Insights regarding the normal and abnormal formation of the atrial and ventricular septal structures.

Knowledge of cardiac development can provide the basis for understanding the morphogenesis of congenital cardiac malformations. Only recently, however, has the quality of information regarding cardiac embryology been sufficient to justify this approach. In this review, we show how such knowledge of development of the normal atrial and ventricular septal structures underscores the interpretation of the lesions that provide the basis for interatrial and interventricular shunting of blood. We show that current concepts of atrial septation, which frequently depend on a suggested formation of an extensive secondary septum, are simplistic. There are additional contributions beyond growth of the primary septum, but the new tissue is added to form the ventral buttress of the definitive atrial septum, rather than its cranial margin, as is usually depicted. We show that the ventricular septum possesses muscular and membranous components, with the entirety of the muscular septum produced concomitant with the so-called ballooning of the apical ventricular component. It is expansion of the atrioventricular canal that creates the inlet of the right ventricle, with no separate formation of an "inlet" septum. The proximal parts of the outflow cushions initially form a septal structure between the developing ventricular outlets, but this becomes converted into the free-standing muscular subpulmonary infundibulum as the aortic outlet is transferred to the left ventricle. These features of normal development are then shown to provide the basis for understanding of the channels that provide the means for interatrial and interventricular shunting.

The development of septation in the four-chambered heart.

The past decades have seen immense progress in the understanding of cardiac development. Appreciation of precise details of cardiac anatomy, however, has yet to be fully translated into the more general understanding of the changing structure of the developing heart, particularly with regard to formation of the septal structures. In this review, using images obtained with episcopic microscopy together with scanning electron microscopy, we show that the newly acquired information concerning the anatomic changes occurring during separation of the cardiac chambers in the mouse is able to provide a basis for understanding the morphogenesis of septal defects in the human heart. It is now established that as part of the changes seen when the heart tube changes from a short linear structure to the looped arrangement presaging formation of the ventricles, new material is added at both its venous and arterial poles. The details of these early changes, however, are beyond the scope of our current review. It is during E10.5 in the mouse that the first anatomic features of septation are seen, with formation of the primary atrial septum. This muscular structure grows toward the cushions formed within the atrioventricular canal, carrying on its leading edge a mesenchymal cap. Its cranial attachment breaks down to form the secondary foramen by the time the mesenchymal cap has used with the atrioventricular endocardial cushions, the latter fusion obliterating the primary foramen. Then the cap, along with a mesenchymal protrusion that grows from the mediastinal mesenchyme, muscularizes to form the base of the definitive atrial septum, the primary septum itself forming the floor of the oval foramen. The cranial margin of the foramen is a fold between the attachments of the pulmonary veins to the left atrium and the roof of the right atrium. The apical muscular ventricular septum develops concomitant with the ballooning of the apical components from the inlet and outlet of the ventricular loop. Its apical part is initially trabeculated. The membranous part of the septum is derived from the rightward margins of the atrioventricular cushions, with the muscularizing proximal outflow cushions fusing with the muscular septum and becoming the subpulmonary infundibulum as the aorta is committed to the left ventricle. Perturbations of these processes explain well the phenotypic variants of deficient atrial and ventricular septation.

Características morfológicas y biométricas del músculo papilar septal en corazones de individuos chilenos / Morphological and biometric characteristics of septal papillary muscle in hearts of chilean individuals

Se realizó un estudio descriptivo de las características anatómicas y biométricas del músculo papilar septal en 30 corazones de individuos adultos chilenos, en edades comprendidas entre 18 y 84 años de edad, de ambos sexos, sin aparente patología cardíaca, pertenecientes a la Unidad de Anatomía Humana Normal del Departamento de Ciencias Básicas de la Universidad de La Frontera. Los resultados mostraron que el músculo papilar septal, se presenta en un 83,3 por ciento de los corazones del estudio. De éstos, el 44,0 por ciento presenta un solo músculo, el 28 por ciento presenta dos músculos y el 28 por ciento tres músculos papilares septales. De los músculos papilares septales encontrados, el 71,1 por ciento correspondió a la forma cono libre, el 24,4 por ciento a la forma cono pegado y el 4,5 por ciento a la forma puente. En cuanto a la longitud de forma cono pegado y cono libre, el rango que se encuentra en mayor porcentaje está entre 4,0 mm ­ 5,99 mm; con un 45,5 por ciento y 42,4 por ciento, respectivamente. Este estudio nos demuestra la importancia de incorporar al músculo papilar septal a la nomenclatura anatómica internacional.

A descriptive research study was carried out on the anatomical and biometric characteristics of the septal papillary muscle in 30 hearts of adult Chilean subjects, between 18 and 84 years of age of both sexes and without apparent cardiac pathology, from the Normal Human Anatomy Unit, Basic Sciences Department of the Universidad de La Frontera. Results show that the septal papillary muscle was present in 83.3 percent of the hearts in the study. Of these 44.0 percent show one muscle only, 28 percent show two muscles and 28 percent show three septal papillary muscles. In the septal papillary muscles found, 71.1 percent are free cone-shaped, 24.4 percent were attached cone-shaped, and 4.5 percent were bridge-shaped. Regarding the length of attached cone and free cone shaped, the highest percentage range was between 4.0 mm and 5.99 mm, with 45.5 percent and 42.4 percent respectively. The present study shows the importance of integrating the septal papillary muscle to the International Anatomical Nomenclature.

Normal development of the muscular region of the interventricular septum--I. The significance of the ventricular trabeculations.

The structures that participate in normal ventricular septation, and to what extent they do so, are questions not yet clarified. Even less is known about how much each of the embryonic structures contributes to the topography of the mature interventricular septum (IVS). The aim of the present paper is to investigate the significance of ventricular trabeculations in the normal development of the muscular region (the middle and apical thirds) of the IVS and to determine the direction in which it grows during cardiac septation. Anatomical studies and in vivo labelling were carried out in chicken embryo hearts at stage 18HH, tracing the labels up to stage 36HH. We analysed the results by measuring the distance between the labelled structures at the beginning and end of the experiments. We demonstrate that the muscular region of the septum originates by the fusion of the ventricular trabeculations with evidence that during cardiac development, the IVS as well as the ventricular cavities grow in opposite direction to the atria.

Histology and ultrastructure of pericardial cells of Scaptotrigona postica Latreille (Hymenoptera, Apidae) in workers and queens of different ages.

The paper presents a study of the pericardial cells of Scaptotrigona postica an eusocial Brazilian stingless bee. Light and electron microscopy was used in a comparative study on workers and queens of different ages, exerting different functions in the colony. The pericardial cells are found only in the pericardial sinus, mainly in groups around the dorsal vessel. Each cell is enclosed by the basal membrane and its peripheral region is characterized by folds of the plasma membrane, which form canals and loops. The points where the plasma membrane folds is frequently closed by diaphragms, that along with the basal lamina form a barrier to substances from hemolymph. Along the membrane limiting the canals and loops, an intense endocytic activity through coated vesicles takes place indicating a selective absorption of hemolymph components. In older individuals, workers or queens, the cells exhibit larger quantities of cytoplasm inclusions, heterogeneous vacuoles containing the final products of intracellular digestion, and autophagic vacuoles with concentric membranous structures. The pericardial cells general morphology is in accordance with the role in processing metabolites captured from hemolymph and storage of indigested residues.

[Asymmetric septal hypertrophy associated with "subclinical hypothyroidism"]. / Hipertrofia septal asimétrica reversible asociada a "hipotiroidismo subclínico".

We report a case of hypothyroid cardiomyopathy manifested as reversible asymmetric septal hypertrophy in a 61-year-old white woman diagnosed 20 years previously with primary hypothyroidism.

Functional capacities of marsupial hearts: size and mitochondrial parameters indicate higher aerobic capabilities than generally seen in placental mammals.

This study of marsupial hearts explored the aerobic capacities of this group of mammals; recent information suggests that marsupials possess higher aerobic abilities than previously accepted. Characteristics such as heart mass, mitochondrial features and capillary parameters were examined. A comprehensive study of the heart of red kangaroos was included because of the high maximum oxygen consumption of this species. Goats were also included as a reference placental mammal. Marsupials have a heart that is generally larger than that of placentals. The allometric equation for the relationship between heart mass and body mass for marsupials was M(h)=7.5M(b)(0.944) (M(h) in g and M(b) in kg); the equivalent equation for placental mammals was M(h)=6.0M(b)(0.97). Mitochondrial volume density and inner mitochondrial surface density do not differ between the two mammal groups; although capillary parameters indicated a lower capillary volume in marsupials. Heart size appears to be the major difference between the two groups. The overall pattern seen in marsupials is similar to that of "athletic" placentals and indicates a relatively high aerobic potential.

Morphology of necrosis and repair after transcoronary ethanol ablation of septal hypertrophy.

The transcoronary ethanol ablation of septum hypertrophy (TASH) results in a necrosis of the myocardium. In contrast to the ischemically induced necrosis, there is not any phagocytosis caused by leukocytes and macrophages, nor does transformation into granulation tissue occur. Even six months after the application of ethanol, the myofibrils can be identified as "ghost cells" surrounded by a web of collagen fibers. This special forming of a scar, unknown to this day, is documented by 13 patients who died between 25 minutes and two years after ethanol-induced ablation of the hypertrophic septum in hypertrophic obstructive cardiomyopathiy (HOCM).

Ultrastructural evidence of the protective effect of Na+/H+ exchange inhibition on the in vitro damage induced by ischaemia reperfusion in the interventricular septum of the rabbit heart.

We investigated the effects of the Na+/H+ antiporter inhibitor, dimethylamiloride, on myocardial injury after 1 h global ischaemia and 30 min. reperfusion in the isolated arterially perfused interventricular septum of the rabbit heart. After ischaemia and reperfusion challenge, dimethylamiloride significantly increased the recovery of developed tension in a dose-dependent manner, and significantly decreased the maximal increase in resting tension. Ultrastructural analysis of myocytes submitted to the experimental in vitro model supported functional maintenance of physiologically-like conditions. Where myocardial portions were submitted to ischaemic conditions and reperfusion, myocyte cell damage reached usual characteristics of infarct-like induced lesions. Intracellular oedema, severe disruption of myofibrils with loss of muscle striation and both swelling and fragmentation of mitochondria were the main characteristics observed. Dimethylamiloride treatment clearly modifies ultrastructural findings towards the normalization of cell shape and structure, only a slight-middle intracellular oedema and contraction bands were found. On the basis of the present results, we suggest that the protective effects exhibited by dimethylamiloride on the ischaemic myocardium are compatible with its Na+/H+ antiporter inhibition properties, they diminish Na+ accumulation and then either Ca2+ overload or non-exocytotic noradrenaline release during the ischaemia and reperfusion challenge.

Quantitative studies on myelinated and unmyelinated nerve fibres in the interatrial septal region of aged rat hearts.

Intracardiac nerve fibres from the interatrial septum were studied quantitatively and qualitatively by electron microscopy of transversely sectioned nerve bundles in male Wistar rats of 4 and 24 months. No significant changes were found in the myelinated fibre diameters, myelinated axon diameters, myelin sheath thicknesses, g ratios, myelinated fibre areas, unmyelinated axon diameters and unmyelinated axon areas. However, there was evidence of structural changes to the nerve fibres and Schwann cells at 4 and 24 months, increasing in prevalence with age: some myelinated fibres showed infolds, disruptions and clefts of the myelin sheath and accumulation of electron dense myelin-like fragments in the axoplasm. Unmyelinated axons showed fewer changes in structure but also contained similar fragments in the axoplasm. The numbers of neurotubules and neurofilaments per microm2 in unmyelinated intracardiac axons was significantly greater than in those in samples of the cervical vagal trunk. This may be an adaptation to the continuous mechanical stress experienced by these intracardiac nerves. It is concluded that there is little structural evidence to suggest that the conductive properties of intracardiac nerve fibres are adversely affected in aged rats.

The membranous part of the human interventricular cardiac septum.

The arrangement of collagen and elastic fibers of the membranous part of the interventricular septum (PMS) was studied in hearts from adult humans. Connective bundles formed a network of fairly independent tendons arranged in two layers. The tendinous bundles consisted essentially of type I collagen fibers while type III fibers were visible as a thin network with transversely and longitudinally oriented meshes around the muscle bundles. Cranial and caudal to the PMS were narrow and irregular bands of collagen fibers that apparently represented zones of low resistance to the high blood pressures acting from the left to the right heart chambers. The predominance of fiber bundles arranged in an approximately transverse direction with regard to the arterial cone axis suggests a resistance to enlargement resulting from high aortic blood pressure. Elastic fibers were observed in the transitional zone between the cardiac muscle and the PMS. They were continuous with elaunin fibers and these with oxytalan fibers closely intermingled with the narrow network of type I collagen fibers of the PMS. The successive transformation of elastic fibers, which were very numerous in the muscle-tendon transition, into elaunin and these into oxytalan fibers toward the central portions of the PMS suggests a functional sequence characterized by a high elasticity and consequent mobility of the transition region itself and by a progressive increase of resistance in this portion.

Formation of the atrioventricular septal structures in the normal mouse.

It is sometimes thought that formation of the atrioventricular septum is equated with fusion of the endocardial cushions and that failure of fusion can explain all deficiencies of atrioventricular septation. Clearly, this is simplistic, but the exact contribution of different primordia to atrioventricular septation is not well understood. To clarify this, we studied normal mouse embryos (days 10 to 15 of gestation), which were serially sectioned and examined by light microscopy. Another group of embryos was examined by scanning electron microscopy after microdissection. Our results show that development of the atrioventricular septal area is highly complex. Proper formation requires the following: remodeling of the inner heart curvature, rotation of the horns of the systemic venous sinus around the pulmonary portal, expansion of the right atrioventricular junction, formation of the muscular atrial and ventricular septa, bridging by the dextrodorsal outflow ridge and the superior endocardial cushion, fusion with the inferior margins of the venous valves, and formation of the mouth of the coronary sinus from the cranial muscular wall of the left sinus horn. Multiple primordia contribute to a central mesenchymal mass (the "septum intermedium"), including the mesenchyme on the leading edge of the primary atrial septum, the atrioventricular endocardial cushions, and the cap of mesenchyme on the spina vestibuli. Fusion of these components closes the ostium primum, completing atrial and atrioventricular septation. Additionally, the spina vestibuli has a mesodermal core, which contributes to the muscularization of the lower margin of the oval fossa. This contrasts with the formation of the upper rim, which occurs as a result of an infolding of the atrial wall itself.

Cardiac outflow tract septation process in the mouse model of transposition of the great arteries.

It has been reported that all-trans retinoic acid induces transposition of the great arteries (TGA) at 80-90% in ICR mice. The authors revealed that retinoic acid affects the initial formation of the conus cushions leading to a loss of spirality in the cardiac outflow tract. However, the aberrant process of septation has not been precisely defined. In this study, we observed the hearts of live embryos using a video system followed by scanning electron microscopic examination. First, we found that, in the retinoic acid-treated embryos, the proximal outflow tract cushions, in addition to hypoplasia and dysplasia, did not establish the continuity with the distal outflow tract cushions and could not contribute to the outflow septation. Second, the distal outflow tract did not rotate counter-clockwise, retaining the outflow septum anlage in the superoinferior position. Third, a tongue-like mesenchymal tissue had developed on the right anterior rim of the muscular interventricular septum and was incorporated into the interventricular septum. Altogether, these processes contributed to establishing a reversed relationship between the outflow septum anlage and the ventricular septum anlage. On the other hand, right-ward deviation of one or both of the distal outflow tract cushions, relative to the mesenchymal tissue, gave rise to variable degrees of overriding of the pulmonary artery orifice. We conclude that, due to hypoplasia and dysplasia of the proximal outflow tract cushions and lack of distal outflow tract rotation, the outflow septum anlage took an inverted relationship with the ventricular septum anlage. Various types of rightward shift of the outflow tract cushions produced a morphological spectrum of TGA-type cono-truncal anomalies.

Polysialylated NCAM expression on endocardial cells of the chick primary atrial septum.

Septation of the tubular heart to form the multi-chambered heart involves endocardial cell mesenchymal transformation at discrete sites. These sites include the crests of endocardial cushions at the atrioventricular junction, crests of the spiral ridges within the outflow tract, and the leading edge of the atrial septum. The factors involved in this multi-step inductive process appear to include the neural cell adhesion molecule (NCAM). The down-regulation of NCAM coincident with mesenchymal transformation has been documented at the atrioventricular cushion tissue. In view of the function-regulation properties of polysialylated NCAM (PSA-NCAM), we hypothesized that this form of NCAM would be playing a role during the dramatic changes in cell-cell interactions occurring in the endocardium at the leading edge of the primary atrial septum. Chicken hearts at stages during primary atrial septum development were fixed with paraformaldehyde and either immunofluorescently stained for the light microscope analysis or immunoperoxidase stained for ultrastructural analysis. A monoclonal antibody to an NCAM polypeptide epitope (5E) was used to detect all forms of NCAM, while a monoclonal to the polysialic acid (5A5) was used to detect that subset of NCAM which is highly polysialylated (PSA-NCAM). By light microscope level analysis, an increase in immunostaining for NCAM and the appearance of PSA-NCAM was detected on embryonic chicken endocardial cells at the leading edge of the growing atrial septum. The ultrastructural analysis revealed that there is also a change in the pattern of NCAM and PSA-NCAM from a polarized localization to a more ubiquitous distribution over the endocardial cell surface as these cells send out processes, form multiple layers, and sink or move into the underlying extracellular matrix. PSA-NCAM was also detected along cell appositions of cells within the matrix. Both NCAM and PSA-NCAM levels were reduced on cells deep within the matrix. These findings indicate that during primary atrial septation, PSA-NCAM may be deployed on endocardial epithelial cells in order to down-regulate cell-cell interactions and allow the detachment and migration of some of these cells into the underlying matrix.

Influences of aortic pressure gradient and ventricular septal thickness with systolic coronary flow in aortic valve stenosis.

This study evaluates flow patterns of the left anterior descending and circumflex coronary arteries by multiplane transesophageal echocardiography in 25 patients with aortic valve stenosis, and assesses the relation between coronary flow characteristics and anatomic and hemodynamic parameters.

The structure of the mouse heart in late fetal stages.

Because of the opportunities for genetic manipulation, the mouse has become the major species for models of human disease. Recently, targeted and insertional mutations have induced many novel models of developmental abnormality, including several of congenital heart defects. Interpretation and use of such models requires a precise understanding of the similarities and differences between mouse and human in terms of cardiac development and structure. To this end, we have characterised the late fetal mouse heart using scanning electron microscopy and serial histological sections. Right atrial anatomy is dominated by the venous valves, which separate the orifices of the caval veins from the musculature of the primary atrium. Their structure and location suggest that the pulmonary vein is unlikely to develop from the venous sinus. The pectinated wall of the appendage serves to distinguish the morphologically right atrium, in that it runs around the atrioventricular junction, from the left atrium in which this vestibular region is smooth-walled. The persistence of the left superior caval vein draining to the right atrium, along with a solitary opening for the pulmonary vein in the left atrium, distinguishes the atrial anatomy of the mouse from that of the human. The flap valve of the oval foramen is extensive and represents the embryonic primary atrial septum. The superior rim of the foramen is an infolding of the atrial roof, as has been described in the human, showing that, contrary to orthodox opinion, there is no extensive formation of a secondary atrial septum. The region of the membranous septum seen in the human heart is a relatively thick structure in the late fetal mouse, and is located exclusively in an atrioventricular position. Unlike the human, there is little distinction between the apical trabeculations of the left and right ventricles of the mouse heart.

Lipomatous hypertrophy of the atrial septum presenting as a right atrial mass.

Lipomatous hypertrophy of the atrial septum (LHAS) has been associated with cardiac arrhythmias and is defined as fatty infiltration > 2 cm thick in the atrial septum. The clinical and histologic features of surgically excised LHAS have not been previously studied. We studied 11 surgical resections of LHAS and compared them with 13 autopsy cases of LHAS and 24 control autopsy hearts. Of 11 surgical patients, eight were women: patients' mean age was 63 years, and six were described as mildly to overtly obese. Symptoms included congestive heart failure, atrial fibrillation, supraventricular tachycardia, palpitations, syncope, and incidental mass found at surgery. Imagining studies typically revealed a right atrial mass with a mean size of 6 cm (range, 2.5-10 cm). Multivacuolated fat was more extensive in surgical (p = 0.005) and autopsy (p = 0.009) cases of LHAS than in control hearts. Atypical, hypertrophied myocytes were presented in 72% of cases of LHAS compared with 8% of controls (p = 0.0003). In autopsy hearts, histologically abundant multivacuolated fat, heart weight, and body size were independently associated with increased atrial septal thickness. LHAS can be surgically excised, it has a distinctive histologic appearance marked by the presence of abundant multivacuolated fat and hypertrophied myocytes, and it is associated with increased body and cardiac mass.

Diagnosis of patent foramen ovale by transesophageal echocardiography and correlation with autopsy findings.

Transesophageal echocardiography (TEE) is accepted as the method of choice for the diagnosis of the patent foramen ovale (PFO). However, direct anatomic confirmation regarding the presence or absence of a PFO on transesophageal imaging has been obtained in only a limited number of patients. Consequently, this study was performed to assess the diagnostic accuracy of contrast and color Doppler TEE for detection of a PFO by comparing the results of TEE with autopsy. The study population comprised 35 consecutive patients (mean age 64 +/- 14 years) who underwent autopsy and prior TEE with examination of the atrial septum. For diagnosis of a PFO, the following criteria were used: (1) no defect in the continuity of the atrial septum on 2-dimensional imaging; (2) > or = 1 bright microbubble appearing in left the atrium within 3 heart cycles after opacification of the right atrium during contrast TEE; and (3) turbulent color jet within the atrial septum by color Doppler TEE. For estimating the PFO size, positive contrast studies were graded semiquantitatively (from 1 to 3), and the maximal color Doppler jet width was measured within the atrium septum at the area of maximal turbulence. At autopsy, a PFO was present in 9 of 35 patients (26%). All were correctly diagnosed by color Doppler TEE. The color Doppler jet width correlated well with the PFO diameter determined at autopsy (r=0.99, SEE=0.51 mm, p<0.0001). By contrast TEE, 8 of the 9 patients with autopsy-proven PFO were correctly identified. In 1 case with left heart disease and a long interatrial channel, a PFO was missed by contrast TEE but clearly demonstrated by color Doppler TEE. All patients with a PFO diameter >10 mm showed intense left atrial opacification of grade 3. With both methods, there were no false-positive results. Sensitivity and specificity for diagnosis of a PFO were 89% and 100% respectively, for contrast TEE, and both 100% for color Doppler TEE. Thus, contrast and color Doppler TEE are complementary and represent a highly sensitive and specific method for diagnosis of a PFO and for estimation of the PFO size.

Central cardiovascular shunts in the perinatal marsupial.

BACKGROUND: Marsupials are born at an early stage of development after a short period of gestation. In this study the nature and timing of closure of the central cardiovascular shunts was investigated. METHODS: Light and scanning electron microscopy were used to determine changes in central cardiovascular shunts in eight marsupial species with gestation periods of between 12.5 and 36.5 days and birth weights ranging from 12.5 mg to 740 mg. Laboratory mice with a birth weight of about 1,000 mg and a gestation period of 21 days were included for comparison. RESULTS: Marsupials have a ductus arteriosus and an interatrial communication. The former closes rapidly after birth in the marsupial; however the interatrial communication is in the form of a fenestrated septum, which closes as a result of tissue proliferation over a period of days after birth. An additional central shunt, an interventricular foramen, was found to persist in three species for a short time after birth. In one species, the eastern native cat, Dasyurus viverrinus, which has a gestation period of about 19 days and low birth weight of about 12.5 mg, in addition to the two common shunts there was a large interventricular communication and septation of the outflow tract was incomplete. CONCLUSION: In adapting from intra-uterine life, it seems that marsupials have adopted different, but equally effective strategies, with regard to the circulatory system.

Regional mechanical properties of passive myocardium.

There is considerable interest in calculating regional stresses in the heart. This, in turn, requires complete information on regional material properties which, surprisingly, is not available. The specific aim of this work, therefore, was to determine if transmural differences exist in the mechanical behavior of passive myocardium in the equatorial region of the heart. Thus, we performed in vitro biaxial experiments on 28 thin, rectangular slabs of noncontracting myocardium excised from four regions within canine hearts: the middle portion of the interventricular septum (n = 8), and the inner (n = 5), middle (n = 9) and outer (n = 6) layers of the lateral left ventricular (LV) free wall. There were three major findings. First, an existing three-dimensional constitutive relation described the nonlinear and anisotropic behavior exhibited in the four regions equally well. Second, the anisotropy was similar in each region. Third, there were, however, regional differences in the strain-energy stored by specimens during identical finite deformations. In particular, specimens from inner and outer portions of the LV free wall tended to be stiffer than those from the middle of the LV free wall and septum. These findings, together with previous results on excised epicardium, suggest that the mechanical properties of the heart are qualitatively similar from region to region, but quantitatively different.