The giraffe heart has a relative mass similar to other mammals, but generates twice the blood pressure to overcome the gravitational challenge of perfusing the cerebral circulation. To provide insight as to how the giraffe left ventricle (LV) is structurally adapted to tackle such a high afterload, we performed a quantitative structural study of the LV myocardium in young and adult giraffe hearts. Tissue samples were collected from young and adult giraffe LV. Design-based stereology was used to obtain unbiased estimates of numbers and sizes of cardiomyocytes, nuclei and capillaries. The numerical density of myocyte nuclei was 120 × 10(3) mm(-3) in the adult and 504 × 10(3) mm(-3) in the young LV. The total number (N) of myocyte nuclei was 1.3 × 10(11) in the adult LV and 4.9 × 10(10) in the young LV. In the adult LV the volume per myocyte was 39.5 × 10(3) µm(3) and the number of nuclei per myocyte was 4.2. The numerical density of myocytes was 24.1 × 10(6) cm(-3) and the capillary volume fraction of the adult giraffe ventricle was 0.054. The significantly higher total number of myocyte nuclei in the adult LV, the high density of myocyte nuclei in the LV, and the number of nuclei per myocyte (which was unusually high compared to other mammalian, including human data), all suggest the presence of myocyte proliferation during growth of the animal to increase wall thickness and normalize LV wall tension as the neck lengthens and the need for higher blood pressure ensues.
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