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For the first time, the cardiovascular system of a modern, fast-growing turkey line (BUT Big 6 n = 40) and that of its corresponding wild form, the Wild Canadian Turkeys (WCT n = 40) was anatomically and light-microscopically compared to ascertain the influence of age (8 and 16 weeks), sex and genetics on their morphology. In the present study, morphometric parameters of the myocardium and the aortic wall of both turkey lines were collected and evaluated. The macroscopic anatomy of the heart and large arteries was mainly consistent in both turkey breeds, but there are differences which can be responsible for the higher incidence of cardiovascular diseases of highly selected, meat type turkeys compared to their wild form.
In relation to their genetically higher body mass, the domestic turkeys of both genders have a smaller heart (0.487 %) compared to that found in the wild turkeys (0.613 %) at week 8 and (0.447 %) to (0.575 %) at week 16.
In both lines, the relative cardiac mass decreases with age. The decrease in the male turkeys is greater than that in the female animals. The lower growth rate of the heart combined with the increase in skeletal muscle mass induces the lower relative heart weight of the male domestic turkeys. The significantly lower relative heart weight of the domestic turkeys requires the higher performance of the smaller heart to supply the larger body of the BUT Big 6 turkeys.
In relation to body weight, the left ventricular wall and the septum of the wild turkeys are three times, the right ventricular wall four times thicker compared to those of the domestic animals. The relative thickness of the ventricular wall decreases with age in both turkey lines.
The microscopic architecture of the myocardium and the myocardial capillary bed hardly differ in the two turkey breeds.
In this study, the diameter of cardiomyocytes averages 8 μm.
The cardiomyocytes in domestic turkeys hypertrophies with age, whereas the cross-sectional area of the wild turkeys´ cardiomyocytes remains constant.
There is no gender-specific difference in diameter of cardiomyocytes.
The mass increase in domestic turkey hearts is due to hypertrophy of cardiomyocytes, whereas the heart mass of the wild type turkeys increases by hyperplasia of cardiomyocytes.
By contrast, in aging wild turkeys, the number of capillaries in the myocardium of the left ventricle rises from 2400 per mm2 at the age of 8 weeks to 3000 mm2 at the age of 16 weeks, but remains constant by 3000 mm2 in the turkey breed BUT Big 6.
In both turkey lines, there is a significant decrease in the ratio of the number of cardiomyocytes and the capillaries that ensure blood supply.
Whereas the diameter of cardiomyocytes of the domestic turkeys increases significantly with age, the number of capillaries remains constant. In contrast, the diameter of cardiomyocytes of the wild type turkeys remains constant but the number of capillaries increases considerably.
The wild turkeys have a higher relative heart weight than the domestic ones and a constant diameter of cardiomyocytes associated with a rising number of capillaries per mm2 myocardium. Therefore, the reduced intercapillary distance and consequently the shorter diffusion course of oxygen and nutrients lead to a higher cardiac capacity of the wild turkeys in relation to physical exertion and an increased perfusion of the myocardium, respectively.
Gender-specific differences of the myocardial capillary proportion exist only in the 16 weeks old domestic turkeys. Male turkeys have a lower capillary proportion than female ones. Furthermore, these male domestic turkeys have a lower relative heart weight, larger cardiomyocytes and a significant negative interrelation of heart weight and capillary density.
The considerably larger diameter of cardiomyocytes in 16 weeks old domestic male turkeys along with a constant capillary density entail a longer intercapillary distance for the exchange of oxygen and nutrition, and therefore a lower cardiac capacity of the domestic animals.
The parameters detected for the 16 weeks old male domestic turkeys, such as a lower relative heart weight and hypertrophic cardiomyocytes with constant capillary density may, especially under increased physical exertion, be responsible for the higher incidence of exercise-induced cardiovascular diseases.
The structure of the aortic wall differs only slightly in both turkey breeds, but there are quantitative differences in the external aortic diameter, wall thickness, number of elastic fiber bundles and elastic proportion of the tunica media of the aortic wall.
The domestic turkeys have a higher absolute aortic wall thickness than the domestic turkeys.
However, the wild type turkeys have a wall thickness that is significantly, sometimes up to two times, larger than the relative wall thickness of the domestic turkeys. The relative wall thickness decreases in both turkey lines by one third, depending on age, so the 8 weeks old animals have a thicker relative aortic wall than the 16 weeks old turkeys.
At the outflow of the A. ischiadica, the relative aortic wall thickness of both turkey lines decreases to 35 % of the thickness at the aortic arch.
Between the outflow of A. coeliaca and A. ischiadica, the Aorta changes from elastic to muscular wall type in both turkey lines, as the proportion of elastic fibres decreases from 75 % to 30 %.
In the wild turkeys, more elastic fiber bundles are found per mm wall thickness than in the domestic turkeys. In relation to the wall thickness, young turkeys have more bundles of elastic fibers than older ones.
In relation to body weight, the aortic lumen in WCT turkeys is 2-3 times larger than that of the domestic turkeys at all three points of measurement. The diameter of the aortic lumen decreases from proximal to distal, regardless of gender.
Due to the larger diameter of the aortic lumen in the wild type turkeys, the flow resistance decreases exponentially, which entails a lower blood pressure than in domestic turkeys.
These differences observed between domestic and wild turkeys might be the reason for the reduced cardiac capacity and the higher blood pressure of the domestic breed. It is to assume that micro- and macroscopic parameters such as higher body weight and lower relative heart weight, lower perfusion of the myocardium, lower relative aortic wall thickness and lower relative diameter of the aortic lumen in the highly selected meat breed turkeys compared to the wild type turkeys, could be responsible for the higher incidence of cardiovascular diseases in the domestic turkeys. An integration of certain parameters, such as relative heart mass and relative aortic wall thickness, in the breeding objectives of high meat, domestic turkeys that are purpose-bred with an emphasis on meat could reduce the incidence of cardiovascular diseases.