There are literature reports and figures showing the structure of the adrenal gland in various mammalian species, including domestic and wild animals. Existing reports describe the morphology of the pancreas, thyroid gland and thymus. Although more than 100 original studies have been published focusing on the morphological structure of organ systems in the European bison, few of them have analysed the endocrine glands in this species. The European bison morphology has been studied since the 1920s. There was a thick layer of adipose cells at numerous locations in the adrenal capsule of the bison. There were no significant differences in the structure of the adrenal medulla and cortex of the European bison compared to other species of domestic and wild mammals.

The left adrenal gland was significantly longer, narrower and thinner than the right one. Sinusoidal vessels were located in the central part of the medulla. The adrenal medulla was composed of large, dark cells with a highly basophilic cytoplasm in the superficial region and of smaller, lighter cells in deeper layers. There were vacuoles in the cells of both zona fasciculata and zona reticularis, which formed a foamy cytoplasm. Numerous apoptotic cells were observed among regular cells in the zona reticularis. The zona glomerulosa contained cells arranged in bundles and curves. It had a fibrous structure and abundant adipose tissue.

The connective-tissue capsule contained two layers. The adrenal gland consisted of a cortex and a medulla. Reference intervals for adrenal gland size and weight were computed separately for two bison age groups (up to 2 years of age and older than two years). However, there was a difference in the length, width and thickness of the two glands. There was no statistically significant difference in weight between the left and the right adrenal gland. There were no statistically significant differences between ipsilateral adrenal glands of males and females with respect to age. The growth of the adrenal glands corresponded to the growth of the whole body, with the largest increase in size occurring in the first 2 years of the animal’s life, followed by a slower increase in size until the animal was 5–7 years old.

The study was conducted on 97 European bison of both sexes. However, there are no available literature reports describing the morphology and morphometry of the adrenal glands of the European bison ( Bison bonasus). The anatomy of the adrenal glands has been widely studied in many species of domestic and wild mammals.

Adrenal cortex adrenal medulla