Aims: 

The mean arterial pressure of the giraffe is twice as high as humans and other mammals (Brøndum et al., 2009), but heart mass relative to body mass is similar to other mammals (Mitchell and Skinner, 2009). To understand how the giraffe heart generates these high pressures, we studied the structure of the left ventricle using stereology.

Methods: 

Tissue was gathered by systematic uniform random sampling from the hearts of 26 giraffes, including 4 calves (less than six month old), and analysed with a microscope connected to a computer equipped with newCAST software (Visiopharm, Denmark).

Results: 

The mean volume density of left ventricular (LV) cardiomyocytes in the adult heart was 0.89 (0.01) (CVtot) and the total volume was 1053.7 cm³ (0.3). In the immature hearts, these were 0.89 (0.02) and 93.6 cm³ (0.3), respectively. The numerical density of LV cardiomyocyte nuclei was 103.7×10³ mm^-3 (0.25) in the adult hearts and 487.7×10³ mm^-3 (0.17) in the calves. Mean number of nuclei per cardiomyocyte was 3.9 (0.15) and total number of LV cardiomyocytes was 34×10⁹ (0.3). The volume per myocyte was 31025 mm³. Further the length of the myocytes as well as length and surface area of the capillaries will be estimated.

Conclusion: 

The numerical density of myocyte nuclei and number of nuclei per myocyte was significantly larger than other mammals, but it remains to be shown whether this difference is linked to the high pressure. We are currently relating physiological in vivo measurements from the same individuals to our structural findings.

References: 

Brøndum et al (2009). Am J Physiol 297, R1058–1065. Mitchell G. And Skinner J.D. (2009).Comp. Biochem and Physiol, Part A 154, 523–529.