Previous morphological studies of mammalian pancreatic islets have been performed mainly in domestic and laboratory animals. Therefore, the present immunohistochemical investigation was conducted in a wild species, the European bison, using antibodies against glucagon-like peptide-1 (GLP1), glucagon, insulin and somatostatin. Morphological analyses revealed that the mean area of the endocrine pancreas constituted 2.1±0.1% of the whole area of the pancreas, while the mean area of a single pancreatic islet was 13301.5±686.5 µm2. Glucagon-immunoreac- tive cells accounted for 22.4±1.1% and occupied 19.4±0.4% of the average islet area. As many as 14.3±1.4% of pancreatic islet cells were shown to express GLP1, which constituted 12.6±0.8% of the mean area of the islet. Insulin expression was confirmed in 67.6±0.7% of pancreatic islet cells, which represented 62.3±4.9% of the mean total area of the pancreatic islet. As many as 8.5±1.3% of cells stained for somatostatin. The somatostatin-immunoreactive cell area was 4.9±0.3% of the mean pancreatic islet area. In summary, we have determined in detail for the first time the morphometry and islet composition of the European bison pancreas. The distri- bution patterns of immunoreactivities to the substances studied in the European bison show many similarities to those described in other ruminant species.

According to European Bison Pedigree Book, wisent population in Poland by the end of 2016 counted 1698 individuals, including 1455 animals in freedom. Therefore in Poland, live over 25% of presently living wisents, i.e. the largest population of this species in the world. Next to our state borders, there are free ranging populations in Belarus – Białowieska Forest about 480 individuals, in Ukraine – National Park Beskyd Skolyvski – 33, and Slovakia – National Park Poloniny – 27 wisents. Planned is the establishment of new transboundary populations at Landscape Park of Lower Oder River Valley, at Romincka, and Augustowska Forests. In many places where transboundary populations already exist or are planned, their potential joint home ranges are divided by anthro-pogenic (fencing – Ukraine, Belarus) or natural barriers (Oder River – Germany). The basic prob-lem for creating such populations will be either elimination of such barriers or introduction of passages allowing for animals' movements. Benefits provided by transboundary populations are: larger area of available habitats, a possi-bility for the maintenance of larger populations, and a chance for mitigation of isolation among particular herds. However possible problems include: more difficult population management due to differences in formal status of the species in particular countries, complicated budgeting of costs connected with maintenance of such herds and compensation of damages, and a possibility for a transmission of infectious diseases, not occurring at territory of one of neighbouring countries. Therefore, creation of transboundary populations of wisents is desirable regarding the possibility of extension of their home ranges and increase of effective numbers. Effectiveness of such initiatives will however depend upon a possibility for stable international agreements and routine coopera-tion. Very important is an establishment of an uniform formal status of this species in Europe or at last in EU member countries.