M. C. Fernández

Coronary artery anomalies and aortic valve morphology in the Syrian hamster

In the Syrian hamster, anomalies in the origin of the left coronary artery are significantly associated with the bicuspid condition of the aortic valve. In this species, bicuspid aortic valves are expressions of a trait, the variation of which takes the form of a phenotypic continuum, ranging from a tricuspid aortic valve with no commissural fusion to a bicuspid aortic valve with the aortic sinuses located in ventrodorsal orientation and devoid of any raphe. The intermediate stages of the continuum are represented by tricuspid aortic valves with a more or less extensive fusion of the ventral commissure and bicuspid aortic valves with a more or less developed raphe located in the ventral aortic sinus. The present study was designed to decide whether there is a gap between tricuspid and bicuspid aortic valves regarding the incidence of coronary artery anomalies, or whether this incidence varies according to the different tricuspid and bicuspid morpho types of the continuum. The study was carried out in Syrian hamsters belonging to a single inbred family with a high incidence of tricuspid aortic valves with fusion of the ventral commissure, bicuspid aortic valves, and anomalies in the origin of the left coronary artery, i.e. single right coronary artery ostium in aorta, anomalous origin of the left coronary artery from the pulmonary artery, and anomalous origin of the left coronary artery from the dorsal aortic sinus. The specimens were examined by means of a stereomicroscope and, in several cases; scanning electron microscopy was also used. The relationships between anomalous coronary artery patterns and aortic valve morphologies were tested using a logistic regression model. The results obtained indicate that there is no discontinuity between tricuspid and bicuspid aortic valves regarding the incidence of coronary artery anomalies. The probability of occurrence of anomalous coronary artery patterns increases continuously according to the deviation degree of the aortic valve from its normal (tricuspid) design. The present findings suggest that in the Syrian hamster, the morphogenetic mechanisms involved in the formation of congenital anomalous aortic valves and anomalies in the origin of the left coronary artery, respectively, are strongly related from an aetiological viewpoint.

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.

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.

Number of Coronary Ostia in Syrian Hamsters (Mesocricetus auratus) with Normal and Anomalous Coronary Arteries

Little attention is being paid to the presence of accessory coronary artery ostia in man and non‐human mammals due to their limited clinical relevance. However, information about their frequency and the cardiac territories irrigated by the vessels arising from them is of interest to obtain an accurate survey of the establishment of the coronary artery system in each species. The aim here was to compare the incidence and significance of the accessory coronary ostia in Syrian hamsters with normal coronary arteries and several coronary anomalies characterized by the absence of a left coronary artery originating from the left aortic sinus. The hearts from 2829 hamsters were examined using a corrosion‐cast technique, micro‐dissection, histochemical techniques, and scanning electron microscopy. Overall, 148 specimens displayed accessory ostia. A limited number of them belonged to the conal artery which supplies the wall of the right ventricular outflow tract. The other accessory ostia led to the septal artery, a vessel which irrigates the most part of the interventricular septum. The incidence of accessory ostia in normal and anomalous coronary artery patterns was quite similar. This suggests that the morphogenetic deviations producing the coronary artery anomalies reported in this study do not alter the connections of the septal and conal arteries to the aorta. The present observations lead to the notion that in the Syrian hamster, the septal artery should be regarded as a third coronary artery.

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.

Rudimentary coronary artery in Syrian hamsters (Mesocricetus auratus).

Congenital underdevelopment of one or more main branches of the coronary arteries has been reported in man, but not in non‐human mammals. In man, this defective coronary artery arrangement may cause myocardial ischaemia and even sudden death. The main goal of this study was to describe the coronary artery distribution patterns associated with the presence of a markedly underdeveloped (rudimentary) coronary artery in Syrian hamsters. Moreover, an attempt was made to explain the morphogenesis of these patterns, according to current knowledge on coronary artery development. Eleven affected hamsters belonging to a laboratory inbred family were examined by means of internal casts of the heart, great arterial trunks and coronary arteries. The aortic valve was tricuspid (normal) in seven hamsters and bicuspid in the other four. A rudimentary coronary artery arose from the right side of the aortic valve in four specimens, from the left side of the aortic valve in a further three, and from the dorsal aortic sinus in the remaining four. In all cases, a second, well‐developed coronary artery provided for all the coronary blood flow. Except for the existence of a rudimentary coronary artery, the present anomalous coronary artery distribution patterns are similar to coronary artery patterns reported in Syrian hamsters, dogs and humans in association with a solitary coronary ostium in aorta. We suggest that an unusual prolonged time interval in the development of the embryonic coronary stems might be a key factor in the formation of coronary arteries displaying significantly dissimilar developmental degrees.

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 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.