undertaken an immunohistochemical study, since we and others have shown that conductive tis- sue retains some embryonic features for a longer period than surrounding myocardium, and expresses e.g. both atrial and ventricular isomyosins (Sartore et al. 1978; de Groot et al. 1985; Sanders et al. 1986). Moreover, we could demonstrate that Purkinje cells and contiguous myocardial cells of the atria express both atrial and ventricular isomyo- sins (de Groot et al. 1985). Hence distinct conductive tissue may exist outside the morphologically recognizable parts of the conduction system. This communication reports the results of an immuno- histochemical study of the isomyosin expression patterns in the atria of the developing chicken hearts and discusses the possible significance of these patterns for the develop- ment of the atrial conductive tissue. Materials and methods The isolation of isomyosins from chicken heart and the production and characterization of the antibodies specific for adult atrial and ventricular myosin heavy chains, respec- tively, were described previously (Sanders et al. 1984; de Groot et al. 1985). Embryos were staged according to Ham- burger and Hamilton (1951). The results of embryos of 3, 4, 5, 7 and 13 days of development are reported as they show substantial change in myosin expression patterns compared to the previous stages. Immersion fixation was performed in a mixture of methanol, acetone, acetic acid and water (36: 36:8:20 v/v) for 2-24 hours, dehydration in dimethoxypropane (BDH) and embedding in Paraplast (Paraplast Plus, Lancer Ox- ford). The embryos were sectioned transversally, i.e. per- pendicular to the body axis in the thoracic region.