Dicephalous v. diprosopus sharks: record of a two-headed embryo of Galeus atlanticus and review of the literature.

As far as is known, this paper gives the first description of a two-headed shark embryo belonging to an oviparous species, Galeus atlanticus (Carcharhiniformes: Scyliorhinidae). The specimen was detected among 797 embryos intended for cardiovascular studies, which represents a defect incidence of 0·13%. Each head had a mouth, two eyes, a brain, a notochord and five gill openings on each side. The two heads fused behind the gills. On the single body, there were four anticipated dorsal fins, two anterior, right and left and two posterior, right and left. Ventrally, the specimen possessed two pairs of pectoral fins, a pair of pelvic fins and one anal fin. Two adjacent notochords, two neural tubes and two dorsal aortas ran along the body, which bent 180° at its posterior portion. There were two hearts, two oesophaguses, two stomachs, two livers, but a single intestine with a spiral valve. Previous reports of conjoined twins in sharks are scarce and only refer to oviparous and ovoviviparous species. Seven dicephalous sharks reported so far were similar to the specimen described here, namely, with two totally separated heads on one body. Instead, only one case of diprosopus shark has been reported; it had a single body and a single head with partial duplication of the face. Two further cases described in the literature as dicephalous or simply as abnormal sharks should be better regarded as diprosopus, while another three cases, also considered dicephalous, showed a mixture of characteristics of diprosopia and dicephalia.

Intimal thickening of coronary arteries in the rabbitfish, Chimaera monstrosa L. (Chondrichthyes: Holocephali).

This study is the first to report the occurrence of intimal thickening of coronary arteries in a holocephalan, namely the rabbitfish, Chimaera monstrosa. The sample studied consisted of five hearts from rabbitfish with body weights ranging between 12 g and 1116 g. The specimens were fixed in formalin, in methanol:acetone:water (MAW), or in paraformaldehyde and were examined by brightfield and polarization light microscopy. Coronary arteries from three larger animals displayed focal intimal thickenings, which were located in the left main coronary artery trunk, at the level of the cono-ventricular junction, and in several intramyocardial ventricular arteries. The vascular changes were characterized by myointimal proliferation, breakage or absence of the inner elastic lamina, and, in one case, by increased collagen within the myointimal proliferation. Taking into account the severity of the coronary arterial changes, their location, and the body weight of the affected animals, we conclude that (1) these changes are age-related in the rabbitfish and (2) local intense mechanical tension seems to be an important factor in their formation in this species.

The Bicuspid Condition of the Aortic Valve Does Not Alter the Incidence of Accessory Coronary Artery Ostia in Syrian Hamsters (Mesocricetus auratus).

In man and Syrian hamsters (Mesocricetus auratus), the prevalence of anomalies in the origin of the coronary arteries is significantly higher in individuals with bicuspid than with normal aortic valves. In hamsters, the incidence of accessory ostia is similar in individuals with normal and anomalous coronary arteries, all of them possessing a normal (tricuspid) aortic valve. In order to evaluate whether or not the presence of bicuspid aortic valves alters the incidence of accessory ostia, 1,050 hearts from hamsters with bicuspid valves were examined. In 594 of them the coronary arteries were normal. The remaining 456 hearts showed coronary artery anomalies characterized by the absence of any artery arising from the left side of the valve. The incidence of accessory ostia was 3.9% in hamsters with normal coronary arteries and 2.2% in those with anomalous coronary patterns. Overall, 3.1% of the accessory ostia were associated with a septal artery and another 0.2% with a conal artery. These data referring to the bicuspid valves were compared with those already published on normal valves. The results of statistical analyses showed that having a bicuspid aortic valve does not alter the incidence of accessory coronary ostia. In the set of tricuspid and bicuspid valves, the incidence of accessory ostia was significantly lower on the left side than on the right side of the valve. This, together with the fact that in the present animal model the coronary anomalies were characterized by the absence of arteries on the left side of the valve, suggests that the embryonic region corresponding to the left side of the aortic valve primordium is particularly associated with preventing the normal development of coronary vessels.

The coronary arteries of the C57BL/6 mouse strains: implications for comparison with mutant models.

There are few detailed descriptions of the coronary arterial patterns in the mouse. Some recent reports on coronary anomalies in mutant mouse models have uncovered the importance of several genes (i.e. iv and connexin43) in coronary morphogenesis. These mutations spontaneously appeared (iv) or were generated (connexin43) in a C57BL/6 background, which is widely used for the development of mutant mice. We have studied the origin and course of the main coronary arteries of two C57BL/6 mouse strains. Unusual anatomical coronary arterial patterns were found, including: solitary ostium in aorta, accessory ostium, high take-off, aortic intramural course, slit-like ostium, sinus-like ostium and origin of a septal artery from the left coronary artery. In humans, some of these conditions are clinically relevant. Most of these patterns, which differ from those observed in wild mice and Swiss albino mice, coincide with those previously found in iv/iv and connexin43 knockout mice. The results indicate that there is variability in the coronary arterial arrangement of the laboratory mouse. Care should be taken when analysing coronary phenotypes of mutant mouse models.

The arrangement of the coronary artery trunks is subject to inheritance factors: A study in Syrian hamsters.

The concept that anatomical variations in the coronary artery tree might be influenced by genes is relatively old. However, empirical evidence on the effect of genotype on the coronary morphology is still scarce. In the Syrian hamster, there is a septal coronary artery which arises from the left or from the right coronary artery and supplies most of the interventricular septum. The aim was to decide whether the anatomical origin of the septal artery is subject to inheritance factors. Overall, 483 internal casts of the heart and coronary arteries were examined. All the hamsters included in this study had normal coronary arteries. The results of 74 crosses were compared statistically to seek for any significant difference between the phenotypes of the offspring and the phenotypes of the parents. The left septal artery was over-represented in the offspring of crosses between parents having both a left septal artery (p < .01), while the right septal artery was over-represented in the offspring of crosses between parents, one with a right and the other with a left septal artery (p < .001), and, more markedly, in the offspring of crosses between parents both with a right septal artery (p < .001). These results are the first to reveal that the coronary artery pattern is influenced by genetic factors, at least in its proximal portion with regard to the aorta.

Anomalous origin of the left coronary artery from the right side of the aortic valve in Syrian hamsters (Mesocricetus auratus).

This study describes the coronary artery distribution patterns associated with the anomalous origin of the left coronary artery from the right side of the aortic valve in Syrian hamsters. The hearts of 15 affected animals were examined by means of a corrosion-cast technique, histology and scanning electron microscopy. The hamsters belonged to a laboratory inbred colony with a high incidence of coronary artery anomalies and bicuspid aortic valves. The aortic valve was tricuspid in eight hamsters and bicuspid in the other seven. In all cases, the right coronary artery was normal, whereas the left main coronary artery trunk arose from the right aortic sinus or from the right side of the ventral aortic sinus when the aortic valve was bicuspid. In 12 specimens, the left main trunk crossed the infundibular septum and then divided into the left circumflex branch and the obtuse marginal branch. In another specimen, the course of the left main trunk was ventral to the right ventricular outflow tract; in the remaining two, it surrounded the aorta dorsally. In man, some of these distribution patterns may cause myocardial ischaemia and sudden death. The present findings prove that the origin of the left coronary artery from the right aortic sinus occurs in primitive mammals such as the Syrian hamster, suggesting that the defect may occur in other mammalian species. Its possible occurrence should be borne in mind in domestic animals, especially in those with signs of myocardial ischaemia after strenuous activity.

Formation of cartilage in congenital bicuspid aortic valves of Syrian hamsters (mesocricetus auratus).

The formation, topographical location and calcification of cartilage in congenital bicuspid aortic valves of 235 Syrian hamsters aged 0--173 days were studied by histological, histochemical and immunohistochemical techniques. In all animals the aortic valve was bicuspid; it had two leaflets, ventral and dorsal, each supported by its own aortic sinus. In 141 valves, a more or less developed raphe was located in the ventral sinus. The remaining 94 valves were devoid of any raphe. The first sign of valvular chondrogenesis was the presence of small groups of cells embedded in a type II collagen-positive extracellular matrix. These cellular groups, which appeared as early as 2 days after birth, became converted into hyaline cartilage or fibrocartilage. A considerable proportion (67%) of the valvular cartilages developed within the first 6 weeks of life. The cartilaginous tissue was capable of forming anywhere along the attachments of the valve leaflets to their supporting sinuses. However, statistical analyses substantiated the observation that the bases of the sinuses and raphes were the valvular regions particularly prone to the development of cartilage. At these sites, the cartilage was usually hyaline and often became calcified. The findings were consistent with the assumption that intense mechanical stimulation plays an important role in the formation of the valvular cartilage. Moreover, these findings supplied new evidence that in the cardiac semilunar valves of Syrian hamsters, cartilage formation does not involve the aggregation of large numbers of cells before their differentiation into chondrocytes. The valvular hyaline cartilages appear to act as competent pivots, resisting mechanical tensions generated during the cardiac cycle. Deposition of calcium in the matrix can be regarded as a reinforcement process of the cartilaginous tissue. Finally, it is hypothesized that the formation of cartilage in the aortic valves of hamsters prevents dystrophic calcification of the valve, a pathological change that causes aortic stenosis in man, especially in patients with a bicuspid aortic valve.

Heart pigmentation in the gray bichir, Polypterus senegalus (Actinopterygii: Polypteriformes).

The occurrence of pigment cells in the heart is well documented in amphibians, birds and mammals. By contrast, information on heart pigmentation in fish is extremely sparse. The aim is to report the presence of pigment cells over the entire surface of the heart in the gray bichir, Polypterus senegalus. The sample consisted of 12 hearts, which, after gross anatomical examination, were studied using histochemical and immunohistochemical techniques for light microscopy, and transmission electron microscopy. The pigment cells were located in the subepicardium, showing a regular distribution pattern across the whole heart, except for the anterior end of the outflow tract, where the pigmentation was much more intense. The cells contained dark, ovoid-shaped organelles which was consistent with a melanosome cell identity. As in other vertebrates, the physiological role of the pigment cells in the heart of the gray bichir is unknown. The absence of such cells in hearts of other polypteriforms suggests that cells containing melanin are not essential for normal fish heart function. Basing on literature data concerning tetrapods, it can be inferred that the pigment cells of the heart of the gray bichir derive from the neural crest. If this were true, our findings would provide the first evidence for the presence of neural crest-derived cells in the subepicardium of adult hearts of early actinopterygians.

Embryological evidence for the formation of a quadricuspid aortic valve in the Syrian hamster.

Congenital quadricuspid aortic valve is a rare anomaly, the morphogenesis of which remains unclear. In this study we report the case of a Syrian hamster embryo that presented an aortic valve with four valve cushions instead of three. The embryo, aged 12 days and 2 hours postcoitum, was at an early stage of valvulogenesis. It was examined using semithin sections of the heart. Two of the four valve cushions were located, one in the dorsal and the other in the left-ventral position, corresponding, respectively, to the dorsal and left valve cushions of a normal aortic valve. The remaining two cushions were situated in the right-ventral position. They were joined at the level of their basal portions and were less developed in size than the other cushions. This report seems to be the first embryological evidence for the formation of a quadricuspid aortic valve. The present findings strongly suggest that the anomalous valve originated from three mesenchymal anlagen and that the supernumerary valve cushion resulted from the division of the anlagen that normally gives rise to the right valve cushion.

Anomalous origin of the left coronary artery from the nonfacing aortic sinus: A study in the Syrian hamster.

A coronary artery that arises from the nonfacing (posterior or dorsal) aortic sinus is regarded as having an anomalous origin. We studied 34 Syrian hamsters in which the left coronary artery originated from the nonfacing aortic sinus. All hamsters belonged to a single family subjected to high endogamous pressure in our laboratory. Twenty-four specimens were examined using a corrosion-cast technique and scanning electron microscopy, whereas the remaining 10 were studied histologically. The aortic valve was normal in 15 cases, whereas it was bicuspid in the other 19 cases. In each specimen of our series, the left coronary artery arose with an acute angle with respect to the aortic valve. Moreover, when the coronary ostium was located in the center of the nonfacing aortic sinus, the proximal coronary artery had an intramural aortic course. These left coronary artery arrangements might underlie an unfavorable coronary circulation physiology. Our morphologic findings agree with the morphogenetic hypothesis that each aortic sinus has the potential to develop coronary artery buds, which hollow out from the aorta to become coronary artery trunks.

Severe congenital stenosis of the left coronary artery ostium and its possible pathogenesis according to current knowledge on coronary artery development.

We report a 72-year-old woman with severe congenital stenosis of the left coronary artery orifice and clinically significant atherosclerotic changes in both the right and left coronary arteries. The stenotic ostium was located at the point at which the left and posterior aortic valve leaflets joined to form the left commissure, just at the distal vertex of the left interleaflet triangle, between the left and posterior aortic sinuses. The right coronary artery was more developed in size than usual, whereas the left coronary artery consisted of a short left main coronary trunk that bifurcated into left anterior descending and left circumflex arteries. The left coronary artery system was filled retrogradely through two vessels proceeding from the right coronary artery, namely, the conal artery and a well-developed branch that ran across the interventricular septum. This abnormal arrangement of the coronary arteries showed striking functional similarities with atresia of the left main coronary artery. Current knowledge on the morphogenesis of the coronary arteries suggests that the present anomalous coronary artery pattern resulted from the penetration of the anticipated left coronary artery system into the aorta at a totally erroneus site. This hindered the normal development of the ostium, which subsisted as a punctiform, practically nonfunctional opening.

Bicuspid aortic and pulmonary valves in the Syrian hamster.

We studied the conditions of the aortic and pulmonary valves of 153 Syrian hamsters belonging to a single family subjected to high endogamous pressure. Semilunar valves of 143 specimens were examined using a stereomicroscope, and in 5 of these cases a histologic study was also performed. The remaining 10 specimens were studied by means of scanning electron microscopy. In 77 specimens both semilunar valves were normal. The aortic valve was bicuspid and the pulmonary valve was normal in 46 animals, while a normal aortic valve and a bicuspid pulmonary valve occurred in 24. In the remaining 6 specimens both semilunar valves were bicuspid. All bicuspid semilunar valves detected herein showed the same morphotype, namely with the leaflets oriented ventrodorsally. The incidence of these anomalous valves did not significantly differ between sexes. Results of a chi 2-test substantiate that conditions of both outflow tract semilunar valves are independent traits. Thus, concurrence of a bicuspid aortic valve and a bicuspid pulmonary valve in an individual is a random event. The present findings support the assumption that bicuspid aortic and pulmonary valves have different morphogenetic origin, although factors producing a bicuspid aortic valve and those leading to a bicuspid pulmonary valve are not mutually exclusive.

Bicuspid aortic valves in hearts with other congenital heart disease.

The bicuspid aortic valve is the most frequent congenital cardiac malformation; it may be isolated or associated with other congenital heart disease. The present investigation consists of a study of bicuspid aortic valves in 1022 heart specimens belonging to the anatomical collection of the Institute of Pathological Anatomy of the University of Padua. A bicuspid aortic valve was observed in 95 specimens. It occurred as an isolated congenital cardiac defect in 28 cases, seven of which had spontaneous laceration of the aortic valve (aortic dissection). It was associated with other congenital cardiac malformations in 67 out of the remaining 994 specimens (6.7%), 41 of which (61.2%) showed obstruction of the aortic arch. The frequency of bicuspid aortic valve in specimens with complete transposition of great arteries was only 1%. Bicuspid aortic valve was particularly frequent in association with ventricular septal defect and was significantly more frequent in cases with (51.1%) than in cases without (20.5%) aortic arch obstruction (p < 0.001). There was no significant relationship between the occurrence of bicuspid aortic valves and left ventricular outflow tract obstructions or mitral valve malformations. The morphology of the pulmonary valve was also examined. Concurrence of a bicuspid aortic and pulmonary valve was detected in 11 specimens, five of these had trisomy-18. Our findings cast doubt on the assumption that altered fetal blood flow through the aortic valve may be the main factor producing the bicuspid condition. Indeed, they rather support the hypothesis that most bicuspid aortic valves are expressions of a developmental complex that affects the aortic arch and the wall of the ascending aorta as well as the aorta valve.

New embryological evidence for the formation of quadricuspid aortic valves in the Syrian hamster (Mesocricetus auratus).

A Syrian hamster embryo, aged 11 days and 4 h post-coitus, had a developing quadricuspid aortic valve. The septation of the cardiac outflow tract was confined to the distal part of the conotruncus. There was a conspicuous recess in the anlage that normally gives rise to the left aortic valve cushion. Globular endothelial cells arranged in several layers were present at the luminal side of the recess. The present findings support the hypothesis that in the Syrian hamster, quadricuspid aortic valves result from the division of one of the three mesenchymal anlagen that give rise to normal aortic valves. In addition, they indicate that the division of the anlage is due to the invagination of the endothelial layer that covers its luminal side. The invagination of the endothelium starts at a very early stage of the valvulogenesis, namely, when the conotruncal ridges begin to fuse at the distal portion of the embryonic cardiac outflow tract.

Anomalous origin of the left coronary artery from the dorsal aortic sinus and its relationship with aortic valve morphology in Syrian hamsters.

The condition of the aortic valve and the origin of the coronary arteries were examined in 2413 Syrian hamsters aged between 1 and 823 days, belonging to nine inbred laboratory families. The specimens were studied with either a stereomicroscope, a corrosion-cast technique, or histological techniques (light microscopy). The aortic valve was tricuspid in 1823 (75.5%) cases, and bicuspid in the remaining 590 (24.5%). In all bicuspid aortic valves there were two aortic sinuses, a ventral and a dorsal, each supporting one cusp. The left coronary artery arose from the dorsal aortic sinus in 71 (2.9%) hamsters, eight of which died unexpectedly between ages 1 and 622 days. In 29 (40.8%) of the 71 cases, the aortic valve was tricuspid and the right coronary artery originated from the right aortic sinus. In the remaining 42 (59.2%) cases, the aortic valve was bicuspid and the right coronary artery arose from the right side of the ventral aortic sinus. Results of a chi 2 contingency test demonstrated that the frequency of left coronary artery arising from the dorsal aortic sinus significantly increased when the aortic valve was bicuspid. This fact, together with previously reported data on coronary artery anomalies in the Syrian hamster, suggests that the left coronary artery arising from the dorsal aortic sinus may be an expression of a single morphogenetic defect which is expressed as bicuspid aortic valve in some cases, anomalous origin of the left coronary artery in others, or in the simultaneous occurrence of these two cardiac abnormalities.

Mucoid dysplasia of tricuspid and congenital bicuspid aortic valves in Syrian hamsters (Mesocricetus auratus).

A histological study was made of the aortic valves of 141 Syrian hamsters aged between 35 and 657 days, belonging to an inbred laboratory colony with a high incidence of congenital bicuspid aortic valves. A tricuspid aortic valve was found in 81 specimens, whereas the remaining 60 possessed a bicuspid aortic valve. In all bicuspid valves there were two aortic sinuses, a ventral and a dorsal, each supporting one cusp. Thirty-three (23.4%) of the 141 specimens showed mucoid dysplasia of the aortic valve. The defect was characterized by thickening of the valve cusps and disruption of the fibrosa layer accompanied by an increased amount of glycosaminoglycans. Ten (12.3%) of the 81 tricuspid aortic valves and 23 (38.3%) of the 60 bicuspid aortic valves were dysplastic. This difference was statistically significant (P < 0.001). The findings indicate that, in the Syrian hamster, the simultaneous occurrence of bicuspid aortic valve and aortic valve dysplasia is not a random event. However, the fact that these valve defects also occur independently suggests that there is no primary morphogenetic dependence between them, but that some other cause predisposes to their concurrence.

Formation of cartilage in cardiac semilunar valves of chick and quail.

The development of cartilage in the aortic and pulmonary valves of chick and quail was studied using histological, histochemical and immunohistochemical techniques. In both species, the earliest evidence of chondrogenesis is the formation of smooth muscle alpha-actin-negative prechondrogenic (type II collagen-negative) cellular condensations in the tunica media of the proximal aorta and pulmonary trunk, in front of or slightly distal to the valvular commissures. Such condensations are present as early as stage 37 of Hamburger-Hamilton in the aortic and pulmonary valves of the chick. In quail embryos, they form somewhat later, namely, at stage 38 in the aortic valves and stage 39 in the pulmonary valves. In the chick, synthesis of type II collagen starts in the central core of the aortic cellular condensations at stage 38. In the pulmonary valves of chick and aortic and pulmonary valves of quail, production of type II collagen does not begin until stage 40. This production then gradually increases toward the periphery of the condensations, which remain devoid of perichondrium prior to hatching. After birth, the condensations become transformed into hyaline cartilaginous foci. In the aortic valves of some chickens and quails, more or less extensive deposits of hyaline cartilage or fibrocartilage form along the attachments of the leaflets to their supporting sinuses. They develop later than the commissural cartilages. The present findings, together with previous data from the literature, suggest that the aortic and pulmonary valve cartilages differentiate from neural crest-derived nonmuscular cells.

Cartilage in pulmonary valves of Syrian hamsters.

The presence of cartilage in the pulmonary valve has been reported in birds, but not in mammals. We describe here the occurrence of cartilaginous tissue in the pulmonary valves of 40 (11.4%) of 351 Syrian hamsters examined using histological, histochemical and/or immunohistochemical techniques. The cartilaginous deposits were located along the fibrous attachments of the valve leaflets to the wall of the pulmonary artery trunk. Our findings indicate that the proximal attachments of the leaflets to their respective sinuses, and especially that of the ventral leaflet, are the most prone valvular regions to develop cartilaginous foci. Nonetheless, the possible function of these foci remains an open question. Formation of cartilage in the pulmonary valve starts within the first month of life, that is during the period in which the valve reaches histological maturation. The earliest evidence of chondrogenesis is the presence of small groups of cells embedded in a type II collagen-positive extracellular matrix. These groups of cells, which can appear as early as one day after birth, increase moderately in size and differentiate into hyaline cartilaginous tissue. The precursors of the cartilaginous cells are presumed to be neural crest-derived elements. However, the factor or factors involved in the differentiation of these precursors into chondrocytes are still unknown. In this regard, our observations cast doubt on the hypothesis that the formation of cardiac cartilages is primarily due to locally intense mechanical stimulation.

Formation of cartilage in aortic valves of Syrian hamsters.

The formation of cartilage in aortic valves of Syrian hamsters was studied using histological, histochemical and immunohistochemical techniques. The sample consisted of 281 specimens aged 0-363 days, all of which had a normal (tricuspid) aortic valve. The first sign of valvular chondrogenesis is the presence of small groups of cells embedded in a type II collagen-positive matrix. These groups of cells, which can appear as early as one day after birth, increase in size and differentiate into hyaline cartilage or fibrocartilage. From the fourth day of life, all hamsters examined displayed cartilaginous foci in the aortic valve. They were located along the fibrous attachments of the valve leaflets to their respective sinuses, including the valve commissures. A considerable proportion (76%) of cartilages formed within the first 40 days of life, that is during the period of time in which the histogenesis of the valve takes place. The present observations are consistent with the assumption that in mammals, the precursors of the aortic valve chondrocytes are neural crest-derived cells. Results of a statistical analysis substantiate that the incidence is significantly higher in (1) the territory that comprises the collagenous condensation of the ventral commissure and the ventro-lateral and proximal fibrous attachments of the right leaflet to its sinus, and (2) the proximal fibrous attachment of dorsal leaflet to its sinus. These findings together with data in the literature concerning the distribution of stress in each leaflet-sinus assembly of the valve during the cardiac cycle, suggest that mechanical action might play an inductive role in the formation of the cartilaginous tissue in the aortic valve of mammals. In addition, they point to the possibility that locally intense mechanical stimulation is responsible for the differentiation of the anticipated cartilaginous tissue into hyaline cartilage.

The coronary arteries of the Syrian hamster, Mesocricetus auratus (Waterhouse 1839).

Bearing in mind that the Syrian hamster provides an animal model for the study of congenital coronary artery anomalies, we decided to undertake a definition of its normal coronary artery pattern. The sample examined consisted of 1204 specimens. They were studied both histologically and by means of a corrosion-cast technique. The course of the coronary arteries in this species is intramyocardial. The right coronary artery has two principal branches: the right circumflex branch and the dorsal interventricular branch. The conal branch usually originates from the main trunk of the right coronary artery. The main branches of the left coronary artery are the obtuse marginal branch, the left circumflex branch, and usually a dorsal ventricular branch as well. The ventral interventricular branch is often absent. When it is present, it always originates from the left coronary artery and seldom reaches the apex of the heart. The interventricular septum is principally supplied by one, or rarely two, septal arteries arising from the right and/or left coronary arteries. According to the number and origin of these vessels, three septal coronary artery patterns were established; namely, the right, the left, and the right-left septal patterns. In the Syrian hamster, the left septal pattern is the most frequent (70.4%). The right septal pattern occurred in 28.1% of the specimens studied, whereas the right-left septal pattern was only found in 1.5% of them.