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Reconstruction of the human sinoatrial node.

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Reconstruction of the Human Sinoatrial Node '
Uepurtment of Anatomy, Temple University School of Medicine,
Philadelphia, Pennsylvania
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-
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.
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.
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).
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
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
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.
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
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
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).
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.
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.
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Cytochein., 4: 87-95.
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heart from the viewpoint of histology and
chemistry. Rev. Lat. Amer. Anat. Path., 2:
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3 78
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node, sinoatrial, human, reconstruction
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