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.

Medulla of adrenal gland