Numerous studies on the shape of the adrenal gland were carried out in various wild and domesticated animal species [9–11, 14–16, 19, 20]. The shape of the adrenal glands may differ not only between species but also between animals of the same species. In all the examined European bison, the left adrenal gland resembled the shape of number “1”, while the right adrenal gland was triangular. A similar adrenal shape was described in domestic cattle and small ruminants . Studies carried out in the Nili-Ravi buffalo showed that the left adrenal gland had a V-shape, while the right gland was C-shaped .
Erdoğan and Pérez  described the shape of the adrenal glands in the pampas deer and found inter-individual differences in the shape of the left gland. Two of the ten studied deer had a V- or heart-shaped left gland, while the remaining eight animals had an oval shaped one. The right gland was oval-shaped in all the studied deer.
There are several morphometric studies of the adrenal gland in literature. They describe the relative and absolute weight, length, width, thickness and circumference of each gland. However, our study is the first to report reference intervals for size and weight of the adrenal glands which may prove useful to clinicians, in particular if abdominal sonography is planned as a diagnostic method in sedated European bison.
The studies carried out in the pampas deer by Erdoğan and Pérez  revealed that the right adrenal gland mean weight was 0.59 ± 0.09 g, and the mean weight of the left adrenal gland was 0.64 ± 0.1 g. The mean length and width of the left adrenal gland (18.04 ± 0.94 mm, 11.83 ± 0.81 mm) was larger than the right gland (17.3 ± 1.58 mm, 10.11 ± 0.42 mm). The mean thickness of the right adrenal gland (5.11 ± 0.45 mm) was greater than that of the left gland (4.59 ± 0.31 mm).
Hussain et al.  presented the absolute and relative weight of the adrenal glands in the Nili-Ravi buffalo. The absolute weight was 23.70 ± 1.12 g, and the relative weight was 0.06 ± 0.001 % in young animals. In old animals, the absolute and relative weights were 33.85 ± 1.17 g and 0.05 ± 0.001 g, respectively. The authors provided the length and width of both the left and the right adrenal glands. The length and width of the right adrenal gland in young animals were 5.03 ± 0.05 cm and 3.84 ± 0.06 cm, and 6.69 ± 0.09 cm and 4.13 ± 0.0015 cm in adult animals, respectively. In the left adrenal gland, those measurements were 5.17 ± 0.08 cm and 2.85 ± 0.05 cm in young animals and 6.85 ± 0.100 cm and 4.82 ± 0.22 cm in adult animals, respectively. The mean length of the adrenal glands was significantly greater in adults than in young individuals. However, the difference in the width of the right adrenal gland between young and adult buffaloes was non-significant .
In the studied European bison population, there was no difference in the weight of adrenal glands with respect to age. However, all three size parameters differed between the left and the right gland. The left adrenal gland was significantly longer, narrower and thinner than the right one in all the age groups.
In the European bison, like in other mammalian species, the capsule consisted of an inner and outer layer, which differed in the amount of connective tissue [19–21]. Interestingly, we found a substantial layer of adipose cells between the inner and the outer layer of the capsule, which may be presumed to be characteristic of wild animals. There are no literature reports describing a similar finding in domestic animals. According to Nabipour , the adrenal capsule in camels (Camelus dromedarius) has dense irregular connective tissue. Al-Bagdadi  reported that the adrenal capsule in camels (Camelus dromedarius) consists of two layers, and the inner layer contains cellular elements. Similarly to domestic animals, the adrenal capsule in the European bison contained a lot of collagen, elastin and smooth muscle fibers [19–21]. Vuković et al.  found mainly collagen fibres and a few elastin fibres. In the European bison, similarly to domestic animals and camels, the adrenal capsule was well developed and formed connective tissue trabeculae that penetrated the cortex and rarely entered the medulla [10, 19–22]. In contrast, Vuković et al.  reported that in marine mammals (dolphins) the thick connective tissue trabeculae did not divide the cortex into pseudo-lobes.
Similarly to other land and marine mammals, the adrenal gland in the European bison consisted of the cortex and medulla. The cortex contained three layers – the zona glomerulosa, zona fasciculata and zona reticularis.
In the zona glomerulosa of the European bison, the cells formed numerous bundles and arches, similarly to domestic ruminants, horse and carnivores [19–21, 23]. The cells of the zona glomerulosa in the European bison were polygonal in shape as in domestic mammals, camels and marine mammals [9, 10, 22].
In the European bison, as in humans and domestic ruminants, the cells in the zona fasciculata were predominantly arranged in bundles . They were oval and arranged in single or multiple arch-shaped strands. On the other hand, in the zona fasciculata of the marine mammals (dolphin), the arches passed into columns and contained one or two cell rows in each cord . Those cells were large and polygonal in marine mammals. Our research showed numerous apoptotic cells in the zona reticularis. A similar finding was described in cattle . In the European bison and camel (Camelus dromedaries), many cells in the zona fasciculata and zona reticulata contained vacuoles, which formed a foamy cytoplasm and indicated the presence of lipids . In the European bison, the structure of the adrenal medulla was similar to that of domestic animals [9, 19–21]. In the superficial layer of the adrenal medulla, there were large dark cells with a strongly basophilic cytoplasm. Small, lighter cells were present in the deeper layer of the medulla. According to Jelinek and Konecny , both types of cells were intertwined in cattle. According to Dellman , the medulla was divided into two parts in domestic animals (horses, cows, sheep and pigs). The outer part was composed of strongly staining epinephrine-secretin cells, while the inner part was formed from cells with a weaker stain affinity (norepinephrine-secreting cells). A similar structure of the adrenal medulla was reported in marine mammals by Vuković et al. .