The Third Branch of the Main Trunk of the Left Coronary Artery in Cercopithecus aethiops sabaeus. Is the Nonhuman Primate Model Appropriate?

Authors


Abstract

The aim of this study was to investigate the morphometry of branching patterns of the main trunk of the left coronary artery (MT of LCA) in nonhuman primates, and comment on the current nomenclature. The biometric study was performed using stereomicroscopic dissection of hearts of healthy and fertile nonhuman primates (Cercopithecus aethiops sabaeus) of both sexes. Our results reveal that the MT of LCA terminates in a bifurcation into the anterior interventricular branch (AIB) and the circumflex branch (CB) (74.6%), trifurcation into the AIB, CB, and diagonal branch (DB) (23.6%), or occasionally quadrifurcation into the AIB, CB, and two DBs (1.8%). This is similar to the case in humans. Furthermore, two morphological aspects of the DB spatial distribution, in addition to its branching pattern, resemble the DB in humans. Myocardial bridges observed over the DB in the Cercopithecus aethiops heart further contribute to the similarity with humans. The resemblance of the DB and its branches to their human counterparts make them a suitable model for experimental study on coronary circulation. Anat Rec, 2011. © 2011 Wiley-Liss, Inc.

INTRODUCTION

Previous studies stated that the blood supply to the monkey heart is usually provided by two coronary arteries, the left and right one, and exceptionally by three (Buss et al,1982, Nikolić et al.,2003,2004,2009). The main trunk (MT) of the left coronary artery (LCA) usually bifurcates into two branches, the anterior interventricular branch (AIB) and the circumflex branch (CxB). According to Nomina Anatomica Veterinaria, the terminal branches of the LCA are named the paraconal interventricular branch (PIB) and the circumflex branch (CxB). Because of the fact that their direction, branching pattern, and irrigation zone are remarkably similar to those of the AIB and the CxB in humans, we adopted these latter names in this article. The MT of LCA in humans less frequently terminates in trifurcation, and occasionally it splits into four or five branches (Vilallonga,2003, Fazliogullari et al.,2010). The third branch of the MT of LCA is not included in the current Terminologia Anatomica and it has various names.

This vessel has unusual anatomical features in humans. It does not run along any cardiac groove but simply slides over the free surface of the ventricle. Although its distribution area is usually small, its existence may decrease the effects of occlusion of the AIB and the CxB. The third branch of the LCA might be a source of arteries at the sternocostal surface of the left ventricle, such as the anterior septal branches and branches for the anterior papillary muscle of the left ventricle (Verna et al.,1988; Vilallonga,2003). As the mass flow rate in blood vessels is proportional to their diameter and angle of termination, the presence of the third branch obviously influences the flow pattern of the LCA. Trifurcation of the left main coronary artery is an anatomical factor which increases the challenge of using angioplasty as a treatment procedure for trifurcation lesions (Shammas,2007).

Because animals used in cardiovascular research reveal an extreme variability in LCA anatomy (Podesser et al.,1997; Colborn,2005; Sahni et al.,2008), finding “the most human type” model would require further investigation. However, in none of the above studies is there any information about of the morphometric parameters of the trifurcation pattern in Cercopithecus aethiops sabaeus. The aim of this study is to provide evidence of the following biometric features in Cercopithecus aethiops monkey heart; (a) branching patterns of the MT of LCA; (b) the luminal diameter and length of the MT of LCA; (c) the angle of division of the MT of LCA; (d) the external diameter of the branches of the MT of LCA; (e) the length of the third branch of the LCA relative to the length of the left ventricle; (f) axial topography and spatial distribution patterns of the third branch of the LCA; (g) presence of myocardial bridges over the third branch of the LCA. In addition, the purpose of this article is to test the Cercopithecus aethiops model in coronary vascular research, by examining these morphometric features, and comparing them with corresponding in human.

METHODS

Animals

The investigation was carried out on 55 hearts of healthy and fertile nonhuman primates (Cercopithecus aethiops sabaeus) of both sexes (35 females and 20 males). The animals, originating from East Africa (Kenya, Uganda, Tanzania) were housed at the Torlak Institute for Immunology and Virology (Belgrade, Serbia) in accordance with the International Guidelines on the Ethical use of Animals. They were all healthy and with body weights between 3,600 and 4,800 g.

There was no artificial lighting, and the daylight was 14 hr. The monkeys were housed in pairs in a climate-controlled environment and fed once a day with a special commercial diet (SDS—Essex, UK) supplemented with fresh bananas and apples. Automatic watering was available.

Necropsy

The animals were deeply anaesthetized by intraperitoneal injection of sodium pentobarbital (80 mg/kg) and sacrificed by exsanguination. The coronary arteries were injected in situ (constant pressure of 18 kPa) through the aortic arch with Indian ink or colored latex (Group A) or Simgal (methyl methacrylate), (Group B). The removed hearts were fixed in 10% formaldehyde solution for two weeks (Group A) or in NaOH solution for three days (Group B).

Tissue Preparation

In Group A, after removal of the periadventitial adipose tissue (with a stereomicroscope), the external surface of the myocardium was exposed and coronary arteries and their branches were identified, dissected, measured, drawn, and photographed. The stereomicroscopic examination included measuring the h and external diameter close to the root of the coronary vessels. We measured the length of the left ventricle according to Ortale et al. (2005). The ventricular length was divided into three equal segments in order to classify the relative length of the third branch from its origin to the point of myocardial penetration. This was classified as short if it was less than one third of the ventricular length, medium length if it was between one third and two thirds of the same reference and long if it was greater than two thirds of the reference. The hearts from Group B, after liquefaction in NaOH solution for three days, were washed with warm water. The corrosion casts thus obtained were used for stereomicroscopic examination and measurements.

Statistical Analysis

We describe the experimental statistics in terms of the central tendency measures, mean, in addition to the standard deviation.

RESULTS

In the Cercopithecus aethiops sabaeus hearts studied, the MT of LCA were observed to terminate in three different ways; (a) by bifurcation into the AIB and the CxB (41 out of 55 hearts, 74.6%); (b) by trifurcation into the AIB, the CxB and the diagonal branch (DB) (13 out of 55, 23.6%); and (c) by quadrifurcation into the AIB, the CxB and two DBs (1 out of 55, 1.8%).

The external diameter of the MT of LCA of all samples varied from 0.60 to 2.70 mm (mean, 1.65 ± 0.39 mm), of which the bifurcation termination type had mean diameter 1.73 ± 0.42 mm, the trifurcation type had mean diameter 1.68 ± 0.45 mm, and the single case where the MT terminated in quadrifurcation had a diameter of 2.30 mm. In the bifurcation, trifurcation, and quadrifurcation type of the LCA termination, the mean LCA length was 4.80 ± 1.15 mm, 5.30 ± 2.38 mm, and 5.70 mm, respectively. The LCA length was defined as short if it was less than 3 mm; medium sized for 3–5 mm, or long if it was more than 5 mm. Applying these criteria to our measurements, we found a short MT of LCA in 11 out of 55 cases (20%), a medium-sized MT in 36 out of 55 cases (65.5%) and a long MT in 8 out of 55 cases (14.5%). The values obtained showed no significant variation between the different termination types.

The average value for the angle of division between AIB and CxB was 88.7°± 22.6° with no statistically significant difference between the different termination types.

The average value of the DB diameter at the origin was 0.95 ± 0.26 mm. In the case with two DBs, their diameters were 0.60 mm and 0.70 mm, for right and left, respectively. The overage values of the AIB and CxB were 1.32 ± 0.32 mm and 1.13 ± 0.31 mm, respectively.

Taking the ventricular length as a reference, we classified the DBs as short, medium or long types. The short type of vessel was present in 8 out of 13 cases (61.6%), the medium type in three cases (23%), while in two cases (15.4%) we found the DB reaching the lower third of the left ventricle, up to the apex (long type). In the heart with two DBs, the left one was short and the right one medium length.

The DB in all cases originated as a branch located between the AIB and the CxB. We distinguished two coronary morphologies based on the path and spatial distribution of the DB. In the first type, which was present in 8 out of 13 hearts (61.6%), the DB followed a course similar to that of the first lateral branch of the AIB, i.e. it descended more or less parallel to the AIB, along the anterior surface of the left ventricle. In the second type, which was present in 5 out of 13 hearts (38.4%), the DB pathway resembled the left marginal artery of the CxB. It travelled obliquely along the left (pulmonary) surface of the heart, toward the left margin (the margo obtusus cordis). In the case with two DBs, there was one of each type.

The branching pattern of the DB varied between the hearts. The DB branched once in 6 out of 13 cases (46.1%), twice in 5 out of 13 cases (38.5%) and five times in 1 case (7.7%) (Fig 1). No branching was evident in one case (7.7%). In the heart with two DBs, the left had three branches and the right had one epicardial branch. Most of these were short branches.

Figure 1.

Trifurcation of the main trunk of the left coronary artery (LCA). Photograph shows long diagonal branch (DB), reaching the apex, with five branches arising from it. Myocardial bridge (mb) over DB is located in the middle portion of the artery. AIB, anterior interventricular branch; CxB, circumflex branch.

In 3 out of 13 hearts (23%), variable degrees of muscular overbridging were observed (Figs. 1 and 2). Myocardial bridges were localized in the middle portion of the artery, while in one case the DB was almost completely overbridged (Fig. 2). In the heart with two DBs myocardial bridging was not detected.

Figure 2.

Trifurcation of the main trunk of the left coronary artery (LCA). Photograph shows long diagonal branch (DB), reaching the apex, taking a course along the anterior interventricular groove, no branches arising from it. DB is almost completely overbridged (mb). AIB, anterior interventricular branch; CxB, circumflex branch.

DISCUSSION

The branching pattern of the MT of LCA is a controversial topic because there is no common consensus on the criteria used to define the third branch of the LCA. The third branch of the MT of LCA is not included in the current Terminologia Anatomica (1998) so it has diverse terminology such as median (or intermedian) branch (Vilallonga,2003; Reig and Petit,2004; Kini et al.,2007; Fazliogullari et al,2010), diagonal artery (Jordao et al.,1999;), diagonal branch (Ortale et al.,2005; Ballesteros and Ramirez2008; Ishizawa et al.,2008) and left marginal artery (Teofilovski Parapid and Kreclović,1998). In our case we named it the diagonal branch, following the nomenclature of the other terminal branches of the MT of LCA. We define a DB of the MT of LCA to be one that: (a) originates at the branching point of the AIB and the CB and lies between them; (b) has a sufficient diameter, and (c) has an area of distribution and corresponding vascular pattern (Figs. 1 and 2).

Comparative anatomical studies of the MT of LCA and its third branch in other animals that were considered to be useful experimental models in cardiovascular research did not show the necessary similarity with human hearts. The concept of the bifurcation/trifurcation classification system of the epicardial branching pattern of the LCA in animals was proposed by Podesser et al. (1997). They stated that in the bifurcation pattern in rabbits, the LCA splits to become the anterior and posterolateral divisions, while in the case of a trifurcation, there is a branch, “coursing down the middle of the anterior surface of the left ventricle,” which could be interpreted as the lateral division. According to Podesser et al. (1997) the MT of LCA in rabbits terminates in bifurcation and trifurcation with equal frequency, while Hadžiselimović et al. (1974) established that the LCA in rabbits divides directly into two branches, the AIB and the CxB. Beside it, in cases when it terminated by bifurcation, it was rarely possible to identify the blood vessel which corresponded to morphofunctional characteristics of the AIB (Podesser et al.,1997). In contrast, Bahar et al. (2007) did not report any bifurcations in Angora rabbits and observed a greater frequency of trifurcation (75%) than quadrifurcation (25%) (Table 1).

Table 1. A comparison of the MT of the LCA branching pattern among various animals
 234
Rabbits   
 Hadžiselimović et al.,1974100%
 Podesser et al.,199750%50%
 Bahar et al.,200775%25%
Pigs
 Weawer et al.,198680%20%
 Kamimura et al.,1996100%
 Jordao et al.,199979%20%1%
 Sahni et al.,2008100%
Monkeys
 Buss et al.,1982  (Macaca radiata)100%
 Teofilovski–Parapid and  Kreclović,1998  (M.fascicularis)82%18%
 Colborn,2005  (squirrel monkey)100%
 Rung et al.,2006  (black gibbon)100%
Present study74.6%23.6%1.8%

The same discrepancy in results is noticeable in the trifurcation pattern of the LCA in swine. Kamimura et al. (1996) in miniature pig and Sahni et al. (2008) have found that the MT of LCA in all cases terminated in two major branches, the AIB and the CxB, while Weawer et al. (1986) and Jordao et al. (1999) reported a 20% incidence of trifurcation of the MT of LCA in swine (Table 1). These findings led us to conclusion that the rabbit as well as pig heart might be useful in very limited conditions in cardiovascular researches.

Our outcomes are in contrast with various studies carried out in monkeys (Table 1), which conclude that the MT of LCA, terminates in a bifurcation into the AIB and the CxB. A trifurcation pattern was reported in Macaca fascicularis monkeys in 18% of cases by Teofilovski Parapid and Kreclović (1998). No reference has been found concerning the anatomical observation of the MT of the LCA in Cercopithecus aethiops sabaeus. The trifurcation pattern has been identified in various human studies with an incidence between 14% and 50%, and quadrifurcation was observed in 4–10% of cases. In contrast, Ishizawa et al. (2008) reported that the DB was present in all of the 103 human hearts which they observed (Table 2). The branching fashion of the MT of LCA observed in this work resembles the one in humans more closely than of other animals, and of other species of monkeys suitable for experimental work (Tables 1 and 2).

Table 2. The frequency of the MT of the LCA branching pattern among various studies in human
Human234
Baptista et al.,199154.7%38.7%6.7%
Reig and Petit,200462%38% 
Ortale et al.,200550%46%4%
Kilic and Kirici,200786%14% 
Ballesteros and Ramirez,200852%42.2%5.8%
Ishizawa et al.,2008 100% 
Christensen et al.,201081%19% 
Fazliogullari et al.,201046%44%10%

Our results for the length of the MT are similar to those found in most of the studies carried out on human hearts, which classified the MT as either “short” or “long“ (Reig and Petit,2004; Ortale et al.,2005; Balesteros and Ramirez,2008).

The average value observed for the angle of division of the terminal branches of the MT of the LCA in Cercopithecus aethiops sabaeus was similar to the corresponding angle in humans (Reig and Petit,2004).

The diameter of the DB was occasionally similar to that of the other terminal branches, but more frequently smaller, which is in agreement with the results in a human study (Balesteros and Ramirez,2008). Such findings among animals are rare. A study in swine found that diameters of the AIB and the CxB were in all but one case greater than that of the DB (Jordao et al.,1999). Teofilovski Parapid and Kreclović (1998) reported measurements of the other terminal branches of the MT of LCA in Macaca fascicularis monkeys, but not the DB.

We observed a short DB penetrating to the myocardium at the level of the upper third of the ventricular surface in 61.6% of hearts, which is in agreement with Jordao et al. (1999) in swine, and Ortale et al. (2005) in humans, while Baptista et al. (1991) and Ishizawa et al. (2008) reported greater prevalence in humans of medium and long DBs, respectively. Demonstrating the short path of this vessel on the subepicardial plane, it is worth stressing that in the two cases when the DB was long relative to the length of the ventricle, upon reaching the apex it was almost completely overbridged (Figs. 1 and 2).

Depending on its spatial distribution, the DB may play an important role as a collateral vessel in coronary circulation. The DB in humans can be distributed as a lateral branch of the AIB or as a marginal (obtuse) branch of the CxB depending on whether it supplies the anterior or the lateral wall respectively (Reig and Petit,2004; Kini et al.,2007; Ballesteros and Ramirez,2008), with a greater prevalence of the first variety (Reig and Petit,2004). Our results agree with this, since we observed the DB following a course similar to that of lateral branch of the AIB in 61.6% of hearts. There is an apparent lack of other reports in the literature regarding the path of the DB in monkey hearts.

With regard to the branching pattern of the DB in Cercopithecus aethiops sabaeus, we observed various numbers of branches from zero to five branches, with a slight predominance of the one branch type (46.1%). Two branches were found in 38.5% in accordance with Jordao et al. (1999) in swine. In one case with a long myocardial bridge over the DB, we noticed five short epicardial branches (Fig. 1). The significance of branching in the DB is emphasized by fact that collateral coronary circulation between the AIB and the CxB could be compromised in the presence of the DB. In humans, the DB is described as a source of arteries supplying either the anterior wall of the left ventricle, the anterior portion of the septum, or the anterior papillary muscle of the left ventricle, which is in agreement with our findings.

As far as we know, it is the first study reporting the incidence of myocardial bridging over the DB of the MT of the LCA. In our work, the DB was overbridged in 23%, in accordance with the results of a human study (Ballesteros and Ramirez,2008). The lack of data in the literature among animals is limiting factor for further comparison.

Our results provide evidence that the morphological pattern of the MT of LCA, its branching pattern, spatial distribution and branching characteristics of the DB, as well as myocardial bridging in nonhuman primates, has similarities with that of humans. Furthermore, the nonhuman primate heart model has been shown to mimic the human coronary circulation more closely than other animal models.

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