Reconstruction of the Human Sinoatrial Node ' RAYMOND CARL TRUEX, MARTHA Q. SMYTHE AND MARGARET J. TAYLOR Uepurtment of Anatomy, Temple University School of Medicine, Philadelphia, Pennsylvania ABSTRACT The superior vena cava and adjacent right atrium containing the sinoatrial node in each of five human hearts was studied histologically in serial scction. The tissue block of a 41 year old man was reconstructed in four colors to provide a three-dimensional model of the sinoatrial node, atrial relations, blood supply and nodal configuration. The resulting model demonstrated the curved course of the compact sinoatrial node and the mural relations of its tapered superior and inferior ends. Microscopic measurements of the boundaries of the five human nodes yielded an epicardial to endocardial mean thickness in the compact body of the node of 1.6 mm, that of the lower node was 0.6 mm. The mean length of the five nodes was 7.3 mm. Such microscopic delineation of the node is more accurate than gross measurements and indicates that the size of this vital mass of pacemaker tissue in man is smaller than the larger meafiurements usually given in the literature. Small strands of nodal muscle fibers follow longer or shorter courses before they become continuous with the larger more darkly stained atrial cardiac muscle fibers. We found no histologic evidence within the human node, or along its periphery, of continuity between the small nodal fibers and very large atrial fibers. The existence of a discrete sinoatrial (SA) node has been substantiated by many investigators since it was described originally by Keith and Flack ('07). Microscopic characteristics of the sinoatrial node in man and other animals have been reviewed and illustrated by several investigators (Copenhaver and Truex, '52; Lev. '60; James, '61; Robb, '65). Histochemical studies have demonstrated a high concentration of cholinesterase within the myofibrils of the nodal fibers of this structure (Carbonell, '56, '58; James and Spence, '66). Lev ('54) and James ('61) also have noted a marked increase in collagen fibers within the human sinoatrial node that occurs with advancing age. James ('61) has called attention to the origin of the blood supply, and the variable relations of the sinoatrial node to the nodal artery. Microdissections of the fresh human sinoatrial node are difficult to interpret and often misleading as to the precise shape and size o€ the node. It has a high collagen fiber content, is covered in part or all of its course by epicardial adipose tissue and variable layers of atrial muscle fibers, and its central portion (body) is difficult to extricate from the ramificaANAT. REC., 759: 371-378. tions of the nodal artery. The main portion of the node can be observed as a pale gray mass of fibers encircling the nodal artery, but one cannot delineate the precise boundaries nor the bger-like projections from the node after the most meticulous of dissections. It is equally impossible to follow by dissection the ramifications and terminations of the fine strands which constitute the more superior and inferior tapered ends of the node. Several diagrams and schemas have been presented to illustrate the relations and shape o f the sinoatrial node in man and other mammals (James, '61; Robb, '65; Truex, '66). However, because the human sinoatrial node extends through several hundred microscopic serial sections one cannot accurately synthesize or visualize the entire node from even a careful study of serial sections. To the author's knowledge only two attempts have been made to demonstrate the sinoatrial node in three dimension by wax model reconstruction. Halpern ('54) reconstructed the sinoatrial node of the rat heart to illustrate its re1 This investigation was supported by U.S.P.H.S. Research grant H-7047-05,and Career Award 5-K6- GM-14,092. 371 372 R. C. TRUEX, M. Q . SMYTHE AND M. J. TAYLOR lationship and unusual blood supply, while Robb, Kaylor and Turman (’48) reconstructed a comma-shaped sinoatrial node in a human fetus of 20 weeks gestation. The present reconstruction was undertaken in an attempt to better visualize the gross and microscopic relations of the sinoatrial node in the adult human heart. METHODS Five adult human hearts were obtained at early necropsy and form the basis for this morphologic study. Tissue blocks containing the superior vena cava and adjacent right atrium were fixed in neutralized 4% formaldehyde and prepared for histologic study by the paraffin technic. Each heart was sectioned in either the frontal or transverse plane. Serial sections were cut at 10 11. Slides from each specimen were prepared by the safraninephloxine-hematoxylin, Masson’s trichrome and Holmes’ silver staining procedures. The sinoatrial node (SA) region of a 41 year old male; cut in the transverse plane, resulted in 1500 sections and was selected as best suited for stereological reconstruction. The discrete nodal mass reconstructed represented an area of 5 by 15 mm (503 sections). As in a previous study of the human atrioventricular junction (Truex and Smythe, ’67) every fifth section through the compact SA node was projected and traced in different colors at a magnification of 15 X. These tracings were then transferred to colored dental wax plates 1 rnm in thickness. Connective tissue of the endocardium and epicardium were cut €rom blue plates, while atrial fibers of the myocardium and the arterial blood supply were made from red plates. The entire extent and ramifications of the sinoatrial node were reproduced in white, whereas the adjacent masses of atrial ganglia and related nerve bundles were cut from orange wax plates. Component structures for each level were then fitted together, and appropriate undercutting of the epicardial and myocardial layers were made in order to visualize the Fig. 1 Photomicrograph of Section 236 through tapered superior end of human SA node and adjacent right auricle. Note strands of small pale nodal fibers extending to the right between bundles of larger and more darkly stained atrial muscle fibers. Plane of section shown in figure 4. Male 41 years old. Hematoxylin-phloxine-safraninc stain. X 17. HUMAN SINOATRIAL NODE 373 Fig. 2 Photomicrograph of Section 436 through middle (body) of human compact SA node. Sinall nodal fibers appear on both sides of the artcry shown penetrating the node at this level. Bundles of atrial cardiac muscle fibers lie both superficial and deep to the SA node a t this Icvel. Plane of section is indicated in figure 4. Male 41 years old. Hematoxylin-phloxine-safranine stain. x 17. SA node as it coursed superiorly to inferiorly adjacent to the sulcus terminalis on the anterior wall of the right atrium. In the completed model a part of the right auricle is included to further aid in orientation (figs. 4, 5). RESULTS Many interpretations can result from microscopic examination of sections through different levels of the SA node unless serial sections are visualized in three dimension. There is little doubt that many erroneous impressions of the SA node have resulted from an overstudy of a few ran- dom sample slides of this area, much as the three blind men inspecting the tail, trunk and ears of an elephant. Each blind man felt he had obtained the most valid observations, yet they had all failed to see the total structural mass. A section through the finger-like strands of nodal fibers which ramify into the atrial myocardium posterior to the superior vena cava are identified in figure 1. Such tapering epicardid strands of the superior part of the SA node often are referred to as a part of the “head of the sinoatrial node.” A section cut more inferiorly through the compact portion of the node (body) is shown in figure 2. This 374 R. C. TRUEX, M. Q. SMYTHE AND M. J. TAYLOR Fig. 3 Photomicrograph of Section 636 through inferior end (tail) of human SA node. Note epicardial, myocardial, endocardial and pcriarterial strands of pale nodal fibers. Plane of section is indicated in figure 4. Male 41 years old. Hcmatoxylin-phloxine-safraninc stain. x 17. segment shows also the entrance of the artery into the mass of nodal tissue, as well as the location of the node deeper within the atrial myocardium. A section through the lower part of the SA node (tail) is illustrated in figure 3. In this view the nodal fibers surround the nodal artery, while small tapering strands continue toward the endocardium or within the myocardium for longer or shorter distances. Ultimately such small pale fibers become larger and blend with the more darkly stained atrial cardiac muscle fibers. The approximate levels of these three sections are indicated in the completed model (fig. 4). It should be noted that the nodal artery in this specimen was anomalous in both its course and origin. The vessel arose from the distal part of the right coronary artery as it approached the inferior vena cava and then ascended to penetrate the node. In 55% of human hearts the SA nodal artery arises from the initial part of the right coronary artery (James, '66b). The masses of ganglion cells located along the margins of the sinoatrial node, and shown on the anterior and posterior surfaces of the supcrior vena cava (figs. 4, 5) are presumably postganglionic parasympathetic neurons. Isolated ganglion cells were observed adherent to the margins of the SA node and scattered within thc atrial myocardium, but could not be included in the model (fig. 5). Irregularities and projections appear on the surface and margins of the node (fig. 5). They indicate small finger-like strands of nodal fibers which leave the node to enter and blend with adjacent fascicles of atrial cardiac muscle after longer or shorter distances. In this case, one additional point merits comment, namely, the more epicardial position of the tapered and fenestrated lower node region. In some specjmens this inferior portion of the SA node may be located more deeply in the atrial myocardium or adjacent to the endocardial surface. IIUNIAN SINOATRIAL NODE 3 75 Fig. 4 Photograph of reconstructed model of human SA node and right atriocaval junction. Arrows indicate levels of the three photomicrographs illustrated in figures 1 to 3. Completed model demonstrates size, shape and relations of SA node and its blood supply. The top of the model is slightly inferior to the iunction of the suaerior vena cava and rirrht atrium. Reconstructed niodel represents a 5 X 15 mm area of the iight atrium. Male 4 1 years old. DISCUSSION The completed four color model is similar to the C-shaped reconstruction of the fetal SA node shown by Robb, Kaylor and Turman (‘48). However, it has little resemblance to the block-like node of the rat as reconstructed by Ilalpern (’54). While the present model (figs. 4, 5) accurately depicts the SA node of this particular heart, i t should be born in mind that similar models of other human specimens might be expected to show minor, or even major, variations. It is interesting to note that the dimensions of the human SA node has been given as 15 mm long, 5 mm wide and a maximal thickness of 2 mm (James, ’66a) ; 25-30 mm in length and 2-5 mm thick (Bullard, ’63) ; and several centimeters long and about 1 cm wide (Lev, ’62). Such measurements indicate the futility of delimiting the SA node by semi-gross inspection. We have employed an ocular micrometer to delimit the microscopic extent of the compact portion of the sinoatrial nodes of the five adult human hearts used in this study. The epicardial to endocardial mean thickness of the compact nodal mass (body) was 1.6 mm, the mean thickness of the lower node was 0.6mm. The superior to inferior curved length of the compact Sa node varied from 5.2mm to 8.5 mm (mean, 7.3 mm). The specimen used to make the model (figs. 4 , 5 ) had the largest most beautifully delineated SA node that we have examined to date. It had an epicardial to endocardial thickness of 1.45 mm (head) and 0.58 mm (tail), and a length of 8.5 mm. Thus the mass of human cardiac pacemaker tissue is probably much smaller in size than indicated by the measurements usually given in the literature. James (’66a) has observed large fibers with Purkinje characteristics that leave the margins of the node to enter the atrium. James states, ‘“There is direct continuity between nodal fibers and these larger exiting fibers, sometimes several nodal fibers fusing to form one large fiber and in other instances the small nodal fiber appearing simply to enlarge into a big Purkinje fiber.” We have observed and 3 76 R. C. TRUEX, M. Q. SMYTHE AND M. J. TAYLOR 377 HUMAN SINOATRIAL NODE illustrated (Truex, ’61) large atrial fibers adjacent to the SA node in some human specimens. In addition, we have observed many transitions from nodal to atrial fibers, but never nodal fibers into the large Purkinje fibers as described by James (’66a). Many strands of atrial fibers penetrate the periphery of the SA node and can be observed running parallel to the smaller nodal fibers in sections cut through almost any level of the node (figs. l, 2, 3 ) . Such large and more darkly stained fibers with big nuclei do fuse with nodal fibers. Transitions between nodal and atrial fibers may also explain the centrally located embryonal cells described by James which have large ovoid nuclei that he presumed to be the “leading” or actual pacemaker cells. Similar triangular-shaped junctional fibers with diameters greater than nodal fibers have been observed within the human atrioventricular node (Truex, ’66). The above observations by the present authors apply only to atrio-nodal relations in our specimens of the human SA node. In our rabbit and ferret hearts the morphologic criteria and intranodal fiber relationships of the SA node are quite different. The studies of Walls (’42) in the hamster heart also showed that the SA node was composed of both large and small fibers. However, one must be cautious in transferring such comparative morphologic, as well as physiologic, data to the human heart. We concur whole heartedly with James (’66a) that, “There is no €actual basis on which to presume that myocardial fibers which may appear ordinary by present criteria are incapable of specidized or rapid conduction.” We also agree that, “It would appear wisest for the present to concede that current anatomic criteria are inadequate for definition of all those particular morphologic characteristics which denote ability for exceptionally rapid conduction in myocardial fibers, although newer explorations with histochemical technics and electron microscopy are offering encouraging results €or this purpose.” It should be added that precise correlations between the size of a fiber and its intracellular electrode potential offers an equally promising synthesis of structure and function. Until such data obtained by the newer methods is more fully documented and correlated, one cannot assume physiologic implications from pure morphologic studies such as that presently reported-regardless of whether one is concerned with either large atrial or small nodal fibers. It would also seem prudent to restrict the use of the eponym “Purkinje fiber” to the large endocardia1 fibers of the ventricular conduction system as described by Purkinje (1845). CONCLUSIONS The reconstructed human sinoatrial node presents a curved shape and possesses tapered superior and inferior ends. The arterial blood supply and part of the right auricle are included to better orient the right atrial relationships of this important pacemaker tissue. The superior end of the node has an epicardial location as the small nodal fibers pass posterior to the superior vena cava to blend with the larger, more darkly stained muscle fibers of the right atrium. The major portion of the node (body) contained considerable amounts of collagen, and was surrounded by atrial muscle fibers, while the inferior part of the node (tail) was again more epicardial in position. Masses of intramural ganglion cells are most numerous on the anterior and posterior surfaces of the superior vena cava, and are primarily associated with the two tapered ends of the node. ACKNOWLEDGMENTS The authors are indebted to Mr. Jack Taylor for his photographic assistance, and Miss Marjorie Stodgell, Medical Artist of the Hahnemann Medical College for her aid with the illustrations. We express our appreciation to Dr. Elizabeth V. Lautsch who made available the human material used in this study. LITERATURE CITED Bullard, R. W. 1963 Physiology. E. E. Selkurt, ed. Little Brown and Co., Boston. Chap. 14, pp. 266-284. Carbonell, L. ,M. 1956 Esterases of the conductive system uf the heart. J. IIistochem. 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