Journal of the Anatomical Society of India

Department of Anatomy, JNMC, AMU, Aligarh-202002

Abstract:

Coronary artery is considered to be a medium size (muscular) artery but information about its comparative microanatomy remains scanty. In the present study hearts of buffalo, goat, rabbit and rat were immersion fixed in 10% formalin. Blocks of 1-cm size from the proximal part of the coronary arteries were processed for paraffin embedding. Haematoxylin-eosin and Weigert and Van gieson stained 10m thick sections were observed under light microscope. In all animals the three tunics of the arterial wall were clearly discernible. Total arterial wall thickness was in proportion to the somatic size of the animal. Taking the wall thickness in rat as unit, it was found to be 1.5 in rabbit, 3 in goat and 10 in buffalo. In buffalo subendothelial connective tissue was much more pronounced in the form of 4 to 5 laminated structure while in goat it was just identifiable, in rabbit and rat remains inconspicuous. The internal elastic lamina did not reveal characteristic corrugation. It was just discernible in goat while in buffalo it was in the form of a thick amorphous band and in rabbit and rat appears as undulating or wavy structure. Tunica media in general consisted of circularly arranged smooth muscle layers. In buffalo, however, it had variable picture from intermingled to 5-7 alternate lamenae of circular and longitudinal fibres. Tunica adventitia generally consisted of loose connective tissue surrounded by subepicardial adipose tissue. In buffalo, however prominent trabeculae of adventitial origin penetrated deep into the muscular coat. It was concluded that though in general coronary artery shows a common basic structure across the mammalian species, in buffalo all three tunics present a more differentiated structure with a closer similarity with that of human coronary artery.

Key words: Coronary artery, tunica intima, tunica media and tunica adventitia.

Introduction:

Coronary artery disease is one of the major causes of death in developing countries. More progress has been made in the last few decades than in all foregoing medical history in the management of cardiovascular diseases. In the past Gross et al (1934) studied histology of coronary arteries and their branches in human heart. Henry (1957) studied coronary arteries in human fetuses, infants and juveniles. Sclerotic lesions were not demonstrated in the coronary arteries of fetuses. Margaret et al (1957) studied the collagen and elastin content of the arterial wall in the dog varying in size from the thoracic aorta to saphenous artery. Parker F (1958) have done electron microscopic study of coronary arteries in rabbit. The submicroscopic structure consists of three coaxial coats-tunica interna, tunica media and tunica adventitious. Boucek (1963) studied relation between microanatomy and functional properties of the coronary arteries in dog. Luis Junqueira et al (1977) studied aging changes of the human muscular arteries in general. The arteries undergo progressive and gradual changes from birth to death and it is difficult to say where the normal growth processes end and the processes of involution begin. Roberts et al (1980) studied cross-sectional are of the proximal portions of the three major epicardial coronary arteries in humans with relationship to heart weight, age, sex. Wayne et al (1994) studied developmental and aging changes of the epicardial coronary arteries in human. Awal et al (2001)have done a histological study on the coronary artery of the black Bengal goat. Kumar Keshaw (2001) studied microstructure of human arteries. Jose and Colvee (2001) studied orgin and distribution of the coronary arteries in normal mice and in the mice of the iv/iv strain. Shyu et al (2002) studied ultrastructure of the intramural coronary arteries in pigs with hypertrophic cardiomyopathy. Dhall andChaudhary et al (2003) have done a histomorphometric analysis of coronary arteries in terms of sexual dimorphism. Kalpana (2003) studied principal branches of coronary arteries in humans. The present study was conducted to compare the histological features (morphology and wall thickness) of coronary arteries in different mammals viz. buffalo, goat, rabbit and rat.

Material and Methods

A total number of 40 hearts (i.e. 10 hearts/ species) of buffalo, goat, rabbit and rat were included in this study.The hearts were collected on basis of age and weight of animals. The rats collected varied between 3-4 months of age, 200-250 gms of weight. The rabbits studied between 6-9 months of age, 1-1.5 kg of weight. The goats studied varied between 1-2 years of age, 12-16 kg of weight. The buffaloes varied between 2-4 years of age 90-150 kg of weight.Rabbits and rats were obtained from central animal house, JNMC, Aligarh. Rabbits and rats were anesthetized with ether. A perfusion apparatus was assembled with a drip set and a bottle with perfusate. Animal was laid on its back on dissectings tray with extremities secured. The chest opened and heart exposed. 18 gauze needles were transfixed in its cavity. The right atrium was opened by nick in the wall to enable drainage and perfusion. Initially physiological saline was run to flush out the blood and later followed by 10% formalin. Proper perfusion was ascertained by the fasciculations on extremities. The heart was taken out with the help of pointed scissors. Buffalo and goat hearts were collected immediately after sacrificing the animal from the slaughter house. The right atrium was opened by a nick in the wall. The hearts thus collected were serially numbered 1 to 40 and immediately transferred to specimen bottles containing 10% formalin for 48 hrs for fixation. Proximal part of right and left coronary arteries were dissected out. 80 sections were processed for paraffin sectioning. Haematoxylin-eosin and Weigert and Van gieson stained 10 thick sections observed under light microscope.

Observations

In all animals studied the arterial wall revealed three tunics from within outwards namely tunica intima, tunica media and tunica adventitia. In general the microstructure of tunics were same among different animals. However, on close examination it was found that one or other feature was much more pronounced in some animals. Tunica intima consisted of endothelial layer, subendothelial connective tissue and internal elastic lamina. Tunica media consisted of smooth muscle arranged in concentric layers. The number of layers varies from animal to animal. Tunica adventitia consisted of loose connective tissue surrounded by subepicardial adipose tissue. The connective tissue composed collagen and elastic fibres. The comparative microscopic features of arterial wall in different animals are present in a tabular form as under On qualitative observation it was found that in general total arterial wall thickness was in proportion to somatic size of animal. Taking wall thickness in rat as unit, it was 1.5 in rabbit, 3 in goat and 10 in buffalo.

Fig. 1: Photomicrograph from the coronary artery of buffalo showing tunica intima, thick amorphous proliferating subendothelial connective tissue, internal elastic lamina (?).Tunica media (M) Tunica adventitia (L) containing collagen and elastic fibres. H and E X 10.

Fig. 2: Photomicrograph from the coronary artery of buffalo showing tunica intima and thick tunica media. In the outer most part of media smooth muscles are arranged longitudinally (?). Tunica Adventitia composed of collagenous fibres (ê). H and E X 10.

Fig. 3: Photomicrograph from the coronary artery of goat showing wavy internal elastic lamina (Û), ill-defined subendothelial tissue. Tunica media consists of 5-6 layers of smooth muscle fibres, tunica adventitia consists of connective tissue composed of collagen fibres (ê). Weigert and Van gieson X 10.

Fig. 4: Photomicrograph showing part of transverse section of coronary artery of rabbit, endothelium and subendothelial connective tissue. The smooth muscle cells are identified clearly in media. Adventitia contains collagen and elastic fibres (pink). Weigert and Van gieson X 20.

Fig. 5: Photomicrograph from the coronary artery of rat along with surrounding subepicardial connective tissue. Tunics are hardly identifiable. Media consists of smooth muscle cells internal elastic lamina is not visible. H and E X 4.

Discussion

In man (James and Burch, 1958) and in other species Christensen, (1962) and Rowlatt, (1981) described that the main coronary arteries run subepicardial course. In the present study attempt has been made to compare the histology of coronary arteries in mammals namely buffalo, goat, rabbit and rat. The general pattern of organization of different coats of arterial wall confirms with the description published on the submicroscopic structure of coronary arteries of the rabbit by Benninghoff (1930). Coronary arteries are considered to be a medium size muscular (David, 1987) and the thick wall is due to the large amount of muscle in tunica media. In buffalo subendothelial connective tissue was much more pronounced in the form of 4-5 laminated structure. In goat it was just identifiable while in rabbit and rat it remained inconspicuous. It should be emphasized that in histologic sections, from all animals, the internal elastic lamina did not reveal typical corrugation. It was discernible in goat. In rabbit and rat it appeared as undulating or wavy structure. This is said to be due to contraction of smooth muscle and reduction in the amount of elastin in tunica media (Michael et. al., 1995). Henry (1957) described the internal elastic membrane as a prominent band of homogenous refractile material line between the endothelium and smooth muscle. In buffalo it was in the form of prominent amorphous band. The uniform thickening of intima is believed to be a normal phenomenon of ageing (Peter, 1995).

In the present study intimal thickening in samples from older buffalo revealed a mixed picture that the thickening affect whole intima but more pronounced focally con-committed with hyperplasia of tunica media. The finding of ageing changes in 4 year older buffaloes are in accord with Wayne et al (1994), who described ageing changes in human coronary artery foetal to adolescence. In the coronary of human fetuses intima consists of layer of elongated endothelial cells in close contact with internal elastic lamina and by 1 year intima consists of collection of subendothelial collagen, elastic fibres and musculoelastic layer that eventually is in corporated into mature media. After adolescence intima becomes thicker than the media and in middle age intima may become diseased and markedly thickened by atherosclerotic plaque.

Peter (1995) stated that the mechanics of musculature of media is complex for several reasons because the structure involved have a concentric arrangement. Focal proliferation of tunica media in the deeper part appears to be an interesting feature. Finding of prominent trabeculae of adventitial origin penetrated deep into the muscular coat in buffalo require further study and explanation.

Conclusion

Buffalo coronary artery having similarities with human coronary artery in terms of:

a) Prominent subendothelial connective tissue and internal elastic lamina as well as longitudinally running fascicles of smooth muscle in tunica media.

b) Age-dependent changes in tunica intima appear as early as 2-4 years of age.

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